Species diversity, primary productivity, and photosynthetic pathway in grasslands of eastern Pennsylvania

We examined the relationship between primary productivity and diversity for grassland, meadow, and savanna (GMS) vegetation in northeastern Pennsylvania, USA, where the landscape is primarily forests, agriculture, and urban or suburban development. We surveyed primary productivity, plant species diversity, invertebrate order diversity, and avian abundance and species diversity for 14 grasslands and open areas that were actively managed and three that apparently occurred naturally. Four grasslands were dominated by warm-season grasses with the C-4 photosynthetic pathway, nine were dominated by C-3 grasses, forbs, and shrubs, and four had a mixture of both types. We found hump-shaped relationships for plant species richness and bird species richness as a function of productivity but not for invertebrate order richness. Richness of invertebrates and birds increased linearly with the number sampled at each site. The number of plant species declined with increasing proportion of warm-season grasses, as did the number of invertebrate orders collected in sweep samples, avian abundance, and avian species richness. Multiple regression showed that both landscape and vegetative characteristics influenced the bird community. Bird species richness increased with the area of the sites, the ratio of area to edge, and the distance to the nearest open area. There was no evidence for the influence of the relative abundance of native plant species on bird or invertebrate richness. GMS restoration of farmlands and mine lands with plantings of warm-season grasses, while possibly beneficial for some forms of wildlife, seems to be less effective in maintaining the diversity of invertebrates and birds than a mixture of warm-season grasses, cool-season grasses, forbs, and shrubs.