Reciprocity in Restoration Ecology: When Might Large Carnivore Reintroduction Restore Ecosystems?

Carnivore reintroduction is often expected to revert community and ecosystem properties to their natural states via risk effects and the direct killing of prey. Because large carnivore extirpation and reintroduction are usually believed to have symmetric and offsetting effects, fulfilling this “assumption of reciprocity” is crucial to realizing the potential of large carnivores to passively restore community structure and ecosystem function. We were unable to find any study in which the assumption of reciprocity was rigorously tested in predator-prey systems featuring large carnivores, their ungulate prey, and primary producers through a comprehensive literature search. We therefore used studies involving (1) the reintroduction of any native apex predator (including but not limited to large mammalian carnivores); and (2) the removal of any introduced apex predator (also including but not limited to large mammalian carnivores) to examine the assumption of reciprocity. Reintroduction of native apex predators did not consistently affect any of four trophic groups (mesopredator, omnivore, herbivore, primary producer) in a positive or negative way, but removal of introduced apex predators consistently increased the abundance and biomass of mesopredators. Further, outcomes of apex predator reintroduction and removal were variable across systems, regardless of system complexity (ranging from single predator-single prey to multiple predator-multiple prey systems). We suggest that the assumption of reciprocity—in which predator extirpation and reintroduction are believed to have consistent, counterbalancing effects—is unsupported by current evidence, and perhaps unrealistic. We discuss potential directions for research that might illuminate when and why the assumption of reciprocity would be valid.