Effects of depleted competitors and prey on hunting, movement, and space-use of African wild dogs

Abstract

Prey depletion is an increasing threat to many of the world's endangered carnivores, by impacting some of the few remaining protected areas critical for species' population persistence. Prey depletion intuitively reduces the density of apex carnivores because their populations are tightly correlated to prey density. Effects of prey depletion on subordinate competitors like African wild dogs are less intuitive, because their populations are strongly limited by their dominant counterparts. Recent research has demonstrated that the effects of prey depletion can impact subordinate carnivores in a similar manner to dominant guild members, with subordinate carnivore populations declining in parallel and not showing benefits of competitive release, despite the low densities of dominant competitors in prey depleted systems. Mechanisms that prevent competitive release and cause population reductions for subordinate carnivores have never been tested, yet are vitally important for conservation and understanding carnivore guild dynamics under an increasingly common threat to carnivore populations. We used long-term spatial, movement, and observational data from African wild dogs to test hypotheses regarding top-down and bottom-up effects of prey depletion, and made inferences in the context of long-term demographic analyses from both ecosystems, to identify the mechanistic causes of reduced wild dog population density and fitness as a result of prey depletion. In prey depleted ecosystems, our results indicate that wild dog movements continue to be affected by long and short-term risk of encountering lions despite their low densities and wild dogs continue to avoid lions, and areas with the highest densities of preferred prey. Prey depletion negatively affects every stage of hunting for wild dogs, and the combination of low prey density and loss of larger bodied ungulates, results in larger annual home-ranges and smaller pack size. These results indicate that prey depletion has negative bottom-up effects on wild dog populations while top-down limitations from dominant competitors remain strong, explaining the lack of competitive release and parallel reduction in density. This work provides new insights for large carnivore guild dynamics in prey depleted ecosystems and is the first to show compounding mechanisms that prevent competitive release for a subordinate competitor when dominant competitors are reduced.