Evidence that dingoes limit abundance of a mesopredator in eastern Australian forests

1. Aggressive behaviour of top predators may have strong effects on the distribution and abundance of mesopredator species. Such interactions between predator species can reduce the intensity of predation on vulnerable prey. Suppression of mesopredators by top predators is a potentially important process that could protect small prey species from unsustainable predation. 2. There is some evidence that in Australia, the dingo Canis lupus suppresses populations of the red fox Vulpes vulpes. This interaction could be significant to biodiversity conservation because while dingoes have been in Australia for several thousand years and coexist with a wide range of small mammals, the fox is a recent arrival which has caused declines and extinctions, and continues to threaten many prey species. 3. However the strength of the effect of dingoes on foxes is unclear, and some published data have been interpreted as demonstrating no relationship between abundance of the two species. These data come from forested habitats in eastern Australia, and may suggest that negative relationships of dingoes and foxes do not occur in complex habitats. 4. We re-analyse published data on fox vs. wild dog (i.e. dingoes plus, potentially, feral dogs and hybrids) abundance in eastern forests. These data reveal a triangular relationship of fox to wild dog density: when wild dogs are abundant, foxes are consistently rare, while when wild dogs are rare, foxes may be abundant but are not always so. This suggests that the abundance of wild dogs sets an upper limit on the abundance of foxes, but does not fully determine fox abundance. 5. Standard regression and correlation methods are not appropriate for analysing such triangular relationships. We apply two statistical methods that can be used to characterize the edges of data distributions, and use these to demonstrate a negative relationship of maximum fox abundance to the abundance of wild dogs. 6. Synthesis and applications. Our analysis adds to evidence that dingoes may have negative effects on red foxes in a wide range of habitats, and therefore, that dingoes may be significant to conservation of mammal biodiversity in Australia. It also illustrates problems and solutions in the statistical analysis of abundance of one species as a function of the abundance of another species with which it has a strong interaction.