Vertebrate herbivores strongly influence the composition of plant communities. Through grazing, trampling, and rooting, herbivores disturb soil, alter plant community structure and succession, and increase invasive species establishment. Grazers can increase species diversity by creating heterogeneity in native vegetation or facilitate the introduction of nonindigenous species. Specifically, terrestrial vertebrate herbivores are important due to their ubiquity, broad diet, large size, and mobility. We tested the enemy release hypothesis: nonnative species lack co-evolved enemies and so native vertebrate herbivores will reduce natives while increasing exotic plant species richness. We also tested the reciprocal hypothesis: nonnative animals reduce exotics with concomitant increases in native richness. Last, we tested individual herbivore species effects on native and exotic plant species richness, with the expectation that species-specific impacts will vary. For our meta-analysis, we used the ISI Web of Science (1965 – present) to collect articles describing 125 exclosure studies that reported plant species richness with means, standard deviation, and sample size. Effect size (Hedges d) was analyzed in MetaWin 2.1 using random categorical structure. A positive or negative effect size indicates an increase or decrease in species richness, respectively, while the value indicates the strength of the biological effect (e.g. d=.2 is weak, d=.5 is moderate, d=.8 is strong).
Results/Conclusions
Native herbivores increased native plant richness (d=.2, p=.02), but did not have a significant impact on nonnative plant richness (d=.3, p=.4). Nonindigenous grazers did not significantly affect native (d=.1, p=.6) or exotic richness (d=.3, p=.6). However, individual herbivore species had significantly different effects on native richness (p=.004), with cattle (d=.2) and moose (d=.4) increasing richness, and hog (d= -.1) and elk (d= -.2) decreasing native plant richness. Herbivore species did not have significantly different effects on nonnative species richness. The enemy release hypothesis is not supported because native animals slightly increased native plant species richness. Additionally, nonnative herbivores did not have significant effects on native or nonnative plant richness. Broad generalizations about the effects of native and exotic grazers on plant species richness are not supported. Instead, species-specific traits such as behavior, feeding habit, size, and seed dispersal may be more useful for understanding the effects of terrestrial vertebrate grazers on plant communities. More studies are needed with larger sample sizes for nonindigenous plant communities, non-grassland ecosystems, and naturally stocked grazer densities. Finally, environmental factors like productivity and succession along with the co-evolutionary history of the grazers and plant communities need closer examination.
