Macroecological perspectives on the functional impacts of late-Quaternary megafauna extinctions and potential restoration through trophic rewilding

Background/Question/Methods: During the last 100,000 years, approximately 50% of terrestrial mammal species with body mass ≥10 kg (megafauna s.l.) have been in lost in a selective extinction event increasingly strongly linked to the global expansion of Homo sapiens. Hereby, current mammal assemblages in much of the world are megafauna-poor and deviate from the norm across the mid to late Cenozoic. There are likely strong functional consequences, but these are poorly understood, as are their consequences for restoration efforts. We address these questions through a macroecological approach, with a focus on large herbivores given their importance for ecosystem structure and functioning in many areas and their strong, but controversial role in conservation and restoration efforts. To this end, we compiled a new global dataset containing habitat, body mass and diet information on all late-Quaternary mammal herbivore species ≥10 kg that existed during the last 130,000 years, using it to 1) compare the functional composition of current herbivore assemblages with that of no-extinction assemblages, and 2) assess the extent to which reestablishment of extirpated herbivores via trophic rewilding efforts have potential to restore ecosystem functioning (assessing the impact of varying restoration baselines: historical (1500 AD), prehistorical Holocene, Late Pleistocene).

Results/Conclusions: We find that roughly 41% of terrestrial ecosystems contain herbivore assemblages that are functionally non-analogous to their no-extinction counterparts. They lack major functional types (generally down-sized and with greater dominance by browsers), and their functional composition is regionally more heterogeneous, suggesting the late-Quaternary extinctions have fundamentally altered the functional composition of herbivore assemblages worldwide. Furthermore, they imply that many parts currently experience herbivory regimes to which the local biota are not evolutionarily adapted. We furthermore find these functional losses could be partially restored by re-establishing extant species within their native ranges, especially for the deeper baselines, but only partially so due to global extinctions (e.g., still strongly reduced assemblage median and maximum body mass). These findings show that without using non-native species as functional replacements or so far unfeasible deextinction, restoration can only achieve partial recovery of herbivore assemblages. Real-world implementation of trophic rewilding furthermore needs to tackle complexities such carnivores, societal acceptability, habitat availability and ongoing climate changes. Nevertheless, we suggest that a fuller consideration of the potential for restoring herbivores towards their pre-extinction typical functional composition would promote self-regulating, biodiverse ecosystems for the large areas needed to overcome the ongoing biodiversity crisis.