Floral resources, body size, and surrounding landscape influence bee community assemblages in oak-savannah fragments

1. Fragmentation of natural habitats due to urban development is predicted to have negative impacts on species diversity. The surrounding landscape (or matrix') of urban or semi-natural habitats can sometimes support biodiversity, but the amount of support will depend on species-specific traits, and on the resources available in the fragment and the matrix. Using data on bees collected from 19 oak-savannah fragments, the question of whether bee communities differ when fragments are embedded in different landscapes (Douglas-fir forest vs. urban residential neighbourhoods) was investigated, and also whether these differences could be attributed to species-specific traits of bees (e.g. body size, specialization) and/or within-fragment floral resources. No differences were found in overall richness or abundance of bees, but there were distinct differences in plant and bee community composition between matrix types. Common wood-nesters and late-flying, small-bodied bees tended to be found in urban-associated fragments, which also had a lower availability of within-fragment floral resources. Forest-associated fragments, on the other hand, had a greater density and richness of early-flowering native plant species, and supported a higher abundance of large-bodied bee species. Bumble bee abundance, in particular, increased with increasing proportion of forest cover in the surrounding landscape. Large-bodied bees appear to respond to increased availability of within-fragment floral resources, but it was also hypothesised that nesting and floral resources in matrix habitat drive the differences in bee community assemblages.