COS 112-5 - Pollen diet breadth varies with scale in three native bee species

Abstract: Bees’ pollen diet breadth is an important functional trait, affecting bee species’ responses to environmental change, the contributions of bee populations to the stability of plant-pollinator communities, and individual bee foragers’ patterns of pollen transfer among flowers. Most studies that measure bee diet breadth do so at only one scale: either locally, relative to other bee species within a single community, or regionally, using museum records collected across a wide region. However, in pollen-generalist bee species, realized diet breadth (ie, the breadth of pollen host plants a bee actually uses in a particular context) can itself vary with scale. Individual bees can specialize on different plant taxa on different foraging trips, such that diet breadth is broader at the population scale than at the foraging trip scale. Similarly, different bee populations can collect pollen from different plant taxa in different places, such that diet breadth is broader regionally, across many populations, than within any given population. As a result, bee species that are generalized at one scale may function as specialists at others, and differences between bee species in diet breadth at one scale may not hold at other scales. Because bees’ diet breadth is rarely studied across spatial and biological scales, though, these issues remain under-explored.

We identified pollen in the pollen loads (i.e., scopal loads; the pollen the bee had collected to feed to her offspring) of 346 female bee specimens of three native bee species, which we collected from 13 study sites spanning a 600 km region of the upper midwestern US. Using a combination of accumulation curves and multivariate methods, we quantified how the pollen diet breadth of each species changed with scale. For all bee species, individual scopal loads were dominated by only 1-2 plant taxa. Pollen diet breadth was 1.5- to 4.5-fold higher at the population (i.e., site) level than at the individual level, and 2- to 4-fold higher at the regional (i.e., across site) level than at the population level. Bee species differed substantially in their diet breadth regionally, though their diet breadth was similar at the population and individual scales on average. This study provides a rare empirical characterization of the biological scaling of bee diet breadth.