Ungulates increase forest plant species richness to the benefit of non‐forest specialists

Abstract

Large wild ungulates are a major biotic factor shaping plant communities. They influence species abundance and occurrence directly by herbivory and plant dispersal, or indirectly by modifying plant-plant interactions and through soil disturbance. In forest ecosystems, researchers' attention has been mainly focused on deer overabundance. Far less is known about the effects on understory plant dynamics and diversity of wild ungulates where their abundance is maintained at lower levels to mitigate impacts on tree regeneration. We used vegetation data collected over 10 years on 82 pairs of exclosure (excluding ungulates) and control plots located in a nation-wide forest monitoring network (Renecofor). We report the effects of ungulate exclusion on (i) plant species richness and ecological characteristics, (ii) and cover percentage of herbaceous and shrub layers. We also analyzed the response of these variables along gradients of ungulate abundance, based on hunting statistics, for wild boar (Sus scrofa), red deer (Cervus elaphus) and roe deer (Capreolus capreolus). Outside the exclosures, forest ungulates maintained higher species richness in the herbaceous layer (+15%), while the shrub layer was 17% less rich, and the plant communities became more light-demanding. Inside the exclosures, shrub cover increased, often to the benefit of bramble (Rubus fruticosus agg.). Ungulates tend to favour ruderal, hemerobic, epizoochorous and non-forest species. Among plots, the magnitude of vegetation changes was proportional to deer abundance. We conclude that ungulates, through the control of the shrub layer, indirectly increase herbaceous plant species richness by increasing light reaching the ground. However, this increase is detrimental to the peculiarity of forest plant communities and contributes to a landscape-level biotic homogenization. Even at population density levels considered to be harmless for overall plant species richness, ungulates remain a conservation issue for plant community composition.

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

“…Increased diversity is not necessarily seen as beneficial if the cause or side effect of this increase is a homogenization of the flora caused by an increase in generalist species and decrease in specialist species (Boulanger et al 2017, but see also Fløjgaard et al 2018). Variation between forest localities gave by far the strongest explanation of understory plant communities explaining 72% of the variation, and only 2% of the remaining variation was explained by the exclusion treatment.…”

“…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.…”

Unprecedented herbivory threatens rear‐edge populations of Betula in southwestern Eurasia

“…, Boulanger et al. , Fløjgaard et al. ), quantification of fossil dung fungal spores had not been used to assess the historical range of variation of grazing disturbance in temperate and Mediterranean Europe so far.…”

Are ungulates in forests concerns or key species for conservation and biodiversity? Reply to Boulanger et al. (DOI: 10.1111/gcb.13899)

“…Boulanger et al. () investigated the effect of ungulate grazing on forest understory plant community composition using exclosures in forests, mainly plantations, over a 10‐year period. They found increasing richness of herbaceous plant species with grazing, promoting light‐demanding species, while shrub richness decreased.…”

“…Boulanger et al. () assess the ecosystem effect using the current managed state of the forest as a baseline, that is, more light‐demanding species in the forest is a change of state and therefore also a concern. However, for biodiversity conservation, we would advocate for baselines informed by a macroecological and evolutionary perspective.…”