Macroecological patterns in mammal abundances provide evidence that an apex predator shapes forest ecosystems by suppressing herbivore and mesopredator abundance

Aim: Apex predators often exert strong, top-down effects on ecosystems, and their removal can result in the dramatic reorganization of ecosystems owing to herbivores and smaller predators becoming the dominant trophic-regulating species. However, field studies designed to understand the influence that apex predators have on ecosystems are hampered by the large spatial and temporal scales required. Here, we use pre-existing datasets to test predictions on the direct and indirect effects of apex predators on mammals and vegetation generated from trophic cascade theory and the mesopredator release hypothesis. Location: Forests of south-eastern Australia. Methods: We applied quantile regression to published data collected at 44 sites during the 1980s and 1990s. We investigated inter-relationships between abundance indices of an apex predator, the dingo (Canis dingo), introduced mesopredators, herbivores, ground-dwelling medium-sized mammals and small mammals, and inter-relationships between habitat complexity and abundances of dingoes and macropods. Quantile regression is well suited for investigating limiting factors of response variables, especially when the observations are made at large temporal and spatial scales. Results: In accordance with trophic cascade theory, dingo abundance was negatively associated with macropod abundance and macropod abundance was negatively associated with habitat complexity; however, dingo abundance was not correlated with habitat complexity. Similarly, in accordance with the mesopredator release hypothesis, dingo abundance was correlated negatively with red fox abundance and red fox abundance was negatively associated with the abundance of ground-dwelling medium-sized mammals, but, dingo abundance was not correlated with the abundance of medium-sized ground-dwelling mammals. Main conclusions: Our results suggest that the direct suppressive effects of the dingo on macropods and foxes occur simultaneously, but that their indirect effects on lower trophic levels are comparatively weak. Our study provides evidence that the effects of apex predators on other species can be exerted across large spatial scales and become manifest on species at multiple trophic levels.