Shifting dominance from native C4 grasses to non-native C3 grasses shows negative effects on community diversity

Grasslands consist of a matrix of a few dominant grasses which structure the environment, affecting the composition and abundance of the sub-dominant species that create the majority of the diversity.  Typically, the tall grass prairies are dominated by one to three native C4 warm season grasses (i.e. Andropogon gerardii, Schizachyrium scoparium, and Sorghastrum nutans).  However, Poa pratensis and other non-native C3 grasses are increasing in abundance and in some areas taking over as the dominant grass species.  This study examines how the shift in the identity of the dominant grasses from native to non-native species influences the diversity of the subdominant plant community.  This study examined a long term data set of 21 abandoned agricultural fields at Cedar Creek Natural History Area.  Each field contains 100 1.0 X 0.5m plots (N=2100).  Plant cover data was collected at the plot level for 6 years (1983, 1989, 1994, 1997, 2002, and 2006).  We used three separate measures of diversity; species richness (the number of species present), species evenness (how equally abundant all the species are), and the Shannon-Weaver diversity index (H), which combines species richness and species evenness into one quantitative variable.  P. pratensis and Agropyron repens (another non-native C3 grass) are significantly and negatively correlated with the diversity of these grasslands, with species richness dropping from about 7 to 4 species.  In contrast, A. gerardii and S. scoparium are either significantly and positively correlated with diversity, with species richness increasing from about 5 to 7 species, or have no significant relationship.