Species that are relatively rare account for most of biodiversity and are disproportionately vulnerable to global change, relative to common species. A possible reason is that species with large populations, broad habitat niches, and large geographic ranges must adapt to thrive in a wide variety of environmental conditions (or conversely, that being so adapted allows a species to achieve commonness), and that this flexibility gives common species a higher chance of successfully exploiting and dominating novel anthropogenic habitats. Additionally, the responses of rare species to human land use are poorly known because rarity is difficult to study. We conducted a 3-year field study to sample bee communities in 36 forested, agricultural, and urban landscapes replicated at a regional scale across New Jersey, New York, and Pennsylvania. We classified the bee species we observed into rarity quantiles based on their frequency in an independent data set compiled from museum collections. We used generalized linear models to ask how the richness and abundance of bees in each rarity quantile differed between forested versus anthropogenic landscapes. In addition to examining numerical rarity, we conducted the same analyses using the outcome variables phenological rarity (short flight season length), and geographical rarity (small range size).
Results/Conclusions
We captured 12,635 specimens of 226 native species from our field study, which represents approximately 45% of the native species known to occur in our study region. The three forms of rarity were uncorrelated, and for each we found that richness of rare bee species in agricultural landscapes was approximately half that in forest (pfrequency=0.03; pphenology=0.05; prange size = 0.02). Richness of phenologically rare bee species in urban landscapes was also half that in forest (pphenology=0.01); however, low frequency and small range species in urban landscapes were not less rich than expected from random species subsets. We still found rare species at every anthropogenic site, and almost half of species in the rarest quantiles were collected exclusively from agricultural or urban sites. Anthropogenic habitats may support these bee species by substituting for some open natural habitats that are no longer available, such as floodplains. However, our main results show that preventing forest loss and degradation is an overlooked priority for bee biodiversity conservation. We suggest that a large proportion of rare bee biodiversity in historically forested regions is adapted to conditions in forest habitat, and these species may not find analogous conditions in contemporary anthropogenic landscapes.