Comparisons of wild and domestic ungulate zoogeochemical impacts rarely evaluate mechanisms underlying functional dissimilarity

Ungulates play vital roles in ecological systems, shaping plant biomass and diversity via herbivory and impacting soil properties through zoogeochemical cycling. As ungulate communities fluctuate worldwide, 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 plants and soil nutrients. We investigated the intrinsic and extrinsic mechanisms researchers identified as driving similarities and differences in ecosystem responses to wild and domestic ungulates, and qualitatively assessed the direction and relative strength of species' impacts within shared environments. We found that surprisingly few studies directly compare the effects of wild and domestic ungulates, and even fewer explicitly consider the mechanisms underlying observed outcomes. Qualitative synthesis suggested that wild and domestic ungulate effects on ecological variables differed in intensity rather than direction, with domestic ungulates exhibiting greater effects on ecosystem responses, perhaps due to their greater densities. More intentional study of the intrinsic and extrinsic factors underlying ungulate effects on ecosystems, and particularly on below-ground processes, is necessary for a more complete understanding of the functional interchangeability - or irreplaceability - of wild and domestic ungulates in a rapidly changing world.