Bumble bees show trait-dependent vulnerability to landscape simplification

Abstract

Agricultural intensification has resulted in large-scale loss of bee pollinators, but while some species have been negatively affected others seem to endure changed conditions. It has been suggested that certain morphological, ecological and life-history traits make some species more vulnerable to landscape changes. Information on which traits make species vulnerable and why may aid conservation of declining species. We performed a comprehensive analysis of how multiple traits related to diet breadth, movement and nesting habits moderate vulnerability of bumble bees to landscape simplification. We surveyed bumble bees in flower-rich non-crop habitats in either complex landscapes (with small crop fields bordered by non-crop habitats), or simple landscapes (with larger fields and therefore less non-crop habitats). We analysed if landscape type interacted with colony size, queen emergence date, colony life-cycle length, nesting habitat, thorax width, proboscis length or variability in thorax and proboscis, to explain bumble bee abundances. Workers and males of species with above-ground nests, small sized colonies and long colony cycle were relatively less abundant in simple compared to in complex landscapes. Simple landscapes hosted fewer males of late emerging species and species with highly variable proboscis length. This suggests that both nesting habitat and spatio-temporal availability of food resources act as ecological filters for bumble bees. Colony size correlated with nesting habitat and queen emergence when correcting for phylogenetic correlations, suggesting that landscape simplification acts through effects on combinations of traits. Our results have consequences for conservation by suggesting that declining bumble bee species can be supported by providing adequate nesting habitats and preferred plant species throughout the season and within short distance from nesting habitats, to allow utilisation also by species having colonies with few workers.