Océane Bartholomée

Abstract Wild bees and hoverflies are key pollinators, but their populations are declining in Europe. As the monitoring of each species would be arduous to implement, a cost-effective solution could be a shift from species level monitoring to an evaluation of the trends of functional groups. However, the lack of centralized trait databases hampers progress, as data are often dispersed across literature, limited to some countries or remain as expert knowledge.

1 Introduction Pollinator declines threaten ecosystem functioning, as animal pollination is crucial for the reproductive success of most flowering plants (Ollerton et al. 2011). A major driver of recent pollinator declines, including bees, is human-driven changes in habitat availability and quality, resulting in loss of foraging resources and nesting sites (Potts et al. 2016; Potts et al. 2010).

INTRODUCTION Understanding what determines species' ecological niches is important for providing insights into the mechanisms that drive community assemblies and can help us predict how community composition will respond to environmental changes (Laughlin et al., 2015; Powney et al., 2014; Suding & Goldstein, 2008).