Effects of mammalian herbivore declines on plant communities: observations and experiments in an African savanna

Abstract

1. Herbivores influence the structure and composition of terrestrial plant communities. However, responses of plant communities to herbivory are variable and depend on environmental conditions, herbivore identity and herbivore abundance. As anthropogenic impacts continue to drive large declines in wild herbivores, understanding the context dependence of herbivore impacts on plant communities becomes increasingly important.2. Exclosure experiments are frequently used to assess how ecosystems reorganize in the face of large wild herbivore defaunation. Yet in many landscapes, declines in large wildlife are often accompanied by other anthropogenic activities, especially land conversion to livestock production. In such cases, exclosure experiments may not reflect typical outcomes of human-driven extirpations of wild herbivores.3. Here, we examine how plant community responses to changes in the identity and abundance of large herbivores interact with abiotic factors (rainfall and soil properties). We also explore how effects of wild herbivores on plant communities differ between large-scale herbivore exclosures and landscape sites where anthropogenic activity has caused wildlife declines, often accompanied by livestock increases.4. Abiotic context modulated the responses of plant communities to herbivore declines with stronger effect sizes in lower-productivity environments. Also, shifts in plant community structure, composition and species richness following wildlife declines differed considerably between exclosure experiments and landscape sites in which wild herbivores had declined and were often replaced by livestock. Plant communities in low wildlife landscape sites were distinct in both composition and physical structure from both exclosure and control sites in experiments. The power of environmental (soil and rainfall) gradients in influencing plant response to herbivores was also greatly dampened or absent in the landscape sites. One likely explanation for these observed differences is the compensatory effect of livestock associated with the depression or extirpation of wildlife.5. Synthesis. Our results emphasize the importance of abiotic environmental heterogeneity in modulating the effects of mammalian herbivory on plant communities and the importance of such covariation in understanding effects of wild herbivore declines. They also suggest caution when extrapolating results from exclosure experiments to predict the consequences of defaunation as it proceeds in the Anthropocene.

Declines in large wildlife increase landscape-level prevalence of rodent-borne disease in Africa

“…Although the effects of susceptible host regulation have been experimentally documented in plant systems (50) and, for zoonotic diseases, have been suggested to result from fragmentation (24), this study is, to our knowledge, the first experimental demonstration of the phenomenon for a rodent-borne zoonosis. In this case, host regulation may be occurring due to both direct (competition) and indirect (vegetation structural changes) effects of changes in abundance of large vertebrates (35,51) rather than via changes in rodent predators (47), the interacting species more often considered (14,38).…”

The spatial distribution of African savannah herbivores: species associations and habitat occupancy in a landscape context

“…What has been more convincing is evidence that the elimination of large-bodied herbivores leads to complex and sometimes unpredictable trophic cascades in savannahs [4,76]. Notably, members of the large mammalian herbivore community appear to lack functional redundancy [77], suggesting that losing individual species will have unique and largely unpredictable effects on communities, capable of cascading from large mammals, to rodents, snakes and ultimately parasites that harbour wildlife disease (e.g.…”

Context‐dependent effects of large‐wildlife declines on small‐mammal communities in central Kenya

“…Agricultural intensity was quantified by the percentage of agricultural crops in all pin drops; lowwildlife agricultural sites all had .10% cover of agricultural crop and lower than median levels of wildlife activity (detailed in Wildlife and livestock activity). Details of vegetation sampling are provided in Young et al (2013).…”