• 106th AnGrazing impacts on plant community composition vary greatly in magnitude globally

KOERNER, SALLY E.; AVOLIO MEGHAN; MELINDA SMITH; BAKKER, JONATHAN D.; WILKINS; LAURA YAHDJIAN

Virtual

Congreso; ?106th Annual Meeting of the Ecological Society of America, Virtual.; 2021

Ecological Society of America

Background/Question/MethodsConsumers are critical for controlling structure and function of plant communities in many of theworld’s ecosystems, particularly in grasslands. Grazers impact plant community richness viachanges in dominance – when grazers reduce dominance, plant community richness increases.However, dominance and richness are just two metrics to measure community diversity andstructure. Here, we explore how plant community composition (measured with multivariatedissimilarity metrics) changes in response to grazing by synthesizing compositional differences inresponse 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 compositionglobally? (2) Do site characteristics (e.g., MAP, MAT) influence the amount of compositionaldifference between grazed and ungrazed plots? And (3) when the community does differ inresponse to grazing, what mechanisms of community difference (e.g., re-ordering, richness,identity, and evenness) cause this compositional difference?Results/ConclusionsThe 252 experimental sites span a wide range of locations as well as large gradients in meanannual precipitation (MAP; 45 to 1511 mm). We found that grazing causes differences in plantcommunity composition with a mean of a 40% change in composition (mean=0.4; boundbetween 0-1) across all 252 sites; however, and perhaps more importantly, we found largevariation in the magnitude of differences (0.05-0.90) among sites. Much of this variation waslinked to site-level climate characteristics. Warmer, wetter sites, and sites with higher productivityand richness all showed greater community difference with grazing. Not surprisingly, higherintensity grazing also caused greater differences than moderate or low intensity grazing.Differences in community composition were primarily driven by reordering of species includingthe shift in the identity of the dominant species. Importantly, changes in richness were not astrong driver of community differences; instead, compositional differences were driven bychanges in the species identity (i.e., species turnover). These results demonstrate that grazinginfluences multiple aspects of community composition and structure, and that species richness isrelatively insensitive compared to other measures of compositional change.