Grazing impacts on plant community composition vary greatly in magnitude globally

Sally E. Koerner, Department of Biology, University of North Carolina Greensboro, Greensboro, NC, Melinda D. Smith and Kate Wilkins, Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO, Meghan Avolio and Ava Hoffman, Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, Lauren M. Porensky, Rangeland Resources and Systems Research Unit, USDA-ARS, Fort Collins, CO, Kimberly Komatsu, Smithsonian Environmental Research Center, Edgewater, MD, Deron E. Burkepile, Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, Santa Barbara, CA, Elisabeth S. Bakker, Department of Plant-Animal Interactions, Netherlands Institute of Ecology, Maarssen, Netherlands, Lauren E. Baur, Scott Collins and Timothy J. Ohlert, Department of Biology, University of New Mexico, Albuquerque, NM, Elisabeth J. Forrestel, Viticulture and Enology, University of California, Davis, Davis, CA, Daniel Griffith, Forest Ecosystems and Society, Oregon State University, Corvallis, OR, Niall P. Hanan, Plant and Environmental Sciences, New Mexico State University, Las Cruces, NM, Jinsong Wang, University of Chinese Academy of Sciences, Beijing, China, Kevin Wilcox, Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY, Laura Yahdjian, IFEVA-CONICET, Facultad de Agronomia, Universidad de Buenos Aires, Buenos Aires, Argentina

Background/Question/Methods
Consumers are critical for controlling structure and function of plant communities in many of the world’s ecosystems, particularly in grasslands. Grazers impact plant community richness via changes in dominance – when grazers reduce dominance, plant community richness increases. However, dominance and richness are just two metrics to measure community diversity and structure. Here, we explore how plant community composition (measured with multivariate dissimilarity metrics) changes in response to grazing by synthesizing compositional differences in response to exclusion of large herbivores for more than 3 years from 252 sites around the world. Specifically, we address three questions: (1) Does grazing affect plant community composition globally? (2) Do site characteristics (e.g., MAP, MAT) influence the amount of compositional difference between grazed and ungrazed plots? And (3) when the community does differ in response to grazing, what mechanisms of community difference (e.g., re-ordering, richness, identity, and evenness) cause this compositional difference?
Results/Conclusions
The 252 experimental sites span a wide range of locations as well as large gradients in mean annual precipitation (MAP; 45 to 1511 mm). We found that grazing causes differences in plant community composition with a mean of a 40% change in composition (mean=0.4; bound between 0-1) across all 252 sites; however, and perhaps more importantly, we found large variation in the magnitude of differences (0.05-0.90) among sites. Much of this variation was linked to site-level climate characteristics. Warmer, wetter sites, and sites with higher productivity and richness all showed greater community difference with grazing. Not surprisingly, higher intensity grazing also caused greater differences than moderate or low intensity grazing. Differences in community composition were primarily driven by reordering of species including the shift in the identity of the dominant species. Importantly, changes in richness were not a strong driver of community differences; instead, compositional differences were driven by changes in the species identity (i.e., species turnover). These results demonstrate that grazing influences multiple aspects of community composition and structure, and that species richness is relatively insensitive compared to other measures of compositional change.