Abstract
Recent studies have reported on dramatic cases of aerial insect population declines by focusing on the measure of the total biomass of caught insects. However, there is currently no consensus about how biomass patterns among sites and habitats might consistently capture the subtleties of changes in aerial insect community structure. Here, we investigated the relationship between the total biomass of wild bees collected using pan traps in urban, agricultural, and semi-natural habitats on one hand, and a spectrum of biodiversity metrics on the other hand, particularly species richness (SR), alpha diversity, functional diversity (FD) and three different forms of phylogenetic diversity (PD). Our results indicate that although biomass is significantly and highly correlated with the abundance of wild bees, it is generally significantly but only moderately and non-linearly correlated to the various facets of wild bee diversity among habitats. By contrast, we also found that all three measures of PD used are consistent across habitats, suggesting that a taxonomic hierarchy based on Linnaean classification could be used as a proxy for the measurement of PD in wild bees, particularly in other well-studied areas such as Western Europe where a multi-gene molecular phylogeny is unavailable as yet. Collectively, our results illustrate the clear limitations of biodiversity monitoring through measures of trapped insects biomass. We advocate for more robust measures of biodiversity trends in wild bees, requiring both standardized surveys, and the identification of caught specimens down to the species level to capture the subtleties of species, traits-based and phylogeny-based community changes across habitats or time. Scaling out this approach is an essential prerequisite for more global conservation planning tailored to the ecological requirements of the targeted insect species.
Highlights
A couple of long-term field studies have recently reported on dra matic declines of aerial insect abundance, both in temperate (Hallmann et al, 2017) and in tropical ecosystems (Lister and Garcia, 2018), raising the alarm about the impacts of such large-scale losses of biodiversity and the associated ecosystem services underlying human well-being (Pimm et al, 1995; Daily, 1997)
These results suggest that the combination of behavioural/ecological traits used in this study for the computation of functional diversity (FD) is highly redundant with measures of phylogenetic diversity (PD), perhaps because (i) of the high number of character states used, and (ii) the overall tendency of wild bees to exhibit rela tively high levels of phylogenetic conservatism in their fundamental behavioural/ecological traits (Michener, 2007), including in Europe (Michez et al, 2019)
Our results suggest that the turnover among these habitat categories is responsible for the non-linear changes in the correlation between in sect biomass and species/functional/phylogenetic biodiversity metrics: the unpredictable species replacement between habitats and the afore mentioned unpredictable trends in biomass imply that there is an equal likelihood of turnover between small and large species of bees
Summary
A couple of long-term field studies have recently reported on dra matic declines of aerial insect abundance, both in temperate (Hallmann et al, 2017) and in tropical ecosystems (Lister and Garcia, 2018), raising the alarm about the impacts of such large-scale losses of biodiversity and the associated ecosystem services underlying human well-being (Pimm et al, 1995; Daily, 1997). The review by Sanchez-Bayo and Wyckhuys (2019a) in particular stirred a vivid wave of criticisms, with international teams of researchers questioning the validity of their conclusions after highlighting similar methodological limitations, including the non-adoption of literature search standards (Komonen et al, 2019; Mupepele et al, 2019; Simmons et al, 2019; Thomas et al, 2019; but see Sanchez-Bayo and Wyckhuys, 2019b)
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