Apex Predators Decouple Population Dynamics Between Mesopredators and Their Prey

The mesopredator release hypothesis (MRH) predicts that the removal of apex predators should lead to increased abundance of smaller predators through relaxation of suppressive, top-down effects. However, apex predators’ effects on mesopredators are also likely to be modulated by interactions with human activities and ecosystem productivity. The exploitation ecosystems hypothesis (EEH) predicts that biomass of apex predators will scale with primary productivity but herbivore and mesopredator biomass will remain constant due to top-down control. Here, we take advantage of the manipulation of dingo abundance across Australia’s Dingo Fence to explore the primacy of top-down and bottom-up effects as drivers of feral cat abundance. Using field data collected across the Dingo Fence, we test the predictions generated by MRH and EEH that cat populations should be bottom-up controlled by prey abundance (a proxy for primary productivity) where top-down control exerted by dingoes was weak but not where it was strong. We examined dingo and cat scats to provide mechanistic support for the idea that dingoes control cats through killing and exploitative competition. Overall, cats were more abundant where dingoes were rare. Cat abundance was correlated positively with prey abundance where dingoes were rare but was not correlated with prey abundance where dingoes were common. Cat remains were present in 1% of dingo scats, and dietary overlap between cats and dingoes was high (0.75–0.82). Our study provides evidence that top-down control exerted by apex predators can decouple population dynamics between mesopredators and their prey and thus have primacy over bottom-up effects.