D3151: Exploring the bimodal latitudinal diversity gradient in bees: Are tropical bees more frequently social, above-ground nesters?

 Many taxa peak in diversity at low, tropical latitudes. However, taxonomists have long observed that bees (Hymenoptera : Apoidea) seem to be an exception to this trend, displaying higher diversity in temperate, xeric habitats. Several hypotheses have been proposed for bees’ low diversity in the tropics, often relating to the interaction between tropical environments, nesting biology, and sociality. The development of sociality may be facilitated in the tropics, where long breeding seasons support the production of multiple broods and cooperative brood care, and where abundant but patchily distributed floral resources may benefit generalist social foragers. Additionally, high humidity in the tropics might favor above-ground nesting to avoid spoiling of pollen provisions in below-ground nests. To investigate the distribution of these life history traits, we mapped the proportion of social and above-ground nesting bees across the Americas and used bee occurrence and climatic data to assess whether sociality and nesting biology were correlated with environmental variables. Proportion of above-ground nesting bee species was highest in Central America and tropical South America, and social bees dominated in Greenland and northern Canada (due to cold-hardy Bombus species) and in tropical South America. However, after controlling for phylogeny we found no correlations between sociality, nesting biology, and climatic variables, suggesting that the distribution of bee diversity is not driven by environmental constraints on sociality and nesting biology.