PS 39-187
Dominance controls diversity: Plant community responses to grazing from a worldwide meta-level analysis

Tuesday, August 11, 2015
Exhibit Hall, Baltimore Convention Center
Sally E. Koerner, Nicolas School for the Environment, Duke University, Durham, NC
Melinda D. Smith, Graduate Degree Program in Ecology, Colorado State University, Ft. Collins, CO
Deron E. Burkepile, Department of Biology, Florida International University, North Miami, FL
The Grazing Exclosure Consortium - GEC, A global group of scientists conducting grazing exclosure experiments

Human alterations of historic disturbance regimes are pervasive in their impacts on patterns of plant diversity, community organization, and ecosystem function. This is especially true in disturbance–dependent ecosystems such as grasslands where loss of large herbivores is common. Responses of herbaceous plant communities to grazing vary widely in grasslands, with grazing having positive, negative, or no effect on diversity. The prevailing theory is that contrasting responses to grazing (or the loss of grazing) occur primarily because of differences in productivity driven by moisture availability; however, not all grasslands conform to this theory. Many studies have examined direct effects of grazing on plant species richness, often evoking grazing effects on dominance as an explanation of their results without explicit tests of this mechanism. Our main objective was to determine how grazing impacts plant richness via its effects on plant dominance and how these patterns vary across productivity gradients. To do so, we conducted a meta-level analysis (based on raw species composition data) comparing grazing exclosure experiments in which large herbivores were excluded for more than 4 years from 94 sites around the world. 


These 94 sites span a wide range of locations as well as large gradients in mean annual precipitation (MAP). The MAP at the 15 North America sites ranges from 190 - 1270 mm. The MAP at the 12 South America sites ranges from 199 - 1495 mm. The MAP at the 41 Eurasia site ranges from 45 - 1380 mm. The MAP at the 12 Africa sites ranges from 196 - 1272 mm. The MAP at the 14 Australia sites ranges from 272 - 960 mm. In general, we found that the effects of grazing on dominance vary across the rainfall gradient. Dominance is negatively impacted by grazing on the wetter end of the precipitation gradient, while dominance increases under grazing on the xeric end of the gradient. Importantly, we also found that the effect of grazing on dominance is strongly related to the effect of grazing on richness. Essentially, richness increases as dominance decreases. This pattern held across all continents. These results demonstrate for the first time across large precipitation gradients and on multiple continents that the mechanism driving grazing effects on richness is dominance.