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

Background/Question/Methods

Open habitats, such as grasslands, meadows, and savannas (GMS), have been declining in Pennsylvania since the beginning of the 20th century. Many animal species associated with such habitats have been declining as well. In eastern Pennsylvania, Both native and restored grasslands can contain warm season grasses with the C4 photosynthetic pathway, cool season grasses (C3 pathway), or a mixture of both along with forbs (C3 pathway). Our objective in this project was to assay plant, bird, and invertebrate diversity at a diverse array of GMS sites to determine which factors influence biodiversity. We hypothesized that grasslands with a mixture of warm season and cool season grasses would support a greater diversity of birds. We selected 17 native and restored grasslands, including those that were dominated by cool season grasses, by warm season grasses, and by mixtures of both types. We estimated aboveground primary productivity by harvesting ten 0.09 m2 quadrats along two transects at each site and sorting the harvested material by species. Plants from each quadrat were dried and weighed. Invertebrates were sampled using sweep nets and pitfall traps. Birds were surveyed by point counts between late May and mid-July when birds were likely resident and breeding. Bird species were identified by song.

Results/Conclusions

We found the highest plant diversity at intermediate levels of primary productivity. Both low productivity sites and high productivity sites had fewer species than sites with intermediate levels of productivity. A least-squares fit of a polynomial equation for number of plant species as a function of primary productivity had a highly significant quadratic term (p <0.01). The number of plant species per quadrat and the number of plant species encountered per grassland showed a humped-shaped relationship with the proportion of primary productivity from C4 grasses. The number of invertebrates collected in sweep samples declined with the increasing proportion of warm season grasses. Multiple regression indicated that both landscape and vegetative characteristics appeared to affect the bird community. Although warm season grasses have been promoted as a means of revegetation, we found that the grasslands most heavily dominated by warm season grasses tended to have lower numbers of species of plants, birds, and invertebrates. To maximize the diversity of plants and birds, grassland sites in eastern Pennsylvania should be restored with a mixture of species, of which no more than 50% should be warm season grasses.