Herbivores alter plant biodiversity (species richness) in many of the world’s ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis—that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally.
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While not all raw species abundances are publicly available because of lack of permission from data owners (contact individual data set owners listed in Supplementary Table 1), all data generated and analysed during the current study (site-level richness response to herbivory, site-level Berger–Parker and Simpson’s dominance response to herbivory, site ANPP, and site MAP) are provided in Supplementary Table 2.
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Funding for this synthesis was provided for by USDA AFRI Foundational Conference Grant (award no. 2018-67013-27400). We would like to thank the National Evolutionary Synthesis Center (Grasslands Working Group), the School of Global Environmental Sustainability at Colorado State University and the National Center for Ecological Analysis and Synthesis for hosting working meetings that led to these analyses. We also thank M. Ritchie, D. Augustine and R. Pringle for helpful comments on an earlier version of the manuscript. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Individual sites acknowledge funding support: Kenya Long-term Exclosure Experiment—NFS DEB 12-56004; Jornada—NSF DEB-0618210; Konza Prairie and Kruger National Park—NSF DEB 0841917; Kruger National Park—NSF DEB 1712786.