ORGANIZATION OF A PLANT-POLLINATOR COMMUNITY IN A SEASONAL HABITAT (BEES, SOCIALITY, FORAGING).

Abstract

The foraging behavior of native solitary and primitively social bees was analyzed by identifying scopal pollen loads. In all species individual bees specialized on one pollen type during single foraging bouts. Generalized foraging behavior at the species level may result from switching pollens on sequential foraging bouts in individuals or from the individuals of a colony simultaneously gathering different pollens. Foraging behavior at the species level had a bimodal distribution, indicating a functional division between specialists and generalists. Though approximately 40% of the generalist species switched pollen preferences between years, no specialist species switched preferences between years. Generalist species have longer seasonal activity periods than specialists. All specialists were found in families (Andrenidae, Colletidae, Megachilidae) or subfamilies (Dufoureinae) in which most species are known to be strictly solitary. Only generalists were found in the subfamily Halictinae which has both social and solitary species. Seasonal variability in flower abundance and phenology was related to foraging preferences of bees. Solitary and primitively social bees, that are univoltine and cannot easily track between-year variation in resources, preferred species with simple flowers and low variability in flower abundance. Bumblebees, with greater behavioral flexibility than solitary bees, used the more abundant and variable flowers when they are available. Foraging behaviors observed in solitary and primitively social bees may result from selection to minimize uncertainty where floral resources are variable and unpredictable between years. The persistence of different foraging behaviors and social behaviors in a bee community may be maintained by the complementary costs and benefits of each behavior. Generalists have greater flexibility in responding to temporal variation, but this flexibility is obtained at the expense of less efficient use of individual floral resources. Specialists do not switch resources and may therefore have greater foraging efficiency, but they will be at a disadvantage when there is high year-to-year variability. Social species can retain both flexibility and efficiency if individual colony members specialize on different resources. However, social bees require a longer period to produce reproductives than do solitary bees, and may have lowered fecundity if the blooming season is unusually short.