Livestock grazing and “overabundance” of large wild herbivores in forested areas have long been perceived as conflicting with silviculture and forest conservation. But certain kinds of herbivory in forests can help to maintain habitat values.
The main aim of this systematic review was to examine how forest vegetation and invertebrates are affected by manipulation of the grazing/browsing pressure by livestock or wild ungulates. The ultimate purpose was to investigate whether such manipulation is useful for conserving or restoring biodiversity in forest set-asides.
Most of the 144 studies included in the review had been conducted in North America, Europe or Australia/New Zealand.
The review found negative responses to herbivory in the abundance of understorey vegetation as a whole, woody understorey and bryophytes, and also in the species richness of woody understorey vegetation. On the other hand, the richness of forbs and bryophytes responded positively. Several effects depended on ungulate origins: 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 duration and intensity of herbivory had few significant effects on vegetation. Among invertebrates the study found negative responses to herbivory in the abundance of lepidopterans and spiders, but no significant effects on species richness
The evidence confirms that manipulation of herbivory strongly influences tree regeneration and the abundance, diversity and composition of understorey vegetation. But the authors also idientify gaps in current knowledge: there are 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.