Different species of large tropical butterflies belonging to the genus Morpho vary dramatically in both the amount of blue color on their wings and the associated irridescence (reflectance). This paper discusses how such a morphological properties may be related to both courtship behavior and effective means of reducing predation (especially by birds) in the low density adult populations. Essentially, the hypothesis is advanced that territorial species of Morpho can afford to possess very conspi
cuous wing coloration that may facilitate courtship interactions, in addition to spacing the territorial male population over the suitable habitat. While territoriality may be favored by natural selection, such behavior can only evolve if the species involved possess effective means of reducing predation, because territoriality in morphos is an extremely predictable form of behavior, toward which predators can easily orient. Two alternate hypotheses are advanced to account for low predation in a territorial morpho, Morpho amathonte, a species in which males are very bright and showy. The first hypothesis, which is more consistent with traditional ideas on the function of bright colors in morpho wings, maintains that predators learn quickly to avoid these butterflies as prey, since they are very difficult to catch. The second hypothesis, suggests that conspicuous territorial male morphos actually employ pursuit-stimuli to invite birds to attack and be subsequently unsuccessful.
Previous authors have used simple models to investigate the relative importance to population increase of variations in the total and age-specific reproductive rates. But while acknowledging that the latter were the product of the age specific birth and death rates, they have used their models only to investigate changes in total or age-specific birth rates and have not been concerned with variations in death rates. This paper extends the use of Lewontin's (1965) model, to a wide range of values
of r, the exponential rate of population increase. It shows how the relative importance of changes in certain life-history features can change with r and be reversed when r is near to zero. It is also shown that variations in mortality rate are not necessarily best expressed in analogous terms to variations in birth rate. If more suitable terms are used it is seen that changes in mortality rate can be of varying importance depending on the existing mortality rate. They can be overwhelmingly important when the mortality rate is high.
The sensitivity to photoperiod of Aelia acuminata after diapause was proved. In order to reproduce females need a continuous stimulation by a diapause averting photoperiod. If kept under a diapause promoting photoperiod they stop ovipositing after about 3 weeks.
The ability of the lime aphid to produce sexuals (males and females) as an alternative to parthenogenetic females is regulated by a timing mechanism (‘interval timer’) which restrains the appearance of these morphs early in the year. In this species there are two ‘interval timers’: one controls the production of sexual females, and the other controls the production of males. Both ‘intervals timers’ are sensitive to day-length and temperature but they respond in different ways.
With the approach
of autumn the waning effect of the ‘interval timer’ inhibiting female production combined with the short day-lengths results in an increasing proportion of the aphids developing into sexual females. The restraining effect of the ‘interval timer’ results in a gradual transition from parthenogenetic to gamic reproduction over a period of several generations and is still operational in the autumn. However, in this species even relatively long day conditions (17 h) can induce the development of oviparae. This low threshold of response to day-length combined with the short generation time results in the sexual morphs appearing very early in the year. This is of considerable adaptive significance in years when, as frequently happens, the aphids disappear locally before the onset of autumn.
The effect of phenological development on the pattern of photosynthate translocation was studied in crested wheatgrass [Agropyron cristatum (L.) Gaertn.] plants grown in a nursery under semi-natural environmental conditions at Logan, Utah. Radiophosphorus was used to trace the photosynthate translocation from April 13, 1968, through December 2, 1968.
In early spring photosynthates were translocated about equally to the roots and younger leaves. Translocation was chiefly upward during the flower
stage but was reversed with raturation of the inflorescence. In late summer the plants appeared quiescent but substantial 32P was transported to the underground portions of the plant. After quiescence was broken in the fall, shoot and root growth were resumed simultaneously with peak 32P movement to the crown and roots.
The concentration of total available carbohydrates in the roots and crowns reached a maximum level just prior to quiescence and decreased during shoot production in the fall. Presumably, the carbohydrates were used in growth and the accumulation of carbohydrates for the winter is apparently not critical in crested wheatgrass. Since the plants have many basal leaves throughout the winter, they may have the capacity to carry out photosynthesis on warm, sunny winter days and are capable of immediately initiating growth in the spring.
Nectar foraging preferences of Colias butterflies in two different mountain ecosystems are examined with respect to plant distribution, nectar quantity, carbohydrate (and amino acid) content of nectar, and visual pattern of the plants utilized and avoided. Colias, and apparently numerous other small, ectothermic, low-energy-demand pollinators, “patronize” plants producing relatively dilute nectars containing a high proportion of monosaccharide sugars and significant amounts of polar, nitrogen-ri
ch amino acids. These plants also converge on a common “target” flower pattern in ultraviolet and human-visible light. High-energy demand, endothermic pollinators, by contrast, appear to require higher concentration nectars and/or higher proportions of di- and oligosaccharide sugars. These results are discussed in the light of water balance and energy budget demands of different pollinator classes. Questions are also raised concerning behavioral aspects of pollinator search for resources and the pertinence of these data to the concept of floral mimicry.
A Markov model of pollinator activity is used to study the effect of constancy on rare plant species. Roughly, constancy is the tendency of the pollinator to continue searching for a flower of the species last visited. The model involves and abundant plant speciesA and two uncommon plant species, B and C, where the pollinator does not distinguish betweenB andC. The effect on the frequency of pollinator visits to species B when C has newly invaded the colony is determined. It is shown that, under
the right conditions, constancy and competition for pollinator services can result in an explosion in the number of specimens of B present in future generations. Conversely, if C becomes extinct constancy and competition for pollinator services can result in the extinction of species B.
Two Colorado populations of Pieris butterflies show a spectrum of larval growth responses to potential foodplant crucifer species growing in montane habitats. Analysis of larval growth responses to this array suggests potential selection for differential utilization of these species: 1) available crucifers vary considerably in the rates of larval survival and growth they support; 2) climatological factors favor larvae which develop rapidly. Food-related larval mortality and climatological factor
s may provide selection for utilization of crucifer species on which larvae develop rapidly.
Larvae of the danid butterfly Danaus chrysippus, fed on the milkweeds Calotropis gigantea and Asclepias curassavica grew successfully during the 9-day feeding period and pupated. In the presence of these milkweeds, the larvae preferred to feed on C. gigantea. Their preference was more pronounced during the final 2 instars than during the initial 3 instars. They consumed 605 mg, defecated 308 mg, and converted 115 mg of C. gigantea; those receiving A. curassavica consumed 563 mg, defecated 287 mg
and converted 85 mg. They assimilated either food with equal efficiency (♀♀: 46%; ♂♂: 50%). But they differed significantly in their efficiency in converting the assimilated food. The presence of a greater amount of latex in C. gigantea than in A. curassavica and/or the nutritional inadequacy of the latter food may perhaps be the reason(s) for the preference of C. gigantea by D. chrysippus larvae over A. curassavica.
The effect of pollinator activity on gene flow in colonies of Viola were examined by measuring pollinator flight distances, the frequency of interplant flights and percent pollination under different plant spacing patterns. Pollinator flight distances were directly proportional to spacing parameters while the frequency of interplant flights and percent pollination were inversely proportional to spacing parameters. These findings show that gene flow is reduced by pollinator activity over a wide r
ange of spacing parameters but in populations with low spacing means highly localized gene exchange can occur within the colony. Isolation of colonies may be expected under these circumstaces and cleistogamy may be the optimal breeding system. However, chasmogamous flowers may be important both in promoting with-in-colony gene exchange and long distance between-colony gene exchange corresponding to the sexual functions proposed in several recent models. Viola colonies appear to be semi-isolated demes with pollinator service which can bring adaptive genes to high localized frequencies, but which maintains low frequency, long-distance gene dispersal. This pattern corresponds to the "Shifting Balance" view of evolution.
A comparison of the floral syndromes, flower biomasses, pollen and total sugar production of the dominant perennial species of two climatically similar but disjunct desert scrub ecosystems was made to assess the degree of convergence in breeding systems. Results indicate that the dominants at the northern site in the Sonoran Desert near Tucson Arizona, USA, possess more diverse floral types, utilize more reproductive methods, produce a greater annual biomass of flowers, provide more rewards for
potential pollinators and employ a wider array of pollen vectors than those at the southern site near Andalgalá, Catamarca, Argentina. The discrepancies in these features can be best explained in terms of the differences in the annual dispersion of rainfall at the two sites. However, when compared to the dominant species of two Mediterranean scrub ecosystems, the breeding systems of the dominants of the desert scrub sites proved to be more similar to one another than to those of a neighboring but different ecosystem type.
This paper examines the reproductive cycles of three ecologically important marine bivalves- Modiolus modiolus (L.), Cerastoderma edule (L.), and Mytilus edulis L. in Strangford Lough, Northern Ireland over a period of almost five years. Whilst Modiolus does not appear to become sexually mature until it is several years old, Mytilus and Cerastoderma can reproduce in their first and second years of life respectively. In Cerastoderrna and Modiolus sexual maturity is preceded by a period of rapid s
omatic growth. The subtidal Modiolus population remained in a more or less futly ripe condition virtually throughout the period of this investigation suggesting that this particular population lacked any marked cyclical reproductive activity. We interpret this as evidence of slow but almost continuous release of gametes throughout much of the year, a suggestion which is supported by recruitment data. A small intertidal population of Modiolus in Belfast Lough monitored over a period of two years exhibited a much more seasonal cycle. Here spawning occurred mainly during the autumn and winter. These data suggest that localised environmental factors are exceedingly important in controlling the annual reproductive cycle of this species. Cerastoderma from the mid-tidal sand flats ripened rapidly during the spring and spawned over a relatively restricted period in the summer. In Strangford Lough Mytilus occurs predominantly in the low-shore and while it spawns mainly in the spring and summer the annual cycle is considerably more protracted and variable than in Cerastoderma. Variations in the duration of the spawning periods in these bivalves can perhaps be explained in terms of both environmental stability and the immediate physical conditions experienced by these particular populations. The reproductive strategies exhibited by Cerastoderma, Modiolus, and Mytilus in Strangford Lough are considered in relation to population stability and to the different patterns of mortality which characterise these species in their respective local habitats.
Pterostichus nigrita undergoes a gonad dormancy which is overcome by the sequence of short-day/long-day in females and by short-day in males. Experiments with abnormal photoperiods showed that only photoperiods of 24 h and their multiples in whole numbers allowed the processes of gonad maturation, which are normally bound to short-day (i.e., previtellogenesis in the females, bundling of sperms to spermiozeugmata in the males). They were mostly suppressed by photoperiods which represent uneven mu
ltiples of 12 h (12, 36, 60). These results permit the conclusion that the short-day measurement is based on an oscillatory (circadian) process. Experiments with “dark breaks” in an extreme long-day (LD 20:4) resulted in two peaks of “short-day” effects. From these findings a model of short-day measurement was derived. It postulates that there are two dark sensitive phases every 24 h during which the beetles require darkness in order for the short-day processes to occur. One of these phases is set by dawn, reaching its peak 15 h thereafter. The other phase is set by dusk and reaches its maximum about 7 h afterwards. Thus, the steps of gonad development bound to short-day are induced if the night is at least about 8–9 h long. The two scotophile phases represent two systems of short-day measurement, which complement each other and strengthen their effects.
Light break experiments revealed that the long-day process of ovarian development (vitellogenesis in the females) is induced if light falls into a photosensitive phase during the second half of the day with its maximum about 15 h after “light on.”
The aims of this paper were to consider the coevolution between bumblebee movement patterns within plants and various properties of the plants such as the spatial distribution of their flowers, and to determine the extent to which the bumblebees and the plants can be considered to be maximally adaptive or optimal. Attention was restricted to plants which have flowers arranged on vertical inflorescences and to the bumblebees which visit these plants.
It was found that the bumblebees tend to comme
nce foraging at the bottom of each infloresence, that they tend to move from one flower to the closest vertically higher flower, that they miss flowers as they move upwards and that they tend to leave each inflorescence before reaching the top. It was also found for the four common plant species considered that nectar abundance per flower decreases with flower height on an inflorescence, that the flowers with receptive stigmas are restricted to the bottoms of the inflorescences while the flowers shedding pollen occur above them, and that the flowers are arranged approximately in spirals on the inflorescences.
The pattern of movements of the bumblebees and the various properties of the plants appear to represent coevolved adaptations. Furthermore the bumblebees' movement patterns appear to be optimal in the sense that they result in the maximum net rate of energy gain to the bumblebees. Further studies are necessary, however, to determine whether or not the plants can be considered to be optimal.
An exception to the above scheme is provided by a plant which is quite uncommon in the study area. This plant also has flowers on vertical inflorescences and appears to be pollinated by bumblebees. However, while the pattern of movements of the bumblebees on this plant species are extremely similar to those on the four common species, this plant species exhibits quite different properties from the other four. Two possible explanations for this exception are presented.
Two species of Banksia (family Proteaceae) studied in Australia were shown to be pollinated by small, non-flying mammals rather than by birds as previously thought, and to possess several adaptations appropriate for mammal-rather than bird-pollination: odor, troughs that channel excess nectar to the ground for attraction, open inflorescence structure for nectar accessibility, hooked wiry styles for effective pollen transfer, crepuscular and nocturnal nectar and pollen presentation, and copious n
ectar. This apparently is the first documentation with quantified data of pollination by non-flying mammals, although many other probable examples exist.
Differences in major selective forces important in early and late successional communities should influence niche breadth and degree of overlap. Early successional species may not experience consistent, strong selection against competition and can be expected to have broader niches with more overlap than later successional species. This paper presents data from 2 early successional winter annuals that show clear niche separation under conditions in which coevolution is unlikely.
L. and Erigeron annuus (L.) Pers. flower in mid-summer and attract the same insect visitors. Erigeron, the more common species, begins to flower in mid-morning when insect visitors are quite active. Lactuca flowers open earlier in the morning and close just before Erigeron flowers open. The early morning hours are not optimum for Lactuca visitation, as most of the visits occur just before floral closing. Both species can selffertilize without a vector, and Lactuca was introduced from Europe. It is, therefore, not possible that niche separation inflowering time and subsequent time of visit in Erigeron and Lactuca is a result of coevolutionary niche differentiation between the two species. This separation is likely to be a preadaptation resulting from coevolution with other species.
It is hypothesized that the body size of a bumblebee will be that size which maximizes its average net rate of energy intake while collecting nectar. A mathematical model is developed with the result that the net rate of energy intake of a nectar-collecting bumblebee is expressed as a function of the body size of the bumblebee. From this model the body size which maximizes the net rate of energy intake (i.e., optimal body size) is found (as the solution of an implicit equation). In this situatio
n the advantage of large size is that larger bumblebees fly faster and hence take less flight time than smaller bumblebees. The disadvantage of larger size is greater energetic costs.
The parameters of the model are estimated using data obtained from the foraging behavior of bumblebees on monkshood (Aconitum columbianum). The optimal body size is then calculated for workers of Bombus appositus which obtained almost all their nectar from monkshood. The observed and expected (i.e., optimal) body size are found to be close and not significantly different.
The model also predicts that, from the bumblebee's point of view, there should be a positive correlation between the size of the bumblebee and the average amount of nectar obtained per flower. Evidence of this correlation is presented and the possible significance of the correlation from the plant's point of view is discussed. A possible extension of the model to general relationships between predator body size, prey size and prey density is discussed.
Desert populations of the evergreen dioecious shrub Simmondsia chinensis exhibit sex-related leaf and canopy dimorphisms not present in populations from more mesic coastal environments. Leaves on female shrubs have characteristically larger sizes, greater specific weights, and greater water-holding capacity than male leaves in desert habitats. In coastal scrub environments no significant difference is present, with leaf characteristics of both sexes similar to those of desert male shrubs. Desert
female shrub canopies are typically relatively open with little mutual branch shading. In male shrubs canopies are more densely branched with considerable mutual shading of branches. Female plants allocate a greater proportion of their vegetative resources to leaves than do male plants. Considering total biomass, male plants allocate 10–15% of their resources (biomass, calories, glucose-equivalents, nitrogen, phosphorus) to reproductive tissues. Female allocation is dependent on seed set. At 100% seed set females would allocate 30–40% of their resources to reproduction, while female reproductive investment would equal that of males at approximately 30% seed set. Sexual dimorphism and the associated physiological characteristics in Simmondsia act as an alternative to differential habitat selection by male and female plants. Female plants respond to limited water resources in desert areas by increasing their efficiency in allocating limited resources to reproductive structures.
There are several sources of potential error in calculating the concentration or energy value of floral nectar. Errors resulting from confusing data become substantial with increasing concentration. The different methods of expressing sugar concentration are here clarified, and the correct methods of converting from one to the other are provided. Refractometers in use in field studies usually read on a weight per total weight basis; this is recommended as the mode of statement. The perils of ove
rsimplifying conversions from this mode, as is often done, are pointed out.
The number of florets visited per unit time was independent of flower head density from 20 to at least 290 heads/m2, in part because the bees utilized more florets per head at low flower head densities, and also because they approached but rejected more flower heads at high rather than at low flower head densities. Previously visited clover-heads were approached but often rejected, while unvisited heads were not rejected. The bees behaved markedly different while foraging in patches of flowers w
hich were available to all foragers, than in those which had been screened and contained on the average 3.9 times more sugar; they tendent to move through depleted areas and to concentrate in rich areas. On successive flower-head visits in depleted areas they moved more forward than backward (82% vs. 18%), while in rich areas they tended to move almost as much backward as forward 47% vs. 53%). The distances of inter-head moves were approximately twice as long in depleted as in rich areas. The bees visited almost as many florets per unit time in the rich as in the depleted areas (32 vs. 35 per min). But in the rich patches they probed on the average into 11.6 florets per head in contrast to only 2.3 florets per depleted head. In an experiment with B. vagans workers foraging from Aconitum napellus inflorescences, the bees did not reject previously visited flowers, and they moved upward in successive flower visits on inverted as well as on unaltered inflorescences. On horizontal inflorescences they moved both right and left. The movements are not a direct response to nectar differences, nor to differences in average nectar distributions. The systematic foraging behavior on vertical inflorescences may thus be a mechanism of reducing the revisiting of just-emptied flowers.
The ability of the biennial herb, Pastinaca sativa L. (wild parsnip), to respond to and compensate for destruction of primary umbel seeds by the larvae of Depressaria pastinacella (Lepidoptera: Oecophoridae) was analyzed by comparing umbel and seed production of damaged and undamaged plants collected from five populations. Plants with a basal stem diameter smaller than 8 mm suffer a reduction in seed set of about 50% when the primary umbel is destroyed but larger plants are able to compensate fo
r loss of primary umbel seeds by increased seed set of tertiary umbels. Depending on plant size, this is due to either an increase in the number of tertiary umbels that reach maturity or an increase in the number of seeds per tertiary umbel. Although seeds of tertiary umbels are significantly smaller than those of primary or secondary umbels, their viability is equivalent to that of secondary seeds and may be greater than that of primary seeds. Characteristics of P. sativa's reproduction, such as the long flowering period and the initiation of more umbels than the plant is normally able to bring to maturity, are important to P. sativa's ability to compensate for the effects of herbivore damage.
Ocotillo, a perennial shrub of Sonoran and Chihuahuan Deserts, produces its red tubular flowers in spring. This timing coincides with northward migration of hummingbirds through desert areas. Observations of visitors, pollen collections, and seed set reductions following exclusion of different flower visitors indicate that both hummingbirds and solitary bees pollinate ocotillo in southern Arizona. Seed sets of flowers on marked plants varied considerably within and between years, and this variat
ion was related to the temporal match between flowering and hummingbird migration. This suggests that selection acts on plants to synchronize flowering with periods of pollinator abundance.
The above-ground heights of inflorescences of 8 species of wild-flowers in a subalpine meadow in the Colorado Rocky Mountains were measured in two successive years. An index of overlap (of height-distributions) was computed for pairwise comparisons of all species. The species were assorted into 4 groups based on their usual pollinators: long-tongued bumblebees (3 species were pollinated by long-tongued bumblebees), short-tongued bumblebees (3 species), hummingbirds (1 species), and solitary bees
(1 species). The values of the sample of overlap indices for plants pollinated by the same animals was significantly smaller than the values for plants pollinated by different animals; plants which share pollinators are less alike in height than those that don't share pollinators. It is suggested that this is a result of selection for enhancement of pollinator fidelity. The selective mechanisms, based on the ‘horizontal’ flight pattern of pollinators and the consequences to the plant of interspecific flights, are discussed.
Flight patterns of honeybees (Apis mellifera ligustica) were quantified as the bees foraged among artificial 'flowers' for sugar solution ('nectar'). Bees exhibited considerable directionality on successive flights which minimized repeat visits to flowers and they usually made short flights to nearby flowers, thus minimizing flight time. The change in direction on successive flights between flowers was independent of the number of immediately preceding consecutive rewarding visits but decreased
as the number of non-rewarding visits increased. Flight distances were short after visits to rewarding flowers but increased as the number of immediately preceding non-rewarding visits increased. The bees' rate of caloric intake (calories/time) was highest at the floral arrays having the highest density, and it was greater at arrays with clumped nectar-distributions than at those with randomly distributed nectar. These findings are explained in terms of the observed flight patterns.
Amaranthus and several other wind-pollinated species of plants are used to test some of the theoretical models of relative reproductive effort towards the male and female sexes. Consistent with these models, in self-compatible, monoecious Amaranthus, Chenopodium, Digitaria, Setaria, and Lepidium, female effort represented over 90% of the total reproductive effort. Also consistent with predictions, Lolium, a self-incompatible wind-pollinated species, was found to have about equal male and female
effort. A method is described here that should prove useful in quantifying male and female effort in both wind and insect-pollinated species of plants.
The pollination biology of the 20 plant species of a treeless, pavement plain in the San Bernardino Mountains of southern California was studied throughout one flowering season.
Occasional insect visitors were a feature of the visitation to nine of the twelve entomophilous plant species and were the sole pollinators for three of these twelve species.
The eight entomophilous plant species which had open, generalized flower morphologies received the heaviest pollinator visitation, while three of t
he four entomophilous species with specialized flower morphologies received little visitation.
Most regular flower visitors, whether bees, flies, or wasps, appeared to be similar with respect to number of plant species visited regularly, purity of pollen load, length of residence and localization of activity on the site.
The question is raised as to whether such similarity of behavior as pollen vectors is a function of the low plant diversity or a feature commonly found when the pollen loads and behavior of different pollinator types are actually monitored.
The floral nectar of angiosperms is primarily a solution of simple sugars, but contains detectable amounts of other solutes, particularly amino acids. These have been regarded as diagnostic, for phylogenetic and taxonomic purposes, and their mean concentrations may be significant in relation to the pollination syndrome of a plant species. However, in several temperate flowers the amino acid concentration varied by an order of magnitude when measured at intervals through a single day. In open, cu
p-shaped flowers, this was partly due to post-secretory equilibration with the varying humidity of the air. But the effects of floral visitation by insects on nectar amino acids were also important, both in open flowers and in those with long corollas and more enclosed nectar. Visitors could add amino acids to nectar by direct contact, by salivation, by damaging the neighbouring tissues causing cell leakage, and by dislodging pollen into the nectar; the importance of each of these effects varied according to corolla morphology and the spectrum of visitors.
Evidence is presented that individuals of a large number of dioecious and subdioecious plant species are able to alter their sexual state in response to changes in the ambient environment and/or changes in size or age. We suggest that lability of sexual expression probably has survival value where a significant portion of the females must otherwise bear the cost of fruit production in unfavorable environments. We demonstrate that in patchy environments of the proper scale and variability in qual
ity, labile sexual expression will enhance an individual's genetic contribution to the next generation.
Lotus scoparius is a drought-deciduous shrub which is an early and abundant colonizer of sites following fire in southern California chaparral. Productivity, seasonal nutrient concentrations, nutrient accumulations and phenology were studied in a 4-year-old burn site in Adenostoma chaparral in which L. scoparius had established 49% cover. Net aboveground primary production for L. scoparius was 105 g m-2 y-1; leaves accounted for 40% of the annual production. The true increment to biomass was onl
y 17 g m-2 y-1; 83% of the net production entered the litter layer or standing dead components. In response to the Mediterranean climatic regime, most of the annual net production and plant activity occurred from May through June when photoperiod and temperatures were favorable and moisture was available. In July leaf abscission occurred in response to the summer drought conditions. Correlation and principal component analysis suggested consistent seasonal behavior in the foliar concentrations of N, P, Zn, and Mn. Nitrogen, P, K, and Zn were strongly reabsorbed from leaf tissues before abscission. Calcium, Mg, and Fe formed a second functional group of elements which increased in concentration throughout leaf maturation and which were not reabsorbed from senescing foliage. The seasonal pattern of nitrogen-containing organic compounds (chlorophylls and proteins) was most associated with the leaf phenology and water stress. The rapid growth of Lotus scoparius plays a role in conserving nutrients that might be lost through runoff and erosion after fire in the chaparral.
The number of flowers produced by inflorescences of Yucca whipplei (Agavaceae) consistently exceeds the number of fruits produced by about one order of magnitude. To determine the factors responsible for low fruit set, the relation between pollinator availability, the amount of resources spent on reproduction (as indicated by inflorescence size), and the number of fruits matured was studied during 1978 and 1979 at 18 locations in chaparral, coastal sage scrub, and desert scrub communities of sou
The following results support the conclusion that pollinators do not usually limit fruit production in Yucca whipplei. Rather, fruit production is limited by the amount of resources available to support developing fruits. (1) Fruit production is positively correlated with inflorescence size both within and between populations. The average size of inflorescence for a population is an excellent predictor of mean fruit production. Furthermore, 54% of the total variance in fruit production of individual plants can be explained by inflorescence size. (2) In contrast, although fruit production within most populations is positively correlated with an index of the number of pollinator visits to an inflorescence, the relative abundance of pollinators for a population is a poor predictor of mean fruit production, and only 9% of the total variance in fruit production can be explained by the visitation index. Furthermore, at four sites studied for two years, there was little change in average inflorescence size or fruit production from 1978 to 1979, despite large differences in relative abundance of pollinators at each of the sites. (3) Based on geographic proximity, and physiographic and vegetational similarities, study sites were grouped into regional clusters. Both inflorescence size and fruit production varied considerably between regions. Of the total variation in fruit production, 27% can be attributed to differences between regions. Most of this variation is the result of regional differences in inflorescence size, which in turn influence fruit production.
Why does Yucca whipplei produce such large inflorescences if so few fruits can be supported? Two relevant hypotheses are discussed: (1) the floral display is the result of selection for pollen dissemination at the expense of fruit set; and (2) the floral display is the result of selection for a bet-hedging strategy either to increase the probability of adequate pollination when pollinators are unusually rare, or to allow individuals to support more fruits when resources are unusually abundant.
Populations of Delphinium nelsonii have been shown to be patchy with regard to standing crop of nectar available to pollinators (Pleasants and Zimmerman 1979). Plants with relatively large amounts of nectar ("hot plants") are associated with other hot plants while plants with relatively small amounts of nectar ("cold plants") are found near other cold plants. Two possible explanations for this pattern exist: 1. Plants in close proximity to one another may have similar nectar production rates or;
2. The patterns of foraging bumblebees might create the observed nectar distribution pattern. By sampling standing crop of nectar in a D. nelsonii population during periods of both high and low bumblebee abundance evidence was gathered indicating that hot and cold spots are caused by bumblebees.
Orchis israelitica, which has no food reward, was found to share the pollinators of Bellevalia flexuosa: Eucera clypeata, Bombylius sp. and possibly also Anthophora sp. Since both species have a similar coloration pattern, flowering period and habitats — it is suggested that O. israelitica mimics the flowers of B. flexuosa as a deceit syndrome of pollination.
Aspects of floral mimicry, with special attention to floral adaptations in the genus Orchis, are discussed.
The adult behavior of the yucca moth, Tegeticula maculata Riley, is finely tuned to the reproductive biology of its specific host plant, Yucca whipplei Torr. The female moths oviposit in the ovaries of the yucca flowers and actively pollinate the same flowers with pollen which they have collected previously. The selective pressures imposed on the moths by 1) the plant's need for pollen transfer via an insect pollinating agent, 2) its partial self-incompatibility, and 3) its ability to regulate s
eed set by aborting excess fruits, have molded the pollinator's behavior in such a way that its offspring have the greatest possible chance of surviving through the early larval stages. The evolutionary responses of the pollinator include the following: 1) the female moths consistently fly to a different plant after collecting pollen, thus insuring cross-fertilization of the flowers, 2) they always pollinate after depositing the first egg in a flower, but not necessarily after subsequent ovipositions, and 3) females emerging near the end of the flowering season frequently oviposit in developing seed pods, as opposed to open flowers which are more likely to be aborted by the plants.
The role of sunbirds (Nectariniidae) in the pollination ecology of Strelitzia nicolai (Musaceae) was studied for one year in a coastal dune forest in Zululand, South Africa. It was found that S. nicolai produced large quantities of low quality nectar (1.74 j/μl); that nectar production was highest during the day-time; and that the flowers displayed several characteristics attractive to bird-visitors. The flowers were large, conspicuous and provided the birds with a perch, facilitating easy acces
s to the nectar. Flowers were visited by four species of sunbirds: Olive Sunbird Nectarinia olivacea, Grey Sunbird N. veroxii, Black Sunbird N. amethystina, and Collared Sunbird Anthreptes collaris. Sunbirds visited the flowers throughout the year, and apparently cued into changes in the flower angle as an indication of nectar flow rates. Sunbirds perched on the flowers in a manner which effected pollination, the pollen being transferred to the stigma via the birds' feet. Besides the sunbirds, there were other visitors (bushbabies, monkeys and insects) to the flowers, but they did not visit the flowers frequently nor did they appear to be significant pollinators. The high seed set of S. nicolai in the study area attests to the efficacy of the sunbirds as pollinators.
Four biennial species (Oenothera erythrosepala, Dontostemon dentatus, Erigeron strigosus, and Erigeron sumatrensis) from the dune system at Azigaura, Japan, were sand-cultured in pots with different nutrient levels, and the critical plant size for the development of a flowering stalk was determined. The critical size decreased in the order O. erythrosepala>E. strigosus>D. dentatus=E. sumatrensis, and corresponded to the order of their distribution along the gradient of soil fertility in the dune
system. This correspondence is discussed, and it is concluded that in a less fertile environment, biennials producing seeds after attaining a larger size have an advantage over those producing seeds earlier.
A field investigation of the mutualistic interaction between a monocarpic perennial plant, Yucca whipplei, and its host-specific pollinator and seed predator, Tegeticula maculata (Lepidoptera: Prodoxidae), was conducted to determine how the resource utilization pattern and population dynamics of the pollinator have influenced the evolution of the flowering and fruiting pattern of the plant. Although the temporal pattern of emergence of pollinators results in a relatively close tracking of flower
abundance within a season, the ratio of pollinators to open flowers does vary significantly within a season, as well as between seasons. At any point in time during the flowering season, the population of adult yucca moths is distributed evenly among the available flowers, so that the number of pollinators on an inflorescence is directly proportional to the number of open flowers available. The relative isolation of individual flowering plants appears to have little effect on the distribution of pollinators among inflorescences. The number of fruits initiated on a plant is directly proportional to the number of flowers produced, and is also partially determined by the time of flowering. Yucca whipplei always produces many more flowers than fruits. Most flowers are not fertilized, and the plants also generally abort and abscise immature fruits after flowering. Fruit production of at least some plants, however, appeared limited by pollination. It is also expected that in some years the relative abundance of pollinators will be low enough that most plants will be pollinator-limited. It is suggested that the pattern of flowering and fruiting of this species has evolved in response to the unpredictability of pollinator availability, both within and between seasons. Resource uncertainty and selection acting on the male component of fitness may also be involved.
Early models of hostplant exploitation by phytophagous insects suffer from unwarranted assumptions and may not be generally applicable. Wordmodels of the co-evolutionary approach may assume unwarranted evolutionary stability in ‘strategic’ explanations, whilst mathematical models derived from earlier optimal-diet studies are unrealistic and unwieldy. A simple arithmetic model synthesises these two approaches, using the two parameters of foodplant suitability and availability. Hostplant use by th
e butterfly Anthocharis cardamines, previously thought to be maladaptively polyphagous, is shown to be optimal under prevailing conditions of short search time. The predictions of the model for hostplant use and community structure of butterflies and other phytophagous insects are tested and, in large part, corroborated. Monophagy and monophagic forms of oligophagy are shown to be favoured by: long adult lifespan; low search costs to females; search images (Whether visual or olfactory); batch-laying of eggs; high differential in foodplant suitability.
Bumblebee foraging behavior was observed on two plant species with similar floral and inflorescence structures. One species produces nectar while the other does not. Bees, upon visiting nectar producing flowers tend to empty them of nectar and by frequently moving between close neighbors, create a patchily distributed resource base. Bees maximize their foraging efficiency in such an environment by using an area-restricted searching behavior and flying distances inversely correlated with the qual
ity of reward received. Pollen collecting bumblebees do not create a patchy environment and maximize their foraging efficiency by more consistently moving shorter distances. Pollen collecting bumblebees are significantly more likely to revisit flowers and to visit more flowers per inflorescence than are nectar gathering bumblebees. These differences in foraging behavior increase the neighborhood size for nectar producing species and make it increasingly unlikely that random drift will be a dominant mode of evolution in populations of these species.
Male reproductive effort was estimated from flower, seed and fruit biomass data in populations of the self-compatible plant Gilia achilleifolia that differ in genetically estimated selling rate. Male reproductive effort decreases with increased rate of selling, a finding that is consistent with theoretical arguments pertaining to the allocation of resources to male and female reproductive functions in hermaphroditic organisms.
Two bumblebee species, Bombus bifarius and B. flavifrons, forage randomly with respect to direction when gathering pollen on Potentilla gracilis. Bees avoid revisiting flowers by being able to differentiate recently visited from unvisited flowers. This recognition occurs while bees are flying over open flowers and appears to be a response to the amount of available pollen within flowers. Random foraging with respect to direction is the optimal strategy when the probability of flower revisitation
is low. Bumblebees appear to be moving preferentially between nearest neighbors, again as predicted by foraging theory. This behavior causes the establishment of pollen patches in the P. gracilis population. Unlike other pollinators studied in similar situations, bumblebees on P. gracilis do not forage utilizing an area-restricted searching behavior. Because floral reward quality can be assessed at low cost by bees foraging on P. gracilis, their tendency to move to nearby flowers even after encountering a poor quality blossom apparently yields a higher rate of net energy intake than does area-restricted searching. The data indicate that bumblebees exhibit great plasticity in foraging behavior and that they are able to forage efficiently under a wide range of environmental conditions.
A three year study of Senecio keniodendron (Compositae), a giant rosette species of the alpine zone of Mt. Kenya, demonstrated that individuals which reproduce are more likely to die, and less likely to reproduce in the future if they do survive, than are vegetative individuals of the same size. However, if an individual reproduces, survives and reproduces again, then it produces more seeds during the second reproductive episode than does a plant of the same height reproducing for the first time
, because reproduction is followed by production of lateral rosettes, increasing the number of potentially-reproductive rosettes per plant. Slow-growing rosettes are less likely to reproduce than fast-growing rosettes. For rosettes which do reproduce, rosette size and rate of leaf production, measured before reproduction begins, are good predictors of fecundity.
The andromonoecious plant Aralia hispida has a complicated blooming schedule involving alternations between male and female phases. Nectar and pollen are released gradually through the day. Plants vary considerably in number of flowers per umbel and number of umbels per plant. The major pollinators, bumble bees, show several characteristic behaviors in response to the plant's presentation. 1. Foraging bees preferentially visit umbels that bear large, numbers of open, male-phase flowers. They als
o prefer shoots with large numbers of umbels. 2. If bees have received high nectar rewards at one umbel, they are more likely to visit a neighboring umbel rather than leaving the area. On drained umbels, bees probe more empty flowers before rejecting the umbel if they have been rewarded just previously. 3. Individual bees restrict their foraging to limited areas. Within these areas, they concentrate their visits on certain shoots which they tend to visit in repeatable sequences, or "traplines". It is inappropriate to consider these bees as "searching". 4. We discuss some of the implications of these data for two areas of current theoretical interest: plant reproductive strategies and optimal foraging.
To examine the effects of pollen and resource availability on floral display and fruit set in Chilopsis linearis three different types of experimental manipulations were performed. Pollen availability to individual inflorescences was altered by combinations of hand-pollinations and/or pollinator exclusions. Number of flowers produced per inflorescence was determinate; it was not affected by pollination, although flowers of the unpollinated treatment lasted longer. Fruit set was increased over na
tural levels by an average of 540% by hand-pollinating individual inflorescences. There was also a negative correlation between inflorescence size and percent fruit set in all treatments tested. Attempts to vary resources available to individual inflorescences and entire trees showed no significant effect on fruit set. These results show that, at the level of individual inflorescences, fruit and seed production in Chilopsis linearis are pollen limited and not resource limited. The problem of testing for resource limitation of female reproductive success in iteroparous plants and the effect of inflorescence size on female components of fitness are also discussed.
Seasonal excavations of ramet systems in overlapping natural populations of Typha latifolia and T. angustifolia revealed intrapopulation variation in ramet size and reproduction. Reproductive “states” were recognized based on whether or not a ramet had flowered and the number of offspring ramets it possessed. As the growing season progressed, the populations became increasingly heterogeneous in their reproductive-state composition with the largest size classes of ramets containing the greatest n
umber of reproductive states. Therefore, despite a strong correlation between size and reproduction, average ramet size was considered to be an inadequate predictor of population growth in ramet numbers. Results for vegetative reproduction and ramet mortality indicate that the T. latifolia population was growing while the T. angustifolia population was approximately stable. In contrast to most results reported for other species, the frequency distributions of ramet size classes showed T. angustifolia to have a normally distributed population. A slight degree of positive skewing occurred in T. latifolia with the greatest skewing at the end of the growing season. Despite the lack of strong skewing of ramet weights, evidence from other sources have demonstrated that competition was intense and we hypothesize that competition in clonal populations may not not always act to cause positive skew in weight distributions.
We present results of experiments designed to identify floral characteristics that influence patterns of pollen carryover by hummingbirds visiting lpomopsis aggregata flowers. We used fluorescent dye powders as pollen analogues. For all four experimental treatments considered, amounts of dye deposited on recipient stigmas declined linearly as a function of flower position in a visitation sequence. The decline was significantly steeper when recipient flowers had pollen-carrying anthers than when
they did not; whereas degree of stigma clogging and presence or absence of empty anthers did not influence carryover. From this we conclude that presence of pollen on recipient flowers significantly reduces the average number of subsequent flowers reached by donor pollen. We discuss mechanisms for this effect and its significance for the evolution of floral structure.
The grass Danthonia spicata produces dimorphic seeds from chasmogamous and cleistogamous flowers. The seed remains attached to seedlings so one can identify the reproductive origin of seedlings. The proportion of chasmogamous and cleistogamous seedlings becoming established varies widely between populations. The types of seedlings becoming established was generally consistent from year to year and correlated with the proportion of flower types produced. Mown populations showed especially high le
vels of cleistogamous seedling establishment. Germination and seedling survival were experimentally investigated. Chasmogamous and cleistogamous seeds had different germination behaviors. Cleistogamous seedlings had higher survival rates than chasmogamous seedlings.
A direct measure of pollinator effectiveness, relying on seed set, is proposed, where: PEi = (Pi − Z) / (U − Z). Pi = mean number of seeds set / flower by a plant population receiving a single visit from species i. Z = mean number of seeds set / flower by a population receiving no visitation. U = mean number of seeds set / flower by a population receiving unrestrained visitation. The utility and limitations of the proposed meas
ure are discussed and exemplified with a population of Ipomoea trichocarpa (Convoculaceae).
The effects of foodplant species and maternal food type on larval growth, development, and digestive parameters were examined for larvae from an oligophagous colony of Euphydryas chalcedona butterflies. Broods of larvae from areas containing two different foodplants, Diplacus aurantiacus and Scrophularia californica, were divided. One group was fed their “native” host and their siblings were fed the other species. Growth and digestion parameters were measured from hatching until larvae entered d
The efficiency of pollination and rates of embryo initiation were compared in two species of Cryptantha (Boraginaceae) which differ greatly in their patterns of seed (nutlet) production. Cryptantha flava normally matures only one of the four ovules per ovary while C. flavoculata often matures all four. The general floral morphology and composition of insect floral visitors were found to be nearly identical in the two species, but C. flavoculata produces twice as much pollen per flower as C. flav
a. Despite this difference, the amount of pollen deposited on stigmas by pollinators is similar. This may be due to the larger stigmatic surface area in C. flava. In both species, the abortion of fertilized ovules, rather than insufficient amounts of pollen, appears to be the primary factor regulating the number of seeds per flower. The pollination ecology of these species is discussed in relation to current theories regarding the allocation of resources to the production of pollen and seeds.
The time required for a bumble bee to visit a flower is affected by the length of the bee's glossa and its body weight, and by the depth of the flower and the volume of nectar it contains. Probing time is comprised of two components: access time and ingestion time. Access time increases linearly with flower depth, but ingestion time varies with flower depth only in flowers deeper than the length of the bee's glossa, due to a decline in the rate of ingestion of nectar. Probing time therefore incr
eases gradually with increasing depth for flowers shallower than the bee's glossa, but beyond that depth it increases much more rapidly. The relation of probing time to flower depth influences the foraging efficiency and choice of flowers by bumble bees.
Characterstics of Australian endemic Helichrysum bracteratum and H. viscosum suggest that foraging ants act as “guards” of developing flowerheads, protecting capitula from seed predators: (1) extrafloral nectar is secreted from leaves subtending the capitula and from bracts encircling the floral disc during pre- to post-flowering periods; (2) capitula are attended by ants; and, (3) encounters between ants and other capitula visitors, including predispersal seed predators such as Tephritis sp. (D
iptera), can be frequent. In experiments to test the ant-guard hypothesis, exclusion of ants from plants increased abundance of other insects on the developing capitula. The difference between ant-access and ant-exclusion treatments was related to ant abundance on the access plants. These effects were statistically significant in spite of the large variation in insect activity between sites and through the season.
The increased abundance of insects on capitula following ant-exclusion did not, however, result in significant increases in the number of adult seed predators observed on capitula, the number of immature seed predators in capitula, or capitula damage as estimated between ant-access and exclusion treatments of either H. bracteatum or H. viscosum. Further, the ant-exclusion treatment on H. bracteatum had no significant influence on pollination as measured by seed set or on the degree of parasitism of Tephritis sp. by Megastigmus sp. Site and season most strongly affected numbers of immature seed predators and damage to capitula.
We discuss these findings in relation to the ant-guard hypothesis and suggest that generalization of the protection hypothesis to all plants with extrafloral nectaries is premature.
Brink (1982) characterizes the distribution of standing crop of nectar for Delphinium nelsonii as bonanzablank, based on comparison with a Poisson. He then discusses possible effects of standing crop variability on pollinator foraging behavior. We disagree with the use of the Poisson and the resulting conclusions. The expected distribution should not be based on doling out random amounts of nectar to flowers, but based on random return times to flowers by pollinators (elapsed time=nectar accumul
ated). When this model is used, standing crop variance does not differ markedly from expectation. What differences do exist can be accounted for by variability in nectar production rates of individual plants. We also take issue with the use of the bonanza-blank terminology. As originally formulated this refers to nectar production differences within a plant rather than standing crop differences among plants.
Photosynthetic rates by flowers of the shrubs Enceliafarinosa and E. californica were studied during three phenological stages of flower development. Both gross photosynthesis and dark respiration rates in the flowers were of similar magnitude and decreased with floral development. Floral photosynthetic rates were saturated by an irradiance equivalent to one half full noon sunlight. Net photosynthesis of flowers was rarely positive and decreased substantially with increased flower temperature. G
as exchange by the flower was unaffected by the water vapor pressure gradient. These results are analyzed in terms of the microclimate experienced by the flowers and the relative contribution of floral photosynthesis to the economy of the flower.
The energy allocation patterns to reproductive structures (RA) and the propagule output (PN) of five annual weedy or ruderal fox-tail species (Gramineae) - Setaria viridis (2 x ) (including vat. pachystachys (2 x )), S. X pycnocoma (2 x ), S. faberi (4 x ), S. glauca (4 x, 8 x ), and S. pallide-fusca (8 x ) - were studied in relation to their biomass on the materials from ten different wild populations in Toyama and Fukui Prefectures, Japan. In addition, the relationship between the RA and PN wa
s also critically examined. For all five fox-tail species, proportional partitioning of dry matter into total reproductive structures (caryopses, bristles, inflorescence axis and other persistent floral organs at the fruiting stage) (RA) shows considerable overlaps, but more or less constant within each taxon and independent of the biomass. On the other hand, the number of propagules produced per plant (PN) is remarkably dependent on the size of plants and thus a function of the biomass. This high efficiency of energy allocation to propagules is evidently determined by the breeding system (i.e., predominant inbreeding) of these weedy annual species. However, the relationship between the RA and PN showed no conspicuous trend, and thus the reproductive output is also independent of RA. The reproductive strategy found in Setaria is perhaps typical in such weedy annuals which grow chiefly in frequently disturbed, unpredictable environments. Another notable discovery was that, in spite of differences in ploidy levels (from 2 x to 8 x ), all five species showed a very similar reproductive strategy, although slight but clear decrease was noted in the RA in response to the increase in ploidy levels, ranging from 49.52% (in diploids) to 27.53% (in tetraploids and octoploids). Seed weight (per gram), on the contrary, clearly increases in response to the increase in ploidy levels, i.e., ranging from 7.09 x 10-4g (in diploids) to 41.87 x 10-4g (in octoploids).
This study describes an experimental analysis of a five-fold plant-insect interaction, with the primary aim of determining the net effect of insect-insect interactions on the host plant. The participlants are the extrafloral nectary-bearing host plant Acacia decurrens; the resident sap-sucking membracid treehopper Sextius virescens; the ant Iridomyrmex sp. which attends both EFN and treehoppers; external herbivores including grasshoppers and gall wasps; and predators and parasitoids of the membr
acids. The main results are as follows. 1. The membracids alone decrease plant growth and seed set. 2. In the absence of membracids, the ants defend the plant against external herbivores, and their presence increases plant growth and seed set. 3. The interaction between ants and membracids exerts an overall negative effect on plant growth and seed set. This has not been demonstrated for any system previously and is the main result of the study. 4. Ant attendance does not increase membracid populations or feeding rates significantly, or reduce mid-season mortality. 5. Ant attendance decreases late-season membracid mortality significantly. This has been shown previously for Lepidoptera and nymphal Homoptera only. 6. The mechanism of the interactive effect in (3) is apparently that the honeydewsecreting membracids attract the ants away from the EFN and decrease the efficacy of the plant's ant defence.
Seeds collected from parents that flowered at different times were dispersed onto experimental plots at different times during the normal dispersal season. Parental flowering and dispersal times, which are correlated with each other, independently affected offspring germination, growth, and time of reproduction. Estimated population growth rates were highest for offspring that were dispersed early in the dispersal season and that came from early flowering parents. The data provide evidence that
1) an individual's fate is determined by the environment of the previous generation, and that 2) an individual's fitness should be calculated from life history data that span more than one generation.
To explore the mechanical determinants of feeding strategies for nectar feeders, we develop a fluid dynamical and behavioral model describing the mechanics and energetics of capillary feeding in hummingbirds. Behavioral and morphological data for Calypte and Archilochus are used to test and illustrate this model. We emphasize the important differences between capillary and suction mechanisms of fluid feeding. Model predictions of nectar intake rates and nectar volumes per lick are consistent wit
h observed values for Calypte anna. The optimal nectar concentration maximizing rate of energy intake depends on tongue morphology and licking behavior. For hummingbirds exhibiting optimal licking behavior, the optimal nectar concentration is 35–40% sucrose for feeding on large nectar volumes. For small nectar volumes, the optimal concentration is 20–25%. The model also identifies certain tongue morphologies and licking frequencies maximizing energy intake, that are consistent with available observations on licking behavior and tongue design in nectar feeding birds. These predictions differ qualitatively from previous results for suction feeding in butterflies.
The model predicts that there is a critical food canal radius above which suction feeding is superior to capillary feeding in maximizing the rate of energy intake; the tongues of most hummingbirds and sunbirds fall above this critical radius. The development of suction feeding by nectarivorous birds may be constrained by the elastic properties of their flexible tongues. Our results show that, in terms of morphology, scaling, and energetics, different mechanisms of feeding on the same food resource can lead to qualitatively different predictions about optimal design and feeding strategies.
Bumblebees, when foraging for nectar in flowers of Aconitum columbianum, do not consistently drain them of reward. The amount of residual nectar varies among sites, times and species of bee. At times, residual nectar can be a significant percentage of total 24 h production strongly suggesting that, if nectar secretion rates are to be studied, flowers must have their standing crop of nectar drained by hand; assuming that a recently visited blossom is empty can lead to erroneous nectar production
values. The fact that residual nectar exists is consistent with predictions of optimal foraging theory but more detailed work is necessary before it can be concluded that the volumes left behind in A. columbianum flowers are optimal.
A model is developed to elucidate the determinants of sugar concentrations in flower nectars. This model analyses the efficiency of sugar intake, or energy flux, which for nectarivores closely approximates the rate of net energy gain. For both steady state and some non-steady flows of nectars, this energy flux is shown to be maximal at particular sugar concentrations referred to here as the maximum flux concentration. Higher concentrations actually yield lower energy intake rates because the con
comitant rapid increase in viscosity sharply reduces the rate of fluid intake. For pure sucrose solutions, the maximum flux concentration is 22%. For flower nectars, which are chemically more complex, the maximum flux concentration is predicted to be closer to 26%, using the first viscosity measures obtained for flower nectars. This concentration is shown to be essentially independent of the pollinator's feeding organ morphology and of the type of potential inducing nectar flow. It is proposed that this concentration applies for virtually all pollinators that select nectars with maximal energy flux.
However not all pollinators are expected to select such nectars because this 26% concentration is not necessarily “optimal”. The model predicts that optimal sugar concentrations vary for particular pollinators as a function of two primary factors: (1) the energy flux derived from the nectar, as discussed above, as well as (2) the relative contribution of transit costs to overall foraging costs. Relatively “dilute” nectars, with sugar concentrations close to the maximal flux value, are predicted for flowers pollinated by organisms that minimize feeding time to reduce high feeding costs, such as that of hovering or of exposure to enhanced predation while feeding. More concentrated nectars are predicted for flowers pollinated by nectarivores that incur high foraging transit costs relative to feeding costs.
Flowers pollinated by hovering pollinators, including many hummingbirds, hawkmoths and bats, have nectars with mean sugar concentrations in close accord with the 26% maximum flux concentration predicted. Moreover, these nectars have relatively low concentrations of nonsugar constituents, which increase viscosity and thereby decrease sugar flux. Over 75% of the flowers examined in this study, which are pollinated primarily by territorial hummingbird species, provide nectars that allow sugar uptake with an efficiency of 90% or greater of the maximal value. According to the model, these data suggest that feeding costs of these pollinators far outweigh foraging transit costs. In contrast, the model suggests that flower nectars taken by traplining hummingbirds and by bees, with sugar concentrations significantly above the maximum flux value, reflect the higher costs of foraging flight relative to costs of feeding for these pollinators.
Increasing temperature decreases nectar viscosity, and thereby increases absolute nectar uptake rates sharply. This leads to a number of predictions regarding foraging behavior as well as flower location, orientation, and color. However, the maximum flux concentration is shown to be practically invariable over a wide range of temperatures-increasing by only 2% sugar from 10°C to 30°C. Thus, contrary to previous expectations, little change in average sugar concentrations of flowers pollinated by particular groups of nectarivores is expected from cooler to warmer regions.
The interactions between the checkerspot butterfly, Euphydryas chalcedona, and two of its principal host plants, Diplacus aurantiacus and Scrophularia californica, were studied to test the hypothesis that feeding behavior in nature reflects food quality in terms of leaf nitrogen and defensive chemical contents. Larvae preferentially fed on Diplacus leaves containing the highest nitrogen: resin ratio in the field and laboratory. Larvae did not feed selectively among Scrophularia leaves, which sho
w little variation in quality. Seasonal timing of feeding activity and larval development rates were closely related to the availability of any Scrophularia leaves and high-quality Diplacus leaves.
Erythronium umbilicatum (Liliaceae) is a common vernal herb of deciduous forests in the southeastern United States whose seed set depends on outcrossing by insects. Although only 40–60% of the ovules mature into seeds, hand-pollination experiments conducted in several populations over four years provided little evidence that reproductive success in this species was limited by pollination. Both honeybeess (Apis mellifera) and two small species of native andrenid bees effectively pollinate E. umbi
licatum, as determined by measurements of seed set and counts of pollen tubes in the styles of flowers kept under cages and allowed a single visit. Full pollination of E. umbilicatum is promoted by: (1) large amounts of nectar and pollen that attract a variety of potential pollinators, (2) the small (2–3) number of visits required to fertilize all of a flower's ovules, (3) extended perianth persistence of unpollinated flowers. The latter two traits are also characteristic of other spring wildflowers and may contribute to fitness in a habitat where both physical factors and competition between plants for floral visitors can make pollinator service unpredictable. Causes of the observed deficiency in seed set in E. umbilicatum are complex and probably involve cold weather, low light levels or other unidentified physical factors, genotypes of pollen parents (i.e. quality of pollination), and diversion of resources from current year's seed set to future reproduction.
In common milkweed (Asclepias syriaca), flower nectar volumes, concentration and sugar production varied according to the age of the sampled blossoms. In individual blossoms, nectar production peaked daily at 0800 hr. Peak production during the life of the flower occurred on the second day of flowering, 50 h after anthesis, and nectar production ceased after 120 h. The amount and quality of nectar were affected by microclimatic conditions and varied between clones. However, the same secretory pa
tterns were found in all flowers studied. This age dependent nectar secretion combined with the sequential mode of flowering found on a single stem, results in substantial reward for extended periods to nectar feeders.
The causes and reproductive consequences of individual variation in nectar production rates within a population of Asclepias quadrifolia were investigated. Two parameters were correlated with nectar production rate per flower: the root weight of the plant and the number of flowers in the inflorescence (umbel). Nectar production increased with increasing root weight but levelled off after a root size of about 3 g was reached. Nectar production decreased with increasing umbel size, but only for um
bels that were greater than average size. A total of 57% of the variance in individual nectar production could be explained by these two variables with root weight accounting for 67% of the explained variance. Root weight is a good indicator of a plant's energetic status, indicating the importance of available energy in determining quantity of nectar produced. About 30% of the energy devoted to flowering is utilized in nectar production. Nectar production was significantly correlated with the male component of reproductive fitness, pollinaria removal, but not with the female component, pollinia insertion. Since pod production is limited by resource availability rather than the number of pollinia insertions, nectar production in A. quadrifolia is most closely associated with the maximization of the male function.
The flowers of the annual herb Impatiens capensis have distinct male and female phases. The male phase lasts four times as long as the female phase, and male flowers contain about 50% more nectar than female flowers. This suggests that the bulk of allocation to the flower is designed to ensure the dispersal of pollen rather than the fertilization of ovules. Honeybees, wasps and bumble bees all land on male flowers more often than would be expected by chance, and, having landed, wasps and bumble
bees are more likely to enter a male flower than a female flower. The frequency of male flowers in the diet therefore exceeds their frequency in the population. This preference, although strong and consistent, is only partial, since some female flowers are included in the diet. We propose two hypotheses to account for the observed partial preference, the first based on competition between bees for flowers, and the second asserting that the bees detect nectar levels directly without using floral gender as a cue. The results of an experiment in which the most obvious gender cue, the androecium, was removed are consistent with the second hypothesis.
Flowering patterns of four Heliconia (Heliconiaceae) species in Trinidad, West Indies were examined for their predictability and availability to the nectarivores that rely on Heliconia floral nectar. Principal flower visitors are trapling hermit hummingbirds; inflorescences are inhabited by nectarivorous hummingbird flower mites that move between inflorescences by riding in the hummingbirds' nares.
Heliconia inflorescences flower for 40–200 days, providing long-term sources of copious nectar (30
–60 μl per flower), but each Heliconia flower lasts only a single day. As an inflorescence ages the interval increases between open flowers within a bract; wet-season inflorescences produce open flowers more slowly than dry-season conspecifics.
Estimated daily energy expenditures for hermit hummingbirds demonstrate that slow production of short-lived open flowers plus low inflorescence density preclude territorial defense of Heliconia by the hermits. Heliconia flowering patterns are viewed as a means of (i) regulating reproductive investment by the plants through staggered flower production over long periods of time, and (ii) maintaining outcrossing by necessitating a traplining visitation pattern by its hummingbird pollinators. I suggest that Heliconia exhibit a two-tiered pollination system by using hermit hummingbirds primarily for outcrossing and using hummingbird flower mites primarily for self-pollination.
We examined net seed production for the selfincompatible, monocarp, Ipomopsis aggregata, by monitoring pre-pollination seed parasite (Hylemya sp.) oviposition and hummingbird mediated fruit set on 21 plants of variable height. Both pollination and seed predation increased with inflorescence height. Net seed production (incorporating seed predator mortality) also was positively related to height, and this would have been the case if pollination or seed predation were doubled. Although results sug
gest Ipomopsis aggregata should be under selective pressure to maximize inflorescence height, generation time and resource limits could result in advantages for inflorescences of intermediate height.
Experimental pollinations of Costus allenii (Zingiberaceae) were conducted to assess the effects of pollen composition on fitness. Plants were selfed, outcrossed with the first nearest neighbor, and outcrossed with pollen mixtures obtained from the nearest 2, 3, and 5 plants. Cross type had a significant effect on seed production, seed weight and total-plant dry weight. Progeny from crosses with 3, and 5 parents grew significantly larger than selfed progeny, or those from 1-parent crosses. Compe
tition experiments indicated the superiority of progeny from 3-, and 5-parent crosses over progeny from 1-parent crosses, but no differences in competitive ability were observed between progeny from 3-, and 5-parent crosses. Relative fitness, based on 1) seed production, 2) percent germination, and 3) dry weight, varied significantly among crosses, and was greatest for crosses with 3 parents and lowest for selfs. The relative fitness of progeny from 5-parent crosses was lower than that of all other outcrossed classes. We suggest that the significant effect of pollen composition on fitness results from variation in the genetic similarity of seed and pollen parents, which is a function of spatial distribution and population structure.
The egg-laying behaviour in the wild of 51 butterflies in Sweden is studied: three different patterns emerge. Firstly, although the majority of butterflies deposit their eggs on the plants on which their larvae later feed, butterflies that overwinter in the egg stage and use herbaceous host plants tend to avoid laying their egges on host plants
Colony foraging activity of four Melipona species (Apidae: Meliponinae, tribe Meliponini) was studied during the dry season, when many plants flower in central Panama. The efficiency of sucrose solution uptake by Melipona was compared to that of domesticated European Apis mellifera. Dynamics of nectar foraging were also recorded for 3 of the Melipona visiting the forest shrub, Hybanthus prunifolius (Violaceae). 1. Sugar concentration in nectar brought to nests averaged from 21 to 60% sugar for 1
5 colonies of M. fasciata, M. compressipes triplaridis, M. fuliginosa and M. marginata micheneri. Concentrations ranged from 19 to 72%, and all species collected nectars ranging at least between 24 and 63% sugar. However, M. compressipes and M. marginata preferred higher concentrations and foraged less on dilute nectars. Peak colony nectar harvest occured in late morning or early afternoon; peak pollen harvest was in early morning. 2. Imbibing rates of bees given 20, 30, 45, 60 or 70% sucrose solutions were highest at ≦45% sucrose, but caloric intake was most rapid at 60% sucrose for all species. All but M. marginata displayed greater net intake rates than domesticated European Apis mellifera. A foraging choice model incorporating caloric reward and imbibing rates of bees suggests M. compressipes and M. marginata should specialize on richer nectars. Rate of caloric intake per forager weight was higher for all Melipona (0.03–0.13g) than for A. mellifera (0.10 g). 3. The nectar of Hybanthus prunifolius (Violaceae), a shrub pollinated exclusively by Melipona, progressed from 35 to 60% sugar during the day. Bees foraged most when nectar was below 60% concentration, a pattern best explained as the result of intercolony competition and greater availability of lower quality nectar. 4. Sugar concentration in nectar harvested by colonies rose from lower to higher values through the day for Melipona. The increasing caloric reward of nectar is adaptive in exploiting foraging preferences of such bees. As standing nectar crop is depleted by competing bees, a gradual shift to more rewarding nectar should promote increased bee foraging range, more flower visits during a foraging trip, floral constancy, and genetic outcrossing. 5. The nectar load capacity of A. mellifera is greater than that of Melipona. Other factors being equal, Africanized A. mellifera, now a permanent resident of neotropical forests, should visit more flowers during a foraging trip. Additional species differences in foraging behavior are analyzed.
Nectar plant utilization by butterflies was compared with the abundance of flowering plants on some SW Swedish meadows. The transportation of pollen grains by individual butterflies was analysed using a scanning electron microscope.
For the majority of plant species, butteflies were of minor importance as pollinators. Some flowers with protruding sexual organs, e.g. Knautia arvensis, when heavily utilized by butterflies, might also be pollinated by these.
For Dianthus deltoides and Viscaria vulg
aris, butterfly pollination was found to be important.
Several butterfly species, e.g. Plebicula amanda, visited legumes regularly. In spite of this, legume pollen was rarely transported by the butterflies. This suggests these butterflies act as nectar robbers among the legumes.
On both artificial flowers in the laboratory and certain plant species in the field, bumblebees often closely approached flowers and then departed without probing for nectar. In laboratory experiments where nectar rewards were associated with subtle visual or olfactory cues, bumblebees approached and avoided non-rewarding flowers. Flowers that bees entered and probed for nectar contained rewards much more frequently than predicted by chance alone. When there were no external cues associated with
nectar content, bees visited rewarding flowers by chance alone, provided rewarding flowers were not spatially clumped. In the field, bumblebees approached and rejected a large proportion of dogbane flowers and red clover inflorescences. On both species, flowers or inflorescences probed by bees contained more nectar than those rejected by bees or those that I chose at random. On fireweed and monkshood, bees rarely or never approached and rejected healthy-looking flowers. Predictions generated by an optimal foraging model were tested on data from four bumblebee species foraging on red clover. The model was highly successful in qualitatively predicting the relationship between handling time and proportion of inflorescences rejected by individual bees, and the relationship between threshold nectar content for acceptance by bees and average resource availability. Thus, bees appeared to use remotely perceived cues to maximize their rates of nectar intake.
Successful cross-pollination of Monarda fistulosa is the result of a complex interaction among flower opening, the pollen-bearing areas of the pollinators and/or their behavior, and the maturation of the stigmas. The flowers open continuously from 0800–2000 h providing a temporally predictable rich source of nectar and pollen. Recently opened flowers may reduce the ability of bees to discriminate between resource rich and poor patches and encourage systematic foraging within patches. The continu
ous opening of flowers coupled with protandry also results in some flowers of most capitula being in the staminate and others in the pistillate phase. Autogamy is highly unlikely due to strong protandry and the spatial separation of anthers and stigmas. Geitonogamy, at least that mediated by Bombus is unlikely because the pollen is spread over a relatively large area of the wings, which reduces the likelihood of a stigma contacting just deposited pollen. Because pollen is transferred from the much smaller coxal area of Anthophora and other bees that mistake the stigmas of early pistillate phase flowers for stamens some geitonogamy seems inevitable. However, the delayed receptivity of young stigmas to self-pollen decreases the likelihood of self-pollen germinating on such stigmas. Older stigmas are equally receptive to self- and cross-pollen and the number of pollen grains germinating and pollen tubes reaching the base of the style increases with flower age.
The distribution of nectar sources is shown to affect both the distribution of adult Euphydryas chalcedona and their offspring. We suggest that nectar sources thereby influence the selection of oviposition host plant species in habitats where those species are spacially separated.
Different subsets of mainland nectarivores visited Quassia amara (Simaroubaceae), a self-compatible, predominately bird-pollinated treelet, at three islands and the mainland in Panamá. Factors correlated with reproductive success, defined as seed to ovule ratio, included the species pollinating and robbing flowers, visitor activity, pollinator response to nectar robbing, and internal regulation of fruit production. The absence of robbers and former pollinators on an island separated from the mai
nland during the holocene was associated with shifts in flower size, nectar production, and 3–4fold increases in population reproductive success and pollinator efficiency (=seeds produced per visit). Exclusion of robbers at three sites resulted in seed production 4–12 times greater than control flowers, at which robbers accounted for 52–98% of all visits. Although 36% of buds and over 83% of all flowers were robbed, this had no direct influence on the recorded 36–61% respective abortion rates of buds and flowers. Opportunistic avian robbers appeared where normal robbers were absent; three avian robbers extensively used floral perforations made by Trigona bees, and all ancillary pollinators also robbed. Selection pressures from nectar robbers are discussed that may relate to plant reproductive fitness.
Relationships between pollen loads, resource availability, and fruit and seed production were determined for Lysimachia quadrifolia ramets in two adjacent sites (the scrub site and the open site) in 1982, 1983, and 1984. Pollen loads limited % fruit set and seed production in the open site in 1982 and 1983. Reproduction in the scrub site was resource limited in 1982, as shown by an increase in % fruit set when one-half of the flowers on a ramet were removed prior to fruit initiation. In the scru
b site in 1983, pollination of one-half of the flowers on a ramet decreased the % fruit set of the remaining, unpollinated flowers. Fruit production in the same site was limited by pollen in 1984. Addition of nitrogen fertilizer to the scrub site in 1984 had no effect on fruit and seed production. There was more variation in fruit set between sites than between years. There was no trend to greater fruit set or number of seeds/fruit on early flowers compared to late flowers on the same ramet. Correlations between measures of reproduction were positive or insignificant. These results demonstrate year-and site-specific variation in the factors that limit plant reproduction.
In a riparian population of Erythronium americanum (Liliaceae) in central New Jersey, experimentally self-pollinated plant produced markedly fewer fruit and fewer seeds per fruit than hand-outcrossed and open pollinated plants, even though differences were not evident between pollen tubes that penetrated stigmas from self or foreign pollen. This weak self-compatibility and a positive relation between the percentage of seeds set by outcrossed plants and the distance between pollen donor and recip
ient plants indicate that this population could be susceptible to inbreeding depression.
Limited resources for seed development apparently constrained maximal seed production, based on low seed set (40.6%) by hand-pollinated plants and positive correlations for these plants between plant size and the number and size of seeds set. In contrast, naturally-pollinated plants set a smaller proportion of their ovules, suggesting that limited pollinator service reduced the quantity of seeds produced in this population. Free-foraging bees usually removed more than half of the available pollen in a single visit, so that individual plants probably have few opportunities to disseminate their pollen.
Even though sexually reproductive ramets produce only a single flower per year, less than a third of variation in floral morphology is associated with variation in plant size. Within the flower, the sizes of some closely associated structures, such as the style and ovary, and the anthers and filaments, vary essentially independently of one another. Production of nectar and pollen, the ultimate attractors of pollinating insects, was positively correlated with flower size.
Aralia nudicaulis L. is a dioecious, perennial, herbaceous plant that is commonly found in the understory vegetation throughout the boreal forest of North America. Female remets have fewer flowers per inflorescence, initiate flowering earlier, and reach peak flowering before male ramets. The consequences of the asynchrony in flowering between the sexes on pollination and seed set were examined during a two-year study. In both years there was significant variation in seed set associated with the
flowering times of individual female ramets. In 1983, seed production was highest in the middle of the flowering season. In 1984, seed production was greatest in the later stages of flowering. Variation in seed set was not attributed to lack of pollination in 1983. In 1984, pollination limited seed set per flower during peak flowering. However, seed production never reached the potential five seeds per flower, suggesting that resource limitation was the most important factor affecting fecundity in both years. The asynchronous pattern of flowering is suggested to be the result of the different inflorescence sizes between the sexes.
Rare “albino” morphs of the montane larkspur Delphinium nelsonii differ from common blue-flowered morphs in overall flower color, and in the strength of a contrasting color pattern at the center of the flower that presumably guides pollinators to concealed nectar. Previous studies showed that bumblebees and hummingbirds discriminate against albinos when presented with mixtures of the 2 morphs, and that it takes these pollinators longer to fly between successive flowers on albino than on blue-flo
wered inflorescences. To explore the link between these observations, we measured pollinator preferences and flower-to-flower flight times (“handling times”) before and after painting flowers in 2 alternative ways that enhanced albino nectar guides. In all of 16 experimental replicates discrimination against albinos was reduced or eliminated after painting, and albino handling times declined toward values for blue-flowered inflorescences. This consistent result indicates that an inferior nectar guide increases the energetic cost of foraging at albinos. Increased cost in turn explains discrimination, under the reasonable assumption that hummingbirds and bumblebees are sensitive to foraging economics.
The purpose of this study was to simultaneously measure pollen dispersal distance and actual pollen-mediated gene-flow distance in a wind-pollinated herb, Plantago lanceolata. The pollen dispersal distribution, measured as pollen deposition in a wind tunnel, is leptokurtic, as expected from previous studies of wind-pollinated plants. Gene-flow, measured as seeds produced on rows of male-sterile inflorescences in the wind tunnel, is non-leptokurtic, peaking at an intermediate distance. The differ
ence between the two distributions results from the tendency of the pollen grains to cluster. These pollen clusters are the units of gene dispersal, with clusters of intermediate and large size contributing disproportionately to gene-flow. Since many wind-pollinated species show pollen clustering (see text), the common assumption for wind-pollinated plants that gene-flow is leptokurtic requires re-examination. Gene-flow was also measured in an artifical outdoor population of male-steriles, containing a single pollen source plant in the center of the array. The gene flow distribution is significantly platykurtic, and has the same general properties outdoors, where wind speed and turbulence are uncontrolled, as it does in the wind tunnel. I estimated genetic neighborhood size based on my measure of gene-flow in the outdoor population. The estimate shows that populations of Plantago lanceolata will vary in effective number from a few tens of plants to more than five hundred plants, depending on the density of the population in question. Thus, the measured pollen-mediated gene-flow distribution and population density will interact to produce effective population sizes ranging from those in which there is no random genetic drift to those in which random genetic drift plays an important role in determining gene frequencies within and among populations. Despite the platykurtosis in the distribution, pollen-mediated gene dispersal distances are still quite limited, and considerable within and among-population genetic differentiation is to be expected in this species.
The flowering of pin (long-styled) and thrum (short-styled) plants in populations of Lithospermum caroliniense (Walt.) McMill. was synchronized and both morphs had an extended period of flowering. The plants produced a large number of flowers that did not set seeds - hence the seed production per plant was very low. The seeds showed strong dormancy. Even under natural conditions, only 19% of the seeds produced seedlings. However, seedling survival was relatively high probably due to rapid root g
rowth to deeper layers where moisture was not limiting. The probability of survival of seedlings increased with age; survival of seedlings, juveniles and adults after 2 years was 39, 68 and 98%, respectively. The dry weight of roots increased from 10% in June to 42% in September whereas the proportion of biomass contained in leaves, stems and hypocotyls decreased with age.
Using dry weight biomass we examined the patterns of investment in male and female functions (prezygotic cost) in plants with different mating systems. All the flower parts of both xenogamous and facultatively xenogamous species were heavier, i.e., larger, than those of facultatively autogamous species. Likewise, the dry weights of all the flower parts of xenogamous species exceeded those of facultatively xenogamous species. On a relative basis, xenogamous species invested less in calyces and mo
re in corollas compared to species with the other mating systems. Facultatively autogamous species invested relatively more in pistils. Xenogamous species invested relatively more in stamens than do facultatively autogamous species. The ratios of dry weight stamens to dry weight pistils were equivalent in xenogamous and facultatively xenogamous species.
The available data from xenogamous species suggest a pattern of resource allocation that is independent of sexual system (perfect-flowered, monoecious, or dioecious) and pollen vector. The cost of mating (prezygotic cost) was male biased and frequently exceeded by parental investment (postzygotic cost). These results are not consistent with models that predict equal allocation of resources to male and female sexual function but are consistent with those that predict unequal allocation of resources to those functions in outbreeding hermaphroditic angiosperms. Two additional lines of evidence are inconsistent with the expectations of sex allocation theory. First, resource allocation to sexual function was not equal in wind-pollinated species. Second, relative amounts of the resources allocated to male vis-à-vis female function did not decrease between xenogamy and facultative xenogamy i.e., with an increase in the selfing rate.
The CO2 costs of producing sexual and vegetative reproductive propagules were calculated for two species of wild strawberry, Fragaria virginiana and F. vesca. Five populations on sites representing a gradient of successional regrowth near Ithaca, New York, USA, were studied for two or three years each. Field studies of phenology, biomass, demography, and environment and laboratory studies of CO2 exchange were integrated using a computerbased model of CO2 dynamics
to estimate costs of propagules. The percentage of plants flowering and the number of flower buds produced were highest in an open, recently disturbed habitat and lowest in a forest habitat. The openhabitat plants had the greatest success in converting flower buds into ripe fruits and also produced the highest numbers of runners and runner plantlets. On the basis of total investments in structure and respiration minus any photosynthetic gain of all reproductive structures, the cost per seed was lowest in the most open habitats and highest and increasingly variable in the more closed habitats. The cost of plantlets also was lowest in the most open habitat. The differences among habitats in cost of plantlets alive after one or two growing seasons increased due to differential survivorship of plantlets, with the open habitat continuing to have the lowest cost per plantlet. Theoretical treatments of life history characteristics such as reproductive effort should recognize that costs of equivalent type and size of propagule may vary among environments.
The association between heliconiine butterflies and Passion flower vines is composed of three or more subassociations, in which each Heliconius species group feeds on a different Passiflora subgenus. The relationships are consistent with the adaptive zone hypothesis of Ehrlich and Raven, which would suggest that (1) species of the subgenus Plectostemma proliferated as a result of chemical barriers to herbivory, which created a herbivore-free adaptive zone in which speciation and diversification
took place, and (2) species of the H. erato-charitonia group overcame these barriers and entered a competitor-free adaptive zone, in which they proliferated and speciated with those plants as hosts. The hypothesis that plant secondary chemicals were responsible for creating such barriers to herbivory was tested using heliconiine species as bioassays, in which reduced growth rates indicated presence of chemical barriers to feeding. Contrary to expectation, plants of the subgenus Plectostemma showed little or no chemical defense against any species of heliconiine caterpillar. In contrast many plants of the "primitive" subgenus Granadilla possessed significant chemical barriers against herbivory by heliconiine larvae, excepting those species in the H. numata-melpomene species group. I concluded that chemical barriers to feeding were not responsible for proliferation and diversification in the subgenus Plectostemma, nor did chemicals create a competitor-free "adaptive zone" in which the H. erato-charitonia species-group could proliferate and speciate. Chemical barriers may have been important in the evolution of the subgenus Granadilla-heliconiine association. I suggest that plant allelochemics are only one of many possible barriers to herbivory which can help create "adaptive zones" for plants and their herbivores, and that the patterns of butterfly foodplant specialization discussed by Ehrlich and Raven (1964) are not necessarily the result of biochemical adaptation and counteradaptation.
The effects of inflorescence pubescence on floral temperatures and fecundity were examined in four species of Puya in the Ecuadorian Andes. These species span an elevational range from 1,980-4,000 m and comparisons among these species showed that pubescence production increases significantly with elevation. Flower temperatures of glabrous or slightly pubescent species of Puya from lower elevations closely tracked air temperature, while those of the pubescent páramo species did not. Pubescence re
moval experiments on Puya hamata, a páramo species, demonstrated that pubescence is an effective insulator, maintaining elevated flower temperatures. In Puya clava-herculis (also a páramo species) elevated flower temperatures were associated with higher seed set. Thus, the greater production of inflorescence pubescence in páramo species of Puya may be an important factor contributing to reproductive success in these higher elevation species.
Male and female flowers of the dioecious perennial herb Rubus chamaemorus L. are similar in general appearance. However, female flowers are somewhat smaller, do not produce any pollen, and contain very small amounts of nectar. Syrphids and bumblebees, which are important pollinators of R. chamaemorus, showed a strong preference for male flowers. Male flowers were also less often rejected by flower visitors than were female flowers, and two different groups of syrphid species stayed longer in mal
e than in female flowers. These observations suggest that female flowers of R. chamaemorus attract pollinators by deceit.
Hand-pollination experiments indicated that pollen availability limited seed production of R. chamaemorus in female dominated habitats but not in areas with an equal floral sex ratio. We suggest that the relative importance of factors limiting female reproductive success is not constant, but is influenced by the floral sex ratio of the population. This should apply also to other dioecious species that show variable sex ratios on either a local or regional scale.
Secondary pollen carryover is defined as the process whereby a pollinator receives previously deposited pollen grains when visiting a flower and transfers them into a new (secondary) carryover sequence. The secondary pollen carryover in a system of ants, Formica rufibarbis, visiting Scleranthus perennis (Caryophyllaceae) was studied using fluorescent dyes as pollen analogues. The mean secondary carryover was found to be 1.2 flowers compared with 4.5 flowers for the primary carryover. The number
of dye grains deposited per flower visited is lower and the frequency of zero deposition is higher in the secondary carryover sequence than in the primary.
Antechinus stuartii has one highly synchronized mating period which occurs at the same time each year. An analysis of the time of reproduction in 162 populations of A. stuartii shows that the onset of the mating period is correlated with the rate of change of photoperiod, rather than with critical photoperiodic length. The rate of change of photoperiod is different for two designated forms of this species and can be used as a predictor of these animals' reproductive timing. A rate of change mode
l further explains the rigid and highly synchronized nature of the mating period and, by providing a mechanism for reproductive isolation, offers an explanation for the evolution of the two forms of this species.
Mortality of an annual plant. Collinsia torreyi, was increased in the vicinity of Pedicularis semibarbata plants. This was a consequence of feeding by Euphydryas editha, a herbivore which feeds on both plant species. Mortality of C. torreyi was better predicted by distance from P. semibarbata than by density of C. torreyi. Since not all herbivores are monophagous, interactions involving distance and density effects on plant survival and on vegetational diversity need to be considered with respec
t to plant and herbivore communities. Some of the complexities of one predator, two prey systems are discussed.
Of 36 plant species surveyed, 6 were significantly associated with nests of the desert seed-harvester ant Veromessor pergandei or Pogonomyrmex rugosus; two other plant species were significantly absent from ant nests. Seeds of two common desert annuals, Schismus arabicus and Plantago insularis, realize a 15.6 and 6.5 fold increase (respectively) in number of fruits or seeds produced per plant growing in ant nest refuse piles compared to nearby controls. Mass of individual S. arabicus seed produc
ed by plants growing in refuse piles also increased significantly. Schismus arabicus, P. insularis and other plants associated with ant nests do not have seeds with obvious appendages attractive to ants. Dispersal and reproductive increase of such seeds may represent a relatively primitive form of ant-plant dispersal devoid of seed morphological specializations. Alternatively, evolution of specialized seed structures for dispersal may be precluded by the assemblage of North American seed-harvester ants whose workers are significantly larger than those ants normally associated with elaiosome-attached seed dispersal. Large worker size may permit consumption of elaiosome and seed.
Inter and intraspecific host plant choice was examined for Mycalesis terminus and M. perseus, two tropical butterfly species of the subfamily Satyrinae. The two species preferred different suites of potential host plant species, but only M. perseus discriminated between different qualities of host. Differences in selectivity between these two species may be associated with the different lengths of time over which their larval resources may be expected to persist. The level of selectivity exhibit
ed by these tropical species appears to be greater than their temperate counterparts. Potential factors controlling differences between oviposition strategies in temperate and tropical Satyrine butterflies and other groups are discussed.
The means by which newly emerged pollen-specific (oligolectic) solitary bees locate their appropriate pollen host plant is not clearly understood. To evaluate the role of flower and pollen odors in this recognition process, preference tests were performed on foraging-inexperienced and, for comparison, field-experienced individuals of the solitary bee Colletes fulgidus longiplumosus. The bees were presented with a choice of four plant species, offered in the form of aromas from flowers, whole pol
len, pollenkitt, and internal pollen lipids. Observed feeding-attempt response patterns suggest: 1) that bees can distinguish between plant species on the basis of odors from flowers, whole pollen, and pollenkitt, 2) inexperienced bees show a preference for the pollen on which they were reared as larvae, and 3) chemicals eliciting feeding responses to pollen appear to be contained in the pollenkitt. Feeding responses by experienced bees differ from those of inexperienced bees in ways which indicate that the insects' odor-based search image of the host plant becomes modified by foraging experience.
Four shrub species of the Australian Proteaceae (Hakea serieea, H. gibbosa, H. suaveolens and H. salicifolia) were introduced to South African fynbos shrublands between 1840 and 1860. H. sericea is highly invasive, H. gibbosa and H. suaveolens are moderately invasive and H. salicifolia is not invasive. The allocation of reproductive energy, germinability, the ability to survive fires and to germinate in burnt and unburnt areas, and the nutrient content of seeds were assessed for the four species
. The information was used to investigate whether the success of H. serieea relative to the other three species could be explained by the superior expression of any trait. The most important trait which separates H. sericea from the other species is its ability to produce a large seed bank in its adopted environment in the absence of seed predators. Seed production in H. sericea shrubs with an above-ground dry mass of 8 kg is four times greater than H. gibbosa and more than 16 times that of H. suaveolens. Although H. salieifolia also produces a large seed bank, its seeds are unable to survive fires due to inadequate insulation by the small follicles. The results are compared to dispersal and seed bank data for indigenous South African Proteaceae, which have low dispersal and suffer high pre-dispersal seed predation. We suggest that potential invasives in the fynbos can be identified as species that have: (i) a potentially high seed production that is limited by specialized predators; (ii) an ability to disperse over long distances; and (iii) are pre-adapted to frequent fires and low soil nutrients. The data also support the current strategy of combatting H. sericea using specialized insect seed predators.
Reproductive success is divided into two phases: preemergent (the number of viable seeds that enter the ambient environment) and postemergent (the percentage of progeny that survive to reproduce). We studied preemergent reproductive success (PERS) in flowering plants by measuring the fruit/flower (Fr/Fl) ratio and the seed/ovule (S/O) ratio in a number of species of outcrossing and inbreeding plants, where PERS=the product of (Fr/Fl) and (S/O). In order to determine the influence of the ambient
environment (including resource availability) we studied pairs of outcrossing and inbreeding species occurring in the same habitat. Among outcrossing species PERS averaged about 22%, whereas in inbreeding species the average was approximately 90%. The progeny/zygote (P/Z) ratio was studied in hand-pollinated populations in Epilobium angustifolium (a strongly outcrossing species) from populations in Oregon and Utah, by direct observation of embryogenesis at twoday intervals throughout the course of seed development. The P/Z ratio in both populations averaged near 30%, and the developing embryos showed a surprising array of abnormalities that resulted in embryo death. During early development >95% of the ovules had normally developing globular embryos, but beginning with differentiation (cotyledon formation) about 70% of the original globular embryos aborted during the course of embryogenesis and seed development. The clustering of developmental lethals during peroids of major differentiation events parallels the animal model of development. We found little evidence that PERS was limited by the ambient environment (including resource availability), pollination, or factors associated with the inbreeding habit. Instead, PERS was found to be inextricably linked to outcrossing plants, whose breeding systems promote genetic variability. The high incidence of developmental lethals in E. angustifolium and the resulting low P/Z ratio (ca. 30%) is attributed to genetic load (any lethal mutation or allelic combination) possibly working in combination with developmental selection (interovarian competition among genetically diverse embryos). Examples of maternally controlled, fixed patterns of ovule abortion with respect to position or number are discussed. However, we found no need to employ “female choice” as a hypothesis to explain our results for the extensive, seemingly random patterns of embryo abortion in E. angustifolium and other outcrossing species. A more parsimonious, mechanistic explanation based on genetic load-developmental selection is sufficient to account for the differential survivorship of embryos. Likewise, the traditional concept of a positive growth regulator feedback system based on the number of surviving ovules in an ovary can account for subsequent fruit survivorship.
In alpine Polemonium viscosum, plants having sweet-scented flowers are primarily pollinated by queens of the bumble bee species, Bombus kirbyellus. In this paper we ask whether two aspects of the pollination effectiveness of bumble bees, visitation rate and pollination efficiency, vary significantly with flower size in sweet-flowered P. viscosum. (i) Bumble bees visited plants with large flowers on 80–90% of encounters, but visited those with smaller flowers on only 49% of encounters. (ii) Howev
er, the gain in pollination that large-flowered plants obtained via increased visitation was countered in part because bumble bees deposited fewer outcross pollen grains per visit on stigmas of large flowers than on those of small ones. When both visitation rate and pollination efficiency are taken into account, the predicted value of a single bumble bee encounter declines from 1.06 seeds for flowers larger than 18 mm in diameter to 0.55 seeds for flowers smaller than 12 mm in diameter. Our results suggest that bumble bee pollinators of P. viscosum prefer flower morphologies that are poorly suited for precise pollination. Such behavioral complexities are likely to place constraints on the evolution of “optimal” floral design.
Most plants are hermaphrodite (cosexual). Charnov et al. (1976) advanced the hypothesis that cosexuality is favoured in plants because a convex fitness set is generated by a non-additive relationship between male and female resource costs. In the first experimental test of this hypothesis, reproductive costs were measured in a male × female factorial design using male, female, cosexual, and neuter cucumber plants. Costs were measured by plant's vegetative growth response to treatments. The resul
ts show that male costs in the system used have negligible effect upon plant growth and female function, and imply a convex fitness set, in accordance with Charnov et al.'s model. Female function (fruit set) has an inhibitory effect upon vegetative growth and male flower production, favouring protandry.
Under conditions where resources are limited, there are often negative correlations between components of maternal yield, or between fruit and flower production. Pollination, in turn, may vary among individuals and influence total maternal expenditure. We examined the impact of variation in pollination thoroughness upon yield components and overall plant growth in wild radish (R. raphanistrum) plants grown in the greenhouse. Plants received different pollination treatments in which 0% to 100% of
all flowers produced were hand-pollinated. Fruit set was increased by hand-pollination, but rarely exceeded 30%, even when more than 50% of the flowers were pollinated. Plants receiving more thorough pollination or having greater proportion fruit set produced significantly smaller seeds. Seed number per fruit was not influenced by pollination treatment. Mean values of yield components and interactions between components often varied among plants from different maternal families. Increasing pollination thoroughness also resulted in dramatic decreases in flower production. If male fitness is related to flower number, there may be a tradeoff between maternal fecundity and successful pollen export operating at the whole-plant level in this species.
In the eastern Andes of Colombia, the shrub Befaria resinosa (Ericaceae) has peaks of flowering that are separated by extended periods of low flower production. The effect that these fluctuations in flower production have on pollen flow was investigated by using fluorescent dye as a pollen analog. Dye applied to open flowers was dispersed over long distances more often during low flower production than during high flower production. Whether enhanced pollen dispersal during flowering lows is of b
enefit to individual plants is not clear. The proportion of flowers that set fruit is positively correlated with flower abundance, negating the possibility that increased pollen dispersal results in a higher rate of fruit production due to outbreeding effects. It is also difficult to attribute the pattern of fruit production to changes in pollinator visitation rates, which are negatively correlated with flower abundance in the case of hummingbirds and not correlated at all with flower abundance in the case of insects. An opportunistic, large-bodied hummingbird (Colibri coruscans) visits B. resinosa during high flowering and may be a particularly effective pollinator, accounting for some of the increase in the proportion of flowers setting fruit. Rainfall is positively correlated with flower production and may be an important factor in shaping flowering phenology, but it is not significantly correlated with the proportion of flowers setting fruits. The possibility that low-level flowering may counteract inbreeding that results from peak flowering is discussed.
The foraging behavior of the pollinators of tristylous Pontederia cordata was studied to determine if differences in floral morphology would lead to preferential visitation of the floral morphs. Although nectar production is not different in the three floral morphs, differences in the production and size of pollen grains produced by the three anther levels results in the morphs offering variable amounts of resources to pollen-collecting insects. Bumblebees (Bombus spp.) and the solitary bee Meli
ssodes apicata used P. cordata primarily as a nectar source and therefore did not seem to exhibit any morph preference. In contrast, honeybees visited flowers mainly for pollen and preferred to forage on long-level anthers of the short-and mid-styled morphs. An analysis of the composition of corbicular pollen loads indicated that, relative to the frequency of production in the population: 1) honeybees collected an excess of pollen from long-level anthers; 2) bumblebees collected the three types of pollen without any apparent preference; and 3) M. apicata preferentially collected pollen from the short-level anthers — presumably because their proboscides are modified by the presence of tiny hairs. The results suggest that P. cordata in Ontario is serviced by a diverse, unspecialized pollinator fauna which is not co-adapted to the tristylous floral polymorphism.
Qualea grandiflora is a typical tree of Brazilian cerrados (savanna-like vegetation) that bears paired extrafloral nectaries (EFNs) along its stems. Results show that possession of EFNs increases ant density on Q. grandiflora shrubs over that of neighbouring non-nectariferous plants. Frequency of ant occupancy and mean number of ants per plant were much higher on Qualea than on plants lacking EFNs. These differences resulted in many more live termitebaits being attacked by foraging ants on Quale
a than on neighbours without EFNs. Termites were attacked in equal numbers and with equal speeds on different-aged leaves of Qualea. The greatest potential for herbivore deterrence was presented by Camponotus ants (C. crassus, C. rufipes and C. aff. blandus), which together attacked significantly more termites than nine other ant species grouped. EFNs are regarded as important promoters of ant activity on cerado plants.
The hills of the Negev highlands, west of Sede Boqer, are typically covered by various half-shrub communities, including the deciduous species of Helianthemum vesicarium Boiss., an Irano-Turanian element that grows only on north facing slopes, and H. ventosum Boiss., a Saharo-Arabian element which grows on both slopes but mainly on those facing south. Upon irrigation, plants of H. vesicarium preserve their natural rhythm of activity during winter and remain deciduous throughout the summer. On th
e other hand, under a similar irrigation treatment, the growth and flowering season of H. ventosum is modified and is extended well into the summer. Thus, H. ventosum shows a high phenological plasticity, while H. vesicarium seems to be very conservative. The different response of the two species to an improved water regime may partly explain the differences in their natural habitats: H. vesicarium on the more humid north facing slopes and H. ventosum mostly on the more arid and extreme south facing ones. Plants of both species which grow near boulders attain larger sizes and denser stands than those on the slopes.
Age analysis based on xylem ring counts of 859 plants of the two natural populations showed that the plants did not exceed 14 years and most of them reached the age of some 5 years only. In general more seedlings are established during rainy year-clusters than during dry ones. However, no direct correlation between seedling establishment and the annual rainfall of specific years could be found.
The rapid turnover of plants in the stands of both species of Helianthemum resembles the behavior of desert annuals rather than that of most desert shrubs.
The orchid Leporella fimbriata is pollinated by pseudocopulation with winged males of the ant Myrmecia urens. This recently studied interaction provides a unique opportunity to examine the two current hypotheses concerning the apparent rarity of ant pollination systems worldwide. The first hypothesis requires a series of specialized growth forms and floral characteristics regarded as adaptations to ant pollination. L. fimbriata does not possess them. The second considers the pollenicidal effects
of secretions from the metapleural gland of ants. These glands are absent in M. urens males and it may be that the occurrence of ant pollination requires the absence of metapleural glands in the vector.
Populations of Lomatium grayi (Umbelliferae), an andromonoecious, perennial herb, differ in growth rates, flowering frequency, and survivorship. Effects of these different life histories on the ontogeny of sex expression were analyzed for plants from two populations grown from seedlings in a common garden and monitored for six years. Plants from Smoot Hill, Washington grew faster, had a higher probability of flowering at each age and size after the first year of growth, and a higher probability
of flowering repeatedly among years than did plants from Clarkston, Washington. The proportion of plants producing some hermaphroditic flowers increased with plant size in both populations. Smoot Hill plants, however, were more likely to begin flowering as small, staminate plants than Clarkston plants. Clarkston plants did not begin flowering until they were older and larger, and most of these plants produced some hermaphroditic flowers when they began reproduction. The positive association between production of hermaphroditic flowers and both plant size and age was consistent with the hypothesis that hermaphroditic flowers are more costly to produce than staminate flowers. Although the populations did not differ in the total number of flowers per plant produced at any age or size, Smoot Hill plants consistently produced a lower percentage of hermaphroditic flowers than Clarkston plants at larger sizes and later ages. Consequently, selection for faster growth rates and higher flowering frequency at small sizes and early ages may have favored the more staminate-biased sex ratios in the Smoot Hill population.
The flowering and fruiting patterns of the dioecious perennial herb Rubus chamaemorus L. were studied in frost-prone (open) and frost-sheltered (Shaded) habitats in northern Sweden over 6 years. The number of ramets with flower buds, the proportion of flower buds that opened, and fruit set varied markedly between years. In the frost-prone populations, the occurrence or absence of detrimental frosts during the development of flowers and fruit could explain much of the variation, both in the propo
rtion of flower buds that developed into flowers, and in fruit set. In the frost-sheltered populations, most female flowers that did not develop into fruit aborted without any signs of physical damage and before any ovules had begun to enlarge. Flower mortality caused by herbivores feeding on reproductive parts was commonly low, but reached values higher than 10% in one of the shaded populations. Hand-pollination increased the proportion of ovules producing seeds in the mature fruits by about 20%, and in one year also increased fruit set significantly in one population. Fruit-producing female ramets had a higher mortality and a lower probability of flowering in the subsequent year than male ramets and non-fruiting female ramets. In R. chamaemorus, the conditions for fruit maturation are highly unpredictable at the time of flower initiation. It is suggested that the apparent “over-initiation” of flower buds is advantageous, as it allows the plant to attain a high reproductive success in years favourable for flowering and fruit development.
In cloud forest at Monteverde, Costa Rica, two common treelets (Palicourea lasiorrachis and Cephaelis elata, both Rubiaceae) depend simultaneously on one hummingbird population (Lampornis calolaema) for pollination. Both species are distylous and self-incompatible. In laboratory experiments, we examined possible effects of indiscriminate foraging by hummingbirds among flowers of both species, as observed in the field, on pollination of Palicourea. In each of 35 trials, captive L. calolaema probe
d 2 flowers from pin plants of Palicourea followed by 20 thrum flowers of the same species, with either 0, 2, or 10 Cephaelis flowers intervening. We assessed pollen transfer by staining and counting pin pollen tubes growing in thrum styles; counts of 0, 1, or ≥2 pollen tubes relate directly to seed output (0, 1, or 2 seeds per fruit, respectively). Intervening Cephaelis flowers sharply reduced pollen receipt by thrum flowers of Palicourea and reduced some aspects of pollen dispersal from pins as well, thereby curtailing maternal and paternal reproductive potential of Palicourea. Such effects of interspecific pollen loss on reproductive output may lead to strong competition among some, though not all, combinations of plant species pollinated by L. calolaema or of other plant combinations that share animal pollinators.
Two carpenter bees (Xylocopa spp.) in southern Israel both use the asclepiad Calotropis procera as a primary nectar source. This plant genus is coevolved with carpenter bees, and aspects of the insect-flower interaction in Israel suggest that the smaller bee, X. sulcatipes, is the natural co-adapted pollinator, a view borne out by the geographical distributions of the species concerned. There are significant mismatches between the plant and the larger X. pubescens, involving physical fit and beh
aviour. These mismatches are particularly evident when the physiologies of the bees and the plant are considered. The different sizes and colours of the two bees lead to different daily activity patterns, only X. sulcatipes being thermally suited to, and thus abundant at, times of maximum nectar production by Calotropis. Similarly the water requirements of X. sulcatipes are finely balanced with the water production in the floral nectar; this bee gains just enough water when foraging to restore its blood concentration and production in the floral nectar; for deposition in the nest. X. pubescens does not incur net water loss in flight and gains too much water from Calotropis flowers, necessitating copious urination and ‘tonguelashing’. Hence physiological information can be of use in deciphering insect-plant coevolutionary patterns, and the water component of nectar is confirmed as a potentially major determinant of foraging activities. The circumstances where this will be particularly true, and cases where it may not apply, are discussed.
In the Mediterranean region of southern France, the fate of flowers and fruits of Cornus sanguinea, a deciduous shrub, was studied in three contrasting habitats: (1) an abandoned olive grove, (2) the edge and (3) the interior of a deciduous forest. Abundance of flowering and fruiting of C. sanguinea differed widely between the three habitats. The fate of the flowers, on the other hand, did not differ significantly between habitats. Taking all three habitats together, 77% of the flowers aborted,
23% initiated fruits, 6% developed mature fruits, and 3% produced ripe fruits that were eaten by birds. Likewise, there were for the most part no significant differences in the fate of immature and mature fruits between the habitats. On average, 58% of the immature fruits were abscised undamaged, 24% matured, 10% were damaged by insects and 8% dried up. Of the mature fruits, on average, 51% were eaten by birds, 23% damaged by insects, 20% dried up and 6% fell to the ground undamaged. Independent of habitat conditions, C. sanguinea seems to regulate the quantity of its seed crop primarily by limiting the number of flowers and secondarily by aborting surplus immature fruits, and the number of flowers is mainly controlled by resource availability and genetic factors. In the case of C. sanguinea, both the fruit/ flower ratio and the proportion of flowers producing mature fruits that are eaten by birds remain constant over a wide range of environmental conditions.
Effects of variation in fire season on flowering of forbs and shrubs were studied experimentally in two longleaf pine forest habitats in northern Florida, USA. Large, replicated plots were burned at different times of the year, and flowering on each plot was measured over the twelve months following fire. While fire season had little effect on the number of species flowering during the year following fire, fires during the growing season decreased average flowering duration per species and incre
ased synchronization of peak flowering times within species relative to fires between growing seasons. Fires during the growing season also increased the dominance of fall flowering forbs and delayed peak fall flowering. Differences in flowering resulting from variation in fire season were related to seasonal changes in the morphology of clonal forbs, especially fall-flowering composites. Community level differences in flowering phenologies indicated that timing of fire relative to environmental cues that induced flowering was important in determining flowering synchrony among species within the ground cover of longleaf pine forests. Differences in fire season produced qualitatively similar effects on flowering phenologies in both habitats, indicating plant responses to variation in the timing of fires were not habitat specific.
Bumblebees foraging on the self-incompatible Anchusa officinalis fly between near neighbour plants and between near neighbour inflorescences within plants. Although many-flowered plants attracted most bumblebees these plants received fewer visits on a per flower basis than smaller plants, and each bumblebee visited a smaller proportion of the flowers. The calculated effective visitation rate per flower was highest on plants of an intermediate size. If pollen-carryover was assumed to be limited t
he most efficient plant was predicted to be smaller since the proportion of fertilized flowers per bumblebee visit is expected to decrease further on the largest plants in relation to the total flower number. These predictions were tested by measuring fruit-set in the field. The percentage fruit-set decreased with plant size at all sizes that were investigated. That the most efficient plant was small indicates that pollen-carryover was indeed limited. However, the low percentage fruit-set associated with large size did not present a serious problem since the total estimated seed production per plant still increased with size. Selection favoring smaller plants may be low or absent in Anchusa.
The ecological potential for multiple mating is high in Amianthium muscaetoxicum. The percentage of long-distance pollinations (20-100 m) is greater than reported for most insect-pollinated systems. Estimations of neighborhood area are at least an order of magnitude larger than any previously reported for plant species. Seasonal effects on fluorescent dust dispersal indicate that neighborhood areas change during the flowering season. The number of flowers marked with fluorescent dust on an inflo
rescence increases with increasing inflorescence size, and the proportions of available inflorescences that are marked decrease with distance from the source. Allozyme analysis indicates that heterozygosity levels are typical of outcrossing plants. The diversity of seed genotypes is increased by increasing the size of the floral display. The present investigation is the first to consider the effects of floral display on seed diversity and adds to existing data indicating that inflorescence size is important to fecundity and/or pollen donation in some systems.
Genetic neighborhood size and area were estimated from pollinator movements over 3 years in a scarab beetle-pollinated clonal herb, Dieffenbachia longispatha (Araceae) at the La Selva Biological Station, Costa Rica. This species was characterized by low densities of reproductive individuals and asynchronous flowering within the population. The pollinator flight distributions were characterized by relatively long mean distances between consecutive visits to inflorescences (83 m) and movements to
the nearest neighboring inflorescence in the appropriate phase of flowering. Pollinator movement distributions between consecutive visits to inflorescences were significantly leptokurtic in 2 of the 3 years. I calculated neighborhood sizes incorporating the levels of kurtosis and found minimal estimates of Ne to be 227–611 ramets and neighborhood area to be 88 000–180 000 m2. The three beetle species that made 94% of the visits (Cyclocephala gravis, C. amblyopsis, and Erioscelis columbica) varied in their flight distributions and in their contributions to the estimates of neighborhood size. Cyclocephala amblyopsis exhibited the greatest degree of kurtosis in its movement patterns, and neighborhood size based on its movement was large relative to Ne calculated from movement distributions of the other two beetle species. Long-distance movements of C. amblyopsis (>300 m) accounted for 68% of the neighborhood size.
The effects of the surface secretions of eight species of ants on three types of pollen were bioassayed by exposure to the integument of undisturbed, living individuals for 20 min. Ant species included Atta texana which cultures fungi by means of various types of secretions. The frequency of grains showing membrane dysfunction, and therefore reduced viability, was quantified by means of a fluorochromatic test. Comparisons of treated and control samples showed that in 46 out of 50 bioassays there
was a reduction in pollen viability following exposure to ants, 38 being statistically significant. Variation in the outcome of bioassays showed differential potency among ant species and differential vulnerability among pollen types. Ant pollination may be uncommon because surface secretions, often from the metapleural glands, cause membrane dysfunction in pollen. Ant species without metapleural glands may be pollinators, but ant pollinated plants may have pollen resistant to the secretion.
We derived an index of reproductive effort (g reproductive tissue per g leaf) from data collected over two seasons on 28 males and 28 females of the dioecious shrub Oemleria cerasiformis. Males produced an average of three times as much flower and flower-stalk tissue as females, but because of their large fruits, females produced four times as much total reproductive biomass. Reproductive effort of both sexes was related to light. Fruit set in females (% carpels producing drupes) averaged 11.2%
and was related to spring light levels. Male-biased sex ratios in this species may be related to the greater reproductive effort of females.
The forest annual, Amphicarpaea bracteata L. can reproduce via aerial chasmogamous, aerial cleistogamous, and subterranean cleistogamous flowers. Both plant size and light intensity influenced the utilization of the three modes of reproduction. Chasmogamous and aerial cleistogamous flower number and the ratio of chasmogamous flowers to the total number of aerial flowers increased with plant size. The latter demonstrated a shift to xenogamy and outbreeding in larger plants. Light intensity indire
ctly influenced reproductive modes through its influence on plant size. Seed set by both types of aerial flowers was low and unrelated to plant size. Subterranean seed number and the total dry weight of subterranean seeds per plant increased with size. The subterranean seeds of Amphicarpaea bracteata are thirty-four times larger than the aerial seeds (fresh weight). Under field conditions, subterranean seeds had greater germination after one year than aerial seeds. The plants arising from subterranean seeds were significantly larger and more fecund than those from aerial seeds. Seeds produced by aerial cleistogamous, hand selfpollinated chasmogamous, and naturally pollinated chasmogamous flowers had equivalent germination rates and produced plants of equal size and fecundity. This suggests that the outbred progeny from chasmogamous flowers have no advantage over the inbred progeny from aerial cleistogamous flowers.
The effectiveness of nectarivorous birds, introduced honey bees and staphylined beetles as pollinators of Banksia menziesii was assessed. Staphylinids removed substantial amounts of pollen but did not deposit any onto stigmata. Abundance of beetles on inflorescences was related to the mean number of florets opening per day. Honey bees collecting pollen were more likely to effect pollination than those collecting nectar which only contacted stigmata when arriving or leaving an inflorescence. Nect
ar-foraging birds probed between florets 10.2±0.8 (±SE) times, contacting 8–16 stigmata during each probe. Bees visited inflorescences ten times more frequently than birds although they deposited only 25% of the pollen that birds did on stigmata. Fruit set was ten times greater on inflorescences visited by birds than on inflorescences visited by bees. Bees were capable of removing as much pollen as birds but, because of direct pollen transfer to birds when florets opened during foraging, actual removal was probably much less. Selection for floret opening during nectar foraging by birds may have resulted from pollen removal by non-pollinating animals, such as staphylinids.
Examination of the gut contents of adult Rhingia campestris reveals that there are considerable differences in the relative amounts of pollen and nectar ingested by individual insects. Pollen, a rich protein source, is required most by females, particularly during those stages of ovarial development when yolk deposition occurs. Nectar, which is mainly carbohydrate, is required in large amounts by males, and is also required by females before and after oogenesis. These results are discussed in re
lation to the likely nutritional needs of the flies, and it is suggested that similar feeding patterns may be found in a variety of other flower visiting insects. Finally, reference is made to the problems of making ecological assumptions about feeding behaviour on the basis of the morphological attributes of the mouthparts.
Can bees accurately gauge accumulating bodily pollen as they harvest pollen from flowers? Several recent reports conclude that bees fail to assess pollen harvest rates when foraging for nectar and pollen. A native nightshade (Solanum elaeagnifolium Cavanilles) that is visited exclusively for pollen by both solitary and social bees (eg. Ptiloglossa and Bombus) was studied in SE Arizona and SW New Mexico. The flowers have no nectaries. Two experiments were deployed that eliminated "pollen feedback
" to the bees by experimentally manipulating flowers prior to bee visits. The two methods were 1) plugging poricidal anthers with glue and 2) emptying anthers of pollen by vibration prior to bee visitation. Both experiments demonstrated that bees directly assess pollen harvest on a flower-by-flower basis, and significantly tailor their handling times, number of vibratile buzzes per flower and grooming bouts according to the ongoing harvest on a given flower. In comparison to experimental flowers, floral handling times were extended for both Bombus and Ptiloglossa on virgin flowers. Greater numbers of intrafloral buzzes and numbers of times bees groomed pollen and packed it into their scopae while still on the flower were also more frequent at virgin versus experimental flowers. Flowers with glued androecia received uniformly brief visits from Bombus and Ptiloglossa with fewer sonications and virtually no bouts of grooming. Curtailed handling with few buzzes and grooms also characterized visits to our manually harvested flowers wherein pollen was artificially depleted. Sonicating bees respond positively to pollen-feedback while harvesting from individual flowers, and therefore we expect them to adjust their harvesting tempo according to the currency of available pollen (standing crop) within Solanum floral patches.
To document seasonal changes in the reproductive behavior of the perfect-flowered, self-incompatible mustard, Raphanus sativus L., we monitored individual survival, flower and fruit production among 58 individuals in a California population over six census dates (cohorts). Population size declined dramatically and mean individual levels of fruit set changed significantly between cohorts. The frequency distribution of flower and fruit production became increasingly skewed over the first four coho
rts. The phenotypic maleness of individuals, a standardized measure of phenotypic gender, oscillated during the reproductive season, peaking in the third and fourth cohorts. We calculated a simple estimate of expected male reproductive success of each plant (the number of fruits sired on conspecifics); this estimate was a function of an individual's flower production and the fruit production of its potential mates in our sampled population. Mean expected male success did not differ significantly among cohorts; expected male success per flower did, however, change significantly among cohorts. Among individuals within each cohort, maternal fruit production and expected male success were both positively correlated with flower production throughout the season. Spearman rank correlation coefficients indicate that the strength of these associations, however, changed during the season. Linear regressions of transformed variables indicated that the shape of several fitness functions also changed over time. In addition, the amount of variation in maternal or expected paternal success explained by flower production declined over the first four cohorts. If typical of wild populations, these temporal changes in these functions suggest that measurements of the intensity of phenotypic selection on flower production will depend on when and how fitness is measured in natural populations.
In 4 common Middle-European mainly bumblebee-pollinated plant species (Impatiens glandulifera, Echium vulgare, Aconitum napellus, Symphytum officinale) the influence of patch size on species composition of the pollinator community was studied. Short-tongued species were most dominant in large patches, while small patches were frequented by middle- and long-tongued bumblebees. This phenomenon was extremely obvious in Symphytum officinale and Aconitum napellus, where short-tongued species had bitt
en a hole in nearly every flower of large patches. Long-tongued species were forced to small patches, where nectarrobbing occurred only exceptionally. In small patches visitationrate (Number of visits per flower per hour) was not lower but either equal or even higher then in large patches. Nectar measurements in Echium vulgare showed, that not only the mean quantity of nectar but also the variance was lower in small patches. As a result, the possible gain can be predicted much more precisely in a small patch than in a large one, and bumblebees have less difficulties in making the right foraging decisions. According to this, foraging strategies depend on patch size. This was confirmed by a computer simulation. The conclusion can be drawn, that many bumblebee species are able to share the same resource by using different patch sizes. Since large flower patches occur mainly in man-made habitats, the dominance of short-tongued species in many bumblebee communities studied by other authors may be unnatural.
Euploea core is a long lived butterfly which lays a few relatively large eggs each day. In such a species it is unlikely that reserves of carbohydrate and amino acid accumulated during the larval instars would be sufficient to last its entire adult life. Female E. core were kept in a large flight cage and assigned to one of four treatments. Each treatment comprised a different concentration of sugar and amino acid in the adult diet of the butterflies. Individuals with 25% sugar in their diet liv
ed for longer and attained higher fecundities than those with 1% sugar in their diet. Butterflies on the 1% sugar diet removed greater volumes of food solution than those on the 25% sugar diet. The availability of amino acids in the adult diet had no marked effect on longevity and, if anything, had a negative effect on fecundity. The composition of the adult diet had no discernable effect on egg weight in this species. Sugar is shown to be an important component of the adult diet of E. core but their requirement for amino acids in their adult diet remains unresolved. Finally, using the known volume of food solution removed each day, estimates were made of the minimum amount of energy required by this species each day and the amount of energy required to produce an egg.
Abundance and flower visitation rate of the pollinators of Lavandula latifolia (Labiatae), an insect-pollinated shrub, were studied over a 6-year period. The objective was to elucidate interspecific patterns in the “quantity” component of the plant-pollinator interaction. A total of 54 insect taxa are considered in the analyses, including hynenopterans, dipterans and lepidopterans. Most pollinators were comparatively scarce, with a few taxa acounting collectively for the majority of individuals.
Pollinators differed broadly in flower visitation rate (0.2–30 flowers/min). Most of this variation was explained by differences in flower handling time (HT). Regardless of proboscis length, hymenopterans had intrinsically shorter handling times than lepidopterans. Within each group, HT decreased exponentially with increasing proboscis length. Abundance and visitation rate were uncorrelated across pollinator taxa. The total number of visits that each pollinator contributed to the plant (NFV) was estimated as the product of abundance x visitation rate. NFV values spanned four orders of magnirade. A small, taxonomically diverse group of species (1 moth, 1 butterfly, 4 bees) accounted for most visits and thus could effectively exert some selection on floral features. Nevertheless, the morphological diversity represented in this group of dominant pollinators probably constrains plant specialization, as they will most likely select for different floral features or in opposing directions on the same traits.
The evergreen tussock-forming Eriophorum vaginatum revealed consistently earlier (c. 1 month) phenology and greater biomass per tiller than the summergreen rhizomatous E. scheuchzeri in all four components measured (vegetative and reproductive shoots and stems) under the same climatic regime in central Alaska over one growing season. Greatest allocation to vegetative shoot growth occurred in mid-summer in both species. The tussock growth form of E. vaginatum raised shoot meristems 25-30 cm above
the soil surface, where temperatures were warmer, permitting shoot growth to begin earlier in spring and continue longer in autumn than in E. scheuchzeri. Consequently, E. vaginatum was able to allocate reserves to reproductive tillers primarily in autumn and early spring, times when minimal reserves were required for vegetative growth. By contrast, the rhizomatous E. scheuchzeri had a more constrained growing season, and allocation to reproduction coincided with allocation to vegetative growth. For this reason, reserves were drawn down more fully in mid-summer in E. scheuchzeri than in E. vaginatum. The more conservative use of nutrient stores in E. vaginatum may relate to its great longevity, reduced allocation to reproduction (including low seedling recruitment), and relatively stable habitats. The mid-seasonal pulse of allocation to reproduction in E. scheuchzeri appears viable only in relatively fertile disturbed sites, where the soil nutrient supply is sufficient to support simultaneous allocation to vegetative growth and reproduction.
Seasonal patterns of insect damage to reproductive tissue of the legume Baptisia australis were studied for three years in native tallgrass priairie. Contrasting seasonal patterns of damage were associated with the major species of insect consumers. The moth Grapholitha tristegana (Olethreutidae) and the weevil Tychius sordidus (Curculionidae), which together infested 80-100% of developing fruits (pods), consistently damaged more seeds on average in early than in late maturing pods. But while la
te opening flowers were less subject to attack from moths and weevils, they were more subject to attack from chewing insects, particularly blister beetles (Epicautafabricii, Meloidae), which destroyed > 80% of all flowers and developing young pods (including moth and weevil larval inhabitants). The blister beetles arrived late in the flowering season and fed particularly on young reproductive tissue, allowing larger, older pods that had developed from early opening flowers to escape destruction. The relative abundances and impacts of blister beetles, moths, and weevils varied from year to year. Adding to the uncertainty of reproductive success of the host plant were the large and variable amounts of damage to immature buds inflicted by insects (including the blister beetles and weevil adults) and late killing frosts. Thus, timing of flowering is critical to success in seed production for B. australis. The heavy impacts of insects and weather can result in a very narrow window in time (which shifts from year to year) during which B. australis can flower with any success. The opposing pressures exerted by insects and weather on floral reproductive success may act in concert with other features of the plant's biology to foster the maintenance of considerable diversity in flowering times among individuals in local populations of B. australis.
The orchid Prasophyllum fimbria is pollinated by nectar-feeding native bees and wasps. The pollinia are spatially separated from the viscidium by a stipe so that pollinia can be labelled with coloured histochemical stains without interfering with pollinarium removal. Pollen flow was monitored by following the movement of the coloured pollen in several populations of P. fimbria in Western Australia. Statistical analysis confirmed that pollen labelling did not interfere with pollinarium removal or
subsequent pollination of the labelled flower. Fifty eight labelled pollinaria were removed by vectors from 16 test spikes, with a total of 125 flowers on 47 spikes receiving labelled pollen. An average of 2 flowers received pollen for every pollinium removed but up to 6 flowers received pollen from a single pollinium. No significant differences between mean vector flights and pollen flow distances were detected. On average, geitonogamous transfers only accounted for 22% of all pollinations. This is a simple and inexpensive technique for the direct labelling of pollen with minimal disruption to the pollination system and may have applications in other plant families.
Temporal mechanisms that influence the synchrony of gender expression and the patterns of withinplant pollen flow were examined in Amianthium muscatoxicum. In this species self-incompatible pollinations can clog stigmas, interfere with the growth of outcrossed pollen tubes, and reduce fecundity. The majority of flowers have partial dichogamy: a two-day period of pollen dehiscence and a four-day period of pollen viability are nested in a six-day period of pistil viability. An indeterminate flower
ing sequence among flowers on the same plant and partial dichogamy within flowers help reduce pollen flow within the whole plant. The combined effects at both of these levels should reduce pollen wastage and lower the incidence of stigma clogging by incompatible self pollen.
We tested the hypothesis that generally higher levels of herbivory on bittercress in sun vs in shade, especially by leaf miners, were related to the earlier phenological development of plants in the sun. Naturally-occurring plants in the sun were taller and had longer leaves than did those in the shade during the first three weeks of the growing season, which corresponded with the timing of adult fly oviposition. We divided individual bittercress plants from the sun into three parts: one part wa
s transplanted into willow shade immediately after snow melt; the other two parts were replanted in the sun and one of these was sprayed with insecticide. The transplant experiment had two primary results. First, bittercress transplanted into the shade suffered significantly higher levels of insect damage than either treatment in the sun. Leaf-mining in the shade also increased and equalled that observed in the sun. These results strongly support the phenology hypothesis; higher damage in the sun is due, at least in part, to the earlier development of plants in sun vs in shade early in the season. Second, the ramets with the greatest damage, e.g. the shade treatment, initiated significantly fewer rosettes than did ramets in the other two treatments. The decrease in vegetative reproduction may have been due to the direct effects of increased insect herbivory on these shade plants. This result is particularly interesting because so little information is available on below-ground, vegetative reproductive response to chronic, above-ground foliage loss to native herbs caused by insect herbivores.