Indirect effects of large mammalian herbivores, while much less studied than those of apex predators, strongly affect communities and ecosystems. The strength of these effects is spatiotemporally variable, and several may be governed in part by primary productivity. However, field and theoretical studies have generated conflicting results and predictions.
We used meta-analysis to characterize the direction and magnitude of herbivore-initiated indirect interactions (HIII), using 67 published studies comprising 456 individual responses. Of these, 41 studies (including 253 responses from 33 locations on 5 continents) were georeferenced to a satellite-derived map of primary productivity. Because predators might alter the impact of herbivores, we conducted a similar analysis to test how large carnivore species richness might affect HIII strength.
Results/Conclusions
Large herbivores generally suppressed the abundance of other consumer species and also tended to reduce consumer richness, although the latter effect was only marginally statistically significant.
Negative (i.e., suppressive, due for example to competition) indirect effects of large herbivores on consumer abundance were weaker in more productive ecosystems. In contrast, positive (facilitative) indirect effect strengths were not correlated with productivity, likely because these were due to a greater variety of mechanisms. Carnivore species richness had no effect on the strength of HIII.
These results will help predict the indirect effects of wildlife declines and irruptions, and how these might be affected by climatically driven shifts in resource abundance. Our findings also contribute to resolving the fundamental problem of ecological contingency with regard to the strength of an under-studied group of species interactions.