Wild Vs. Domestic Ungulate Ecosystem Impacts: Understanding Functional Differences Requires Greater Focus on Mechanisms

Ungulates play vital roles in ecological systems, shaping plant biomass and diversity via herbivory and impacting soil properties through trampling and nutrient deposition. As ungulate communities fluctuate across the globe, the extent to which wild ungulates and domestic livestock can play similar ecological roles is an increasingly vital - and fraught - question. Here, we synthesized the literature directly comparing wild and domestic ungulate effects on above- and belowground ecosystem responses to assess the direction and relative strength of species’ impacts within shared environments. We then investigated the intrinsic and extrinsic mechanisms researchers identified as driving differences in ecosystem responses to wild and domestic ungulates. Overall, our synthetic review revealed that surprisingly few studies directly compare the effects of wild and domestic ungulates, and even fewer explicitly consider the mechanisms underlying observed outcomes. We found that wild and domestic ungulate effects on plant and soil variables are overwhelmingly similar in kind, differing in intensity rather than direction, with domestic ungulates exhibiting stronger effects on ecosystem responses. Specifically, livestock appear to reduce plant biomass and cover more than wild species, but wild ungulates exhibit more positive effects on plant diversity. Diet and stocking density were by far the most frequently referenced mechanisms explaining differences between wild and domestic ungulates, and other mechanisms (e.g. behavior, movement, veterinary treatments) were rarely considered, let alone tested explicitly. Thus, more intentional study of the intrinsic and extrinsic factors underlying ungulate effects on ecosystems, and particularly on belowground processes, is necessary for a more complete understanding of the functional interchangeability - or irreplaceability - of wild and domestic ungulates in a rapidly changing world.