Global Vegetation Restoration Limited by Herbivory, Study Reveals

Excluding herbivores – or reintroducing their predators – may aid restoration efforts in many locations, suggests a new meta-analysis of more than 600 global studies. According to the analysis, herbivores at restoration sites reduced vegetation abundance by 89%, on average, a larger effect than they had at relatively undisturbed sites. Herbivores also suppressed plant diversity at these locations. Vegetation is a primary foundation of most ecosystems. However, in many, it has been dramatically degraded, contributing to the loss of biodiversity and ecosystem services worldwide. Restoring vegetation to degraded areas through active planting or encouraging natural regeneration is becoming a common strategy. However, such efforts aren't always successful. Plant abundance and diversity often do not fully recover to "pristine" conditions, even after decades of management. Previous studies have suggested that the limited recovery at degraded sites may be caused by an underappreciated control of vegetation by herbivores, or indirectly by the loss of predators that control herbivore populations. Nevertheless, the processes that contribute to the success or failure of vegetation restoration aren't fully understood, particularly at the global level. To address this, Changlin Xu and colleagues performed a metanalysis of the results of 2,594 experimental tests published across 610 research articles to better understand how herbivory affects restoration success worldwide. Using the dataset, Xu et al. compared the effects of herbivores on relatively undisturbed terrestrial and aquatic sites to herbivores' effects on sites where restoration of lost vegetation was being encouraged. Xu et al. found that, in contrast to undisturbed sites, herbivory on restoration sites had an overall negative effect on plant abundance and diversity, particularly at sites where restoration was actively promoted through planting. The negative effects were pronounced in regions with higher temperatures and lower precipitation. Thus, managing herbivory may be a promising strategy for enhancing vegetation restoration efforts. According to Xu et al.'s synthesis, excluding herbivores at restoration sites increased vegetation abundance by an average of 93 and 158% at natural regeneration and planted restoration sites, respectively, and introducing predators increased abundance by 138 and 372% at natural regeneration and planted restoration sites, respectively. In a related Perspective, Nacho Villar discusses the study and its implications for managing restoration efforts in greater detail.