Manipulating ungulate herbivory in temperate and boreal forests: effects on vegetation and invertebrates. A systematic review

Abstract

Background: Livestock grazing and 'overabundance' of large wild herbivores in forested areas have long been perceived as conflicting with the aims of both silviculture and forest conservation; however, certain kinds of herbivory can help to maintain habitat values in forest ecosystems. Management of mammalian herbivory in protected forests can, therefore, be a critical tool for biodiversity conservation. The primary 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. Methods: We considered studies of manipulated ungulate herbivory in forests anywhere within the boreal and temperate zones, not only in protected areas but also in production forest. 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 focal/target species. Studies were mainly selected from a recent systematic map of the evidence on biodiversity effects of forest management relevant to protected areas. Additional studies were identified through updated searches online and in bibliographies of existing reviews. Relevant studies were critically appraised, and studies with low or unclear validity were excluded from the review. Quantitative outcomes were extracted from 103 articles, and summary effect sizes were derived by meta-analysis. Results: Most of the 144 studies included in the review 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 long grazed by livestock to ungrazed forests. Effects on vegetation and invertebrates were reported in 135 and 23 of the studies, respectively. We 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, whereas 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-exceptions included woody understorey abundance

Pervasive moose browsing in boreal forests alters successional trajectories by severely suppressing keystone species

“…This contrasts with Hegland and Rydgren (2016) who showed an overall decline in dwarf shrub abundance with increasing red deer density. 6) in accordance with a previous analysis by Speed et al (2014), but in contrast to a recent meta-analysis (Bernes et al 2018). In our study, the two berry-producing and highly dominant dwarf shrub species, Vaccinium myrtillus and V. vitis-idaea, became increasingly associated with the open plots with time ( Fig.…”

“…Finally, based on a recent systematic review (Bernes et al 2018), we expect a negative effect of herbivore exclusion on bryophyte diversity and a weaker negative effect on overall understory plant diversity. We predict that excluding moose would favor all tree species except spruce and increase the dominance from deciduous trees.…”

“…However, a recent meta-analysis showed that ungulates can have idiosyncratic effects on the diversity within different groups of plants that can balance out to no overall effect on diversity (Bernes et al 2018). Previous studies have found remarkably varying results of cervid herbivory effects on vascular plant diversity (see Introduction).…”

“…The effect of wild cervids on understory plant diversity remains an unresolved issue, with studies finding effects that are positive (Chollet et al 2013, Boulanger et al 2017, neutral (Speed et al 2014), negative (Rooney andWaller 2003, Beguin et al 2011), and non-linear (Hegland et al 2013). A recent systematic review by Bernes et al (2018) also highlights the unique responses within different functional groups of plants. Such community responses are always confounded by different plant species having unique responses to herbivory with some species being favored by herbivores and others not (Tremblay et al 2006, Hegland and.…”

“…In any case, the mechanisms behind such diversity effects may be direct through cervid-mediated seed dispersal (Albert et al 2015) or grazing on nutrient rich herbs, or indirect through altered growing conditions such as micro-climate, light regimes, or soil properties. Bryophytes are not eaten by cervids, but have been shown to be positively affected by cervid presence (Chollet et al 2013) and ungulate densities in general (Bernes et al 2018), assumedly due to reduced competition from vascular understory plants or from altered light and moisture conditions associated with a browsing related change in canopy composition.…”