What is limiting the abundance and richness of wild bees on a reclaimed mine land: floral resources or nest sites?

Sustainability of wild bee populations depends on both floral resources and nest sites.  Floral resources have been considered the main determinant of bee diversity and have been the focus of most bee conservation and habitat enrichment activities.  Nest substrates also can be limiting, however.  Reclaimed mines in Ohio have a paucity of native flower species and woody vegetation.  We predicted that bee communities in these areas would be richest and most abundant near woodland edges or in areas where floral resources had been planted.  Over two years, we sampled bee communities, floral resources, and potential nest substrates in 24 locations at varying distances from forest remnants on a reclaimed mine land.  Artificial nest substrates of drilled boards were provided in half of the sites in year one and all sites in year two to test if increasing nest substrates could improve the abundance and richness of twig-nesting species (Megachilidae).  We found positive and significant effects of floral richness and the availability of natural nest substrates on the overall abundance and richness of wild bees.  The abundance of twig-nesting bees was slightly, but not significantly, higher in sites augmented with nest substrates over two years compared to those that only received nest substrates in one year.  Nest substrate augmentation did not affect the richness and diversity of all bee species combined.  Attempts to improve habitat by augmenting floral resources alone may fall short of promoting the richest, most abundant bee communities if nest sites are also limiting.