Wild bees and landcover: bee species' body size does not predict the scale of effect, but bee phenology predicts association with landcover type

Abstract

Habitat is a key aspect of any species' niche and can affect populations at multiple spatial scales. Basic ecology and effective conservation thus require an understanding of which habitats matter and at what scales. Yet, habitat studies are rarely scale-optimized, and what determines the scale(s) at which populations are affected by surrounding habitat (the ‘scale of effect') is poorly understood. In this study, we test the ‘mobility hypothesis,' which predicts that species with larger foraging ranges should have larger scales of effect. The mobility hypothesis is the most popular explanation of what determines species' scales of effect, but empirical support is mixed. We test the mobility hypothesis using wild bee species and, in doing so, also assess landscape-scale habitat associations of 84 bee species. We collected 30 376 specimens of 84 bee species from 165 sites in the northeastern USA and used linear models to determine landcover associations and scales of effect for each species. To test the mobility hypothesis, we asked whether scales of effect varied with two mobility-related traits – body size or sociality, which are the strongest known predictors of bee foraging ranges. Controlling the false discovery rate at 5%, we found 193 significant species–landcover associations across 60 (of 84) species. Scales of effect ranged from 100 to 8000 m (mode = 200 m; median = 1000 m) and, counter to the mobility hypothesis, were not associated with body size or sociality. As a result, we argue that ecologists should reconsider making assumptions about species' scales of effect and should instead explicitly measure scales of effect for their particular study organism and system. Considering the landcover associations themselves, we found these were broadly explained by phenology, with spring-flying bees being associated with forests and summer-flying bees being associated with more open, non-forested habitats.