Trait-based mechanistic approach highlights global patterns and losses of herbivore biomass functional diversity

Mammalian herbivores strongly shape Earth system processes. A lack of global dynamic models of mammal populations limits our understanding of their ecological role at large scales and the consequences of their extinctions. We developed a mechanistic global model of terrestrial herbivore populations simulated with 37 functional groups defined by analyzing eco-physiological traits of all extant herbivores (n = 2599). We coupled this model with a global vegetation model to predict herbivores biomass maximum potential (pre-industrial) and at present. Natural ecosystems could have sustained a wild herbivore wet biomass of 330 Mt, comprised of 192 Mt by large (body mass > 5 kg) and 138 Mt by small species. Anthropogenic activity reduced large herbivores biomass by 57%, estimated now at 82 Mt; consequently, small herbivores now dominate global herbivore biomass with 98 Mt (-29%). Losses vary greatly across climatic zones and functional groups, suggesting that size is not the only discriminant feature of biomass decline. Actual evapotranspiration is the most important driver of total, large, and small herbivore biomass and explains 64%, 59%, and 49% of its variation, respectively. Energy and water dependency is high for large herbivores, whose biomass is greater in hot and wet areas, challenging the notion that large herbivore biomass peaks in savannas only. Outside Africa and the Tropics, potential-biomass hotspots occur in areas today dominated by humans; this could undermine the recovery of larger species in certain areas. These herbivore biomass estimates provide a quantitative benchmark for setting conservation and rewilding goals at large spatial scales. The herbivore model and functional classification create new opportunities to integrate mammals into Earth System science and models.