Changes in adult sex ratio in wild bee communities are linked to urbanization

Gordon Fitch,Ivette Perfecto,Maria-Carolina Simao,Paul Glaum,Jill Matthijs,Benjamin Iuliano,Chatura Vaidya

doi:10.1038/s41598-019-39601-8

Abstract

Wild bees are indispensable pollinators, supporting global agricultural yield and angiosperm biodiversity. They are experiencing widespread declines, resulting from multiple interacting factors. The effects of urbanization, a major driver of ecological change, on bee populations are not well understood. Studies examining the aggregate response of wild bee abundance and diversity to urbanization tend to document minor changes. However, the use of aggregate metrics may mask trends in particular functional groups. We surveyed bee communities along an urban-to-rural gradient in SE Michigan, USA, and document a large change in observed sex ratio (OSR) along this gradient. OSR became more male biased as urbanization increased, mainly driven by a decline in medium and large bodied ground-nesting female bees. Nest site preference and body size mediated the effects of urbanization on OSR. Our results suggest that previously documented negative effects of urbanization on ground-nesting bees may underestimate the full impact of urbanization, and highlight the need for improved understanding of sex-based differences in the provision of pollination services by wild bees.

Highlights

Wild bees (Apoidea: Hymenoptera) are critically important both to agricultural production and the maintenance of angiosperm biodiversity[1,2]
The effect of urbanization, as measured by impervious surface cover, on bee observed sex ratio (OSR) was qualitatively similar regardless of the scale at which it was assessed (500 m, 1 km, 1.5 km, or 2 km), but the model assessing urbanization at the 2 km scale had the lowest AIC value (Table 1; ∆AIC = 1.48 for next-best model measuring urbanization at 1.5 km), so this scale was used for subsequent analyses
We document a shift in observed sex ratio (OSR) of ground-nesting bees along an urbanization gradient, with the relative abundance of female bees declining as urbanization increases while the abundance of male ground-nesting bees remained unaffected by urbanization

Summary

Introduction

Wild bees (Apoidea: Hymenoptera) are critically important both to agricultural production and the maintenance of angiosperm biodiversity[1,2]. Evaluating only aggregate abundance and diversity masks trends in particular guilds of bees; most notably, studies have consistently found reduced abundance and/or diversity of ground-nesting bees in urban areas[10,12,13,14,15]. Life history differences between female and male bees seem likely to result in distinct trends in observed sex ratio (OSR) with increasing urbanization[19]. There are non-exclusive mechanisms by which urbanization may drive changes in OSR, explored in greater detail below: (1) sex-specific patterns of movement and dispersal, (2) labile sex ratios and (3) temperature For most of their life cycle, non-parasitic female bees are central-place foragers, collecting nectar and pollen in order to provision their brood; as a result, most foraging occurs close to the nest site[20]. Systematic changes in the ability of foragers to provision their brood along the urban-to-rural gradient, resulting from changes in either the abundance or distribution of suitable floral resources or brood parasitism rates, could result in OSR shifts along an urbanization gradient

Methods

Results

Conclusion