Extant native and alien herbivores and carnivores retain substantial capacity to restore late-Quaternary trophic structure

Ming Ni, Evan Fricke, Erick Lundgren, Mathias Mistretta Pires, Jens-Christian Svenning

Global ecosystem restoration seeks to recover ecological functions and trophic integrity. However, late-Quaternary extinctions and range contractions of large mammals have simplified modern ecosystems. The extent to which trophic structure can be rebuilt by surviving species remains unclear. We assess the capacity of extant native and already-introduced alien large herbivores and carnivores to restore herbivore diversity, functional potential, and food-web structure across North and South America, Europe, and Australia. Using species distribution models, trait-based functional metrics, and probabilistic predator–prey networks, we find that extant native and alien herbivores occupy limited, largely non-overlapping portions of their climatically suitable ranges. Expanding native herbivores to potential ranges could reduce herbivore diversity deficits by ~38% on average; range expansion of native and alien herbivores restores ~50% of lost herbivore diversity, with aliens contributing complementary grazing functions where native faunas are depauperated. Recovery of total herbivore biomass is nonetheless constrained by megaherbivore loss. Predator distributions strongly shape trophic structures. Only about half of alien herbivores currently co-occur with suitable predators, but carnivore expansion to potential ranges could raise this to ~90%, and recover nearly one-third of pre-extinction trophic links. Even without carnivore recovery, partial, mesoherbivore-dominated assemblages restore more functional potential and trophic connectivity than persistently defaunated systems. Together, these results show that extant native and alien herbivores and carnivores retain substantial capacity to rebuild trophic structure, with larger-bodied herbivores providing disproportionate functional gains and carnivore recovery maximizing regulation and food-web connectivity.