This study examines patterns and causes of variation in the reproductive success of the desert annual Stipa capensis. Three nested scales of variation were analyzed: variation between individuals of the same plot, variation between different plots of the same habitat, and variation between different habitats in the same region. Perturbation experiments (irrigation and neighbors removal) were performed to test the effects of heterogeneity in soil water and neighborhood competition on the magnitud
e of variation in each scale. The results demonstrate that variation of reproductive success was highest within plots, lowest between plots, and moderate between habitats. Soil water heterogeneity contributed to spatial variation in all scales but was most important for differences between habitats. Neighborhood competition increased the variation within plots, but decreased the variation between habitats. The results further demonstrate that water limitation was negatively correlated with the position of the habitat along the run-off/run-on gradient. An opposite trend was obtained for the effect of competition.
Relationships between flowering plants and their pollinators are usually affected by the amount of reward, mainly pollen or nectar, offered to pollinators by flowers, with these amounts usually positively correlated with floral display. The large Oncocyclus iris flowers, despite being the largest flowers in the East Mediterranean flora, are nectarless and have hidden pollen. No pollinators visit the flowers during daytime, and these flowers are pollinated only by night-sheltering solitary male b
ees. These iris flowers are partially or fully dark-colored, suggesting that they gather heat by absorbing solar radiation. Here we test the hypothesis that the dark-colored flowers of the Oncocyclus irises offer heat reward to their male solitary bee pollinators. Floral temperature was higher by 2.5°C than ambient air after sunrise. Solitary male bees emerged earlier after sheltering in Oncocyclus flowers than from other experimental shelter types. Pollination tunnels facing east towards the rising sun hosted more male bees than other aspects. We suggest that floral heat reward can explain the evolution of dark floral colors in Oncocyclus irises, mediated by the pollinators’ behavior.
The vertical inflorescences of the Mediterranean annual Salvia viridis carry many small, colorful flowers, and are frequently terminated by a conspicuous tuft of colorful leaves (‘flag’) that attracts insect visitors. Insects may use the flags as indicators of food rewards in the inflorescences below, as long-distance cues for locating and choosing flowering patches, or both. Clipping of all flags from patches of inflorescences in the field reduced the number of arriving insects, but not the tot
al number of inflorescences and flowers visited by them. The number of flowers visited per inflorescence increased with inflorescence size, and inflorescence and flower visits rates significantly increased with patch size. Six percent of the plants in the study population did not develop any flag during blooming, yet suffered no reduction in seed set as compared to flag-bearing neighboring individuals. Removal of flags from all inflorescences in a patch reduced seed set in comparison with untreated controls, while flag clipping from ten randomly selected inflorescences in a patch did not decrease seed production. These results suggest that flags signal long-distance information to potential pollinators (possibly indicating patch location or size), while flower-related cues may indicate inflorescence quality.
Plants that do not develop flags probably benefit from the flag signals displayed by their neighbors, without bearing the costs of signal production. Greenhouse-grown S. viridis plants allocated a low proportion of their biomass to flags. Plants grown under water stress did not reduce biomass allocation to flags as compared to irrigated controls. Water loss rates of picked flags were lower than those of picked leaves. These findings suggest that the expenses of flag production and maintenance are modest, reducing the selective advantage of individuals that do not carry flags. We discuss additional potential evolutionary mechanisms that may select for flag production.