More losses than gains in ground-nesting bees over 60 years of urbanization

Abstract

Along with agricultural intensification, urbanization is often implicated in bee declines worldwide. While the vast majority of studies investigate how bee assemblages respond to urbanization across different spatial scales, very few attempts have used a temporal comparison to evaluate the impact of urbanization. The city of Curitiba in southern Brazil is unique for having long-term data of bees, and thus is a good model for studying urban temporal changes. The main goal of this study is to investigate how a ground-nesting assemblage responded to six decades of urban growth in Curitiba. In this time lapse the city population grew from 140 thousand to almost 2 million inhabitants. We demonstrate a 94% decline in nest abundance, a 35% decline in species richness, and a 24% decline in phylogenetic diversity from 1955 (T1) to 2018 (T2). Temporal beta diversity values between T1 and T2 were very high and most sampling sites were dominated by losses in nest abundance. All sampling sites changed similarly with losses in nesting substrates (i.e. exposed soil) and a substantial increase in surrounding impervious surface. This study exhibits how depletion of nesting substrates combined with increases in urban cover can affect pollinators, a concerning scenario as most bees nest in friable soil and thus are strongly susceptible to urbanization. We emphasize that urban planning policies to protect ground-nesting bees are essential to preserve the important ecological service of pollination in cities.