Competition for Bumblebee Pollinators in Rocky Mountain Plant Communities
Автор антэкологических публикаций
, 1980. V. 61. No. 6. P. 1446–1459 (14).
Meadow plant communities at four sites in the Colorado Rocky Mountains were studied to evaluate the influence of competition for bumblebee pollinators. The niche relationships among plant species were characterized by overlap with respect to species of bumblebee visitors and times of flowering. Species in each community could be divided into two to three guilds based upon the identity of the species' major bumblebee visitor. Only the members of a guild would potentially be in competition with one another for pollinators. The competition hypothesis is that guild members have segregated blooming periods which would minimize competition. The null hypothesis is that blooming periods are dispersed randomly through time. The competition hypothesis was tested by comparing the actual temporal spacing pattern for a guild with random patterns generated by computer simulation. For 10 out of 11 guilds the sequence of blooming periods was more regular than expected (either at the statistically significant level [five cases] or near it [five cases]). I also examined the relationships among guild members using another measure of the competitive effect of one species on another which is more accurate than temporal overlap. This variable, called competitive load, includes information on the overlap, abundance, and floral attractiveness of competitors. It is an indicator of the number of visits a species loses to competitors. In general species were found to lose approximately half the number of visits they could potentially receive during the time they were in flower. All species received approximately the same number of visits despite large differences in floral abundance. Thus the pool of available pollinator visits appears to be rather evenly divided among the species in a community. This regularity in resource partitioning is further inferential evidence of competition for pollinators. Rare species avoid the potentially heavy loss of visitors to more abundant competitors by having flowers which are more attractive to pollinators. Attractiveness appears to be a function of the nectar production rate of a species' flowers. There are some exceptions to these generalizations. Species which can also reproduce vegetatively are less attractive, and lose more visits that expected. The competitive effect that on species experiences because of the presence of another may be the result of a loss of pollinator visits (exploitation) and/or a disruption of conspecific pollen flow (interference). Species may reduce the interference component of competition by spatial isolation. For the one guild that had a random flowering sequence with broad overlap among species, spatial isolation and competition between guild members were positively correlated. Temporal divergence and differential attractiveness are seen to be primarily a means of avoiding exploitation competition.