Increasing warm season grasses does not increase avian species richness in isolated temperate grasslands

A diverse array of early successional habitats are being created in Eastern Deciduous Forests offering a fantastic opportunity to study the drivers of avian biodiversity. We compared avian diversity at fifteen sites in northeastern Pennsylvania (USA) to area and site shape, site context, land use history, disturbance regime, vegetative composition, primary productivity, canopy structure, insect diversity, and food web structure. Average number of avian species per point count location varied more than threefold from 4.0 to 12.9 species per point. Habitat area and the ratios of edge to area appeared to produce an increase in both numbers of birds and numbers of bird species. The distance to the nearest patch of early successional habitat also had a positive effect on numbers of birds and bird species. For the vegetative characteristics, primary productivity was positively correlated with bird numbers and diversity, whereas the proportion of primary productivity from C4 grasses was negatively correlated with bird numbers and diversity. The food web studies showed that warm season grasses are included in the food webs of sites where they are found in abundance. Nevertheless, even when warm season grasses produce most of the biomass on a site, there is substantial use of grasses and forbs with the C3 photosynthetic pathway, which may explain why sites with mostly warm season grasses tend to have lower numbers of species.