AVAILABLE ENERGY AND SPECIES DIVERSITY: THEORY AND EXPERIMENTS WITH BEES (COMMUNITIES, FLOWERS, FORAGING).

Abstract

A general biogeographic theory of insular species diversity, species-energy theory, is produced by replacing area in species-area models with a measure of available energy. Islands with more available energy support larger populations, which have lower extinction rates. Given similar immigration rates, islands with greater available energy are therefore expected to support greater equilibrium numbers of species. Assuming that total population size is proportional to energy supply, and that species-abundance distributions are lognormal and of similar form, the species-energy relationship is approximated by S = kEᶻ. Species-energy theory explains 70-80% of the variation in species number of angiosperms and of birds on such widely varying islands as Greenland and Jamaica. The effects of energy on the structure of a subalpine bee community in Colorado were investigated. As available nector declined, during mornings and over the season, foraging profitability for Bombus appositus (Hymenoptera: Apidae) decreased. This change was manifested by increased foraging trip durations: nector loads did not change. Total colony profits increased as colonies grew over the season, but profit relative to colony size declined, due to reduced profitability of individual foraging trips. These results support the hypothesis of resource limitation in this species. Assemblages of bees foraging on patches of flowers showed effects of energy availability on species composition and dynamics. Bees foraging in enriched patches had lower departure rates than bees in control patches, and, consequently, increased equilibrium numbers of individuals and species present per patch. Both behavioral and mechanical factors influenced departure rates. A species-specific arrival-departure rate model satisfactorily described the foraging assemblages and their response to enrichment. Experiments performed on 2 species of flowers with different corolla tube lengths demonstrated that bee species respond differently to resources of unequal availability, necessitating a species-level approach. Analogies with island systems are discussed. Energy is important to communities in general and bees in particular on a variety of scales. By implication, human resource diversion from natural ecosystems may have profound impacts on global diversity and extinction.