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

Mammalian herbivores play a pivotal role in Earth System processes by
affecting biogeochemical cycles and ecosystem functioning, potentially
leading to significant repercussions on atmosphere-biosphere feedbacks.
Global dynamic models of mammalian populations can improve our
understanding of their ecological role at large scales and the
consequences of their extinctions. However, such models are still lacking
and mammals are poorly integrated in Earth System science. We developed a
mechanistic global model of terrestrial herbivore populations simulated
with 37 functional groups defined through the analysis of
eco-physiological traits across all extant herbivores (2599 species). We
coupled this model with a global vegetation model to predict herbivores’
maximum potential biomass in pre-industrial and at present-day and to
study the environmental drivers explaining the distribution of herbivore
biomass. Present-day biomass was estimated by accounting for anthropogenic
activity causing habitat and range losses. We show that natural ecosystems
could have sustained a potential wild herbivore wet biomass of 330 Mt [95%
CI 245–417], comprised of 193 Mt [95% CI 177–208] by large species (body
mass > 1–10 kg, depending on functional group) and 138 Mt [95% CI
68-209] by small species. We estimate that the remaining present-day large
herbivores biomass is 82 Mt [95% CI 32–133], reduced by 57% due to
anthropogenic activity; consequently, small herbivores currently dominate
global herbivore biomass with 98 Mt (95% CI 91–106, -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. Distribution of modeled and observed large herbivores’
biomass suggested a high dependency on energy and water with more biomass
in hot and wet areas. These results challenge the notion that large
herbivore biomass peaks primarily in ecosystems with intermediate
precipitation levels such as savannas. Outside Africa and the Tropics,
pre-industrial biomass hotspots occur in areas today dominated by humans;
this could undermine the recovery of larger species biomass 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.