Ecological filtering in scrub fragments restructures the taxonomic and functional composition of native bee assemblages

Predicting the long‐term consequences of habitat alteration for the preservation of biodiversity and ecosystem function requires an understanding of how ecological filters drive taxonomic and functional biodiversity loss. Here, we test a set of predictions concerning the role of ecological filters in restructuring native bee assemblages inhabiting fragmented coastal sage scrub ecosystems in southern California, USA . In 2011 and 2012, we collected native bees in scrub habitat belonging to two treatment categories: large natural reserves and small habitat fragments embedded in an urban landscape. We compared bee assemblages in reserve and fragment sites with respect to their taxonomic and functional alpha diversity, beta diversity, assemblage composition, and mean geographical range size estimated via distribution maps compiled for this study from digitized specimen records. We found multiple lines of evidence that ecological filtering drove bee diversity loss in fragments: a disproportionate loss of functional diversity relative to taxonomic diversity, shifts in assemblage composition driven largely by the preferential extirpation of reserve‐associated indicator species, and disproportionate loss of range‐restricted species. However, we found no evidence of taxonomic or functional homogenization across fragment bee assemblages, suggesting that filtering was not sufficiently strong to cause a subset of functional traits (and their associated species) to dominate assemblages in fragments. Our results suggest that ecological filtering altered bee assemblages in habitat fragments, even when such fragments contained well‐preserved native plant assemblages, underscoring the importance of preserving large areas of natural habitat for the conservation of bees (especially range‐restricted taxa) and their associated ecological functions.