Rabbits killing hares: an invasive mammal modifies native predator-prey dynamics

Invasive species management requires practical evidence of the impacts of introduced species over ecosystem structure and functioning. Theoretical ecology and empirical data support the potential of introduced mammals to drive native species to extinction, indeed the majority of practical evidence comes from insular environments, where conditions may differ from the mainland. We analyzed the effects of an introduced lagomorph, the eastern cottontail Sylvilagus floridanus on two native mammals, the European hare Lepus europaeus and the red fox Vulpes vulpes. We used relative abundances collected over 8 years at 30 protected areas in Italy. A generalized linear mixed model was fit to test various hypotheses about the relationships between cottontails, foxes and climatic conditions over the abundance of native hares. In our model, hare and cottontail abundances did not show a negative relationship and we believe that no direct competition occurs between the two species. However, the relationship between fox and hare abundances, positive when cottontails were scarce, became more and more negative as cottontails increased: this supports the hypothesis that indirect dynamics like apparent competition exists between the two lagomorphs. Climatic conditions, expressed through the North Atlantic Oscillation, did not affect the relationship between cottontail and hare abundances. As the impact of parasites on mammal populations is generally climate dependent, we believe that cottontails do not play a direct role in the cycle of parasites affecting hares. Our results provide a clue that an invasive mammal, the eastern cottontail, is modifying the predator-prey relationship between two native species in a non-insular environment. The existence of such dynamics should lead wildlife managers to account for the effect of introduced species in their decision making, directing control activities on cottontails and not on native foxes.