Pollinator foraging across dynamic resource landscapes
Given widespread declines in managed and native pollinator communities and increasing global reliance on pollinator-dependent crops, there is an acute need to develop a mechanistic understanding of native pollinator population dynamics and foraging biology. Bumble bees (Bombus spp) are one of the most effective native pollinators, and are often the first to be extirpated in human-altered habitats, yet we know little about what landscapes promote their nesting and foraging biology. In this study, we investigate the impacts of local and landscape-level floral resources, specifically during peak bloom and low bloom periods, on colony density and foraging patterns for the common yellow-faced bumblebee, Bombus vosnesenskii. Field work was conducted across two resource periods, peak bloom (April-May) and low bloom (June-July) across three sites in northern California. In each site, we collected ~100 bees per resource period, sampled across a 3km transect, and we also measured local and landscape floral resource density and diversity. We used a microsatellite-based population genetics approach to determine the impacts of floral resource levels on nest densities and foraging patterns across sites and time periods.
Our findings demonstrate that bumble bee foraging is more plastic and extensive than previously believed, and depends on both local and landscape floral resource levels. Specifically, we show that bumble bees forage further in pursuit of high density resource patches and where landscape-scale floral resources are more homogenous. Foraging distances were dramatically different between bloom periods, regardless of colony identity. Further, we find substantial evidence of colony turn-over between the resource periods, with fewer than 20% of colonies present in both time periods. Overall, this study demonstrates that local and landscape floral resource levels have major impacts on bumble bee foraging patterns and colony persistence.