Livestock grazing and ‘overabundance’ of large wild herbivores in forests have long been perceived as conflicting with the aims of both silviculture and forest conservation; however, herbivory can also help to maintain certain forest habitat values. Management of mammalian herbivory in protected forests can, therefore, be an important tool for biodiversity conservation and restoration. We conducted a full systematic review with meta-analyses to examine how manipulation of the grazing/browsing pressure affects forest vegetation and invertebrates. Our systematic review included studies of experimental manipulation of ungulate herbivory in boreal and temperate forests. Non-intervention or alternative levels of intervention were used as comparators. Relevant outcomes included abundance, diversity and composition of plants and invertebrates, tree regeneration, and performance of target species. 144 studies were included in the review, most from a recent systematic map (1) but with updates based on searches online and bibliographies of existing reviews. Most studies had been conducted in North America, Europe or Australia/New Zealand. The intervention most commonly studied was experimental exclusion (or enclosure) of wild and/or domestic ungulates by fencing. Other studies examined culling of wild ungulates or compared forests grazed by livestock to ungrazed forests. We found negative effects of herbivory on the abundance (cover) of understorey vegetation as a whole, woody understorey and bryophytes, and also on the species richness of woody understorey vegetation. In contrast, herbivory had a positive effect on the richness of forbs and bryophytes. Ungulate origin was a significant effect modifier: Understorey abundance responded negatively to livestock and to ungulates introduced into the wild, but not to native ones. In contrast, understorey species richness responded positively to livestock but not to wild ungulates. The effects on woody understorey abundance and richness became increasingly negative with increasing duration and intensity of herbivory, respectively. Despite a paucity of studies reporting on invertebrates we detected a significant negative effect of herbivory on lepidopteran and spider abundance, but not richness. Our review confirmed that, if used carefully, manipulation of ungulate herbivory can be an effective tool for the management of tree regeneration, understorey vegetation or certain invertebrate groups. Important knowledge gaps included few studies of: boreal areas, long-term herbivory effects, impacts on bryophytes, lichens and invertebrates, and effects of manipulation less radical than total exclusion of ungulates.
1Bernes, C., B.G. Jonsson, K. Junninen, A. Lõhmus, S.E. Macdonald, J. Müller, J. Sandström. 2015. What is the impact of active management on biodiversity in boreal and temperate forests set aside for conservation or restoration? A systematic map. Environmental Evidence 4:25 DOI 10.1186/s13750-015-0050-7