Experimental analysis of the impact of foxes on freshwater turtle populations

Common demographic parameters such as survival and fecundity are regularly estimated in freshwater turtles, but large-scale manipulative and comparative experiments are rare. The resilience of a population to stage-specific predation is dependent on density-dependent inversity or compensation. Some species are also more susceptible to predation because of their life-history strategies. Nest predation by introduced red foxes (Vulpes vulpes) on Australian freshwater turtles is a major source of mortality, but its full impact has yet to be evaluated. Freshwater turtles are long lived, and a population decline resulting from nest predation by foxes may take a long time to become evident. We evaluated the impact of foxes on the population growth and patterns of survival of two widely distributed Australian freshwater turtles in the Murray River, where nest predation rates have previously been reported at above 95%. We estimated densities and survivorship of Emydura macquarii and Chelodina expansa from the nest to adult stage. We then conducted a fox removal program (before-after control-impact design) to determine the impact of foxes on populations of these species. We found a contrast in life-history strategies between the two sympatric species. C. expansa was five times less abundant than E. macquarii. A type III survivorship curve described the life history of E. macquarii because nest predation and adult survival rates were >0.9. Densities of C. expansa nests were low and situated away from shore Consequently, nest predation rates were half those of E. macquarii. Projection matrices predicted that E. macquarii populations may be declining slowly but are likely to respond to changes in adult survival, which has the greatest elasticity value. Foxes destroyed a proportion of nesting adult females each year; however, there appears to be no density-dependent compensation in adult survival, and foxes potentially regulate these populations. E. macquarii populations had low productivity and relied on a standing crop of turtles for population maintenance, whereas C. expansa had relatively more juveniles in the population and was less reliant on adult survival. Despite its vulnerable status, C. expansa is relatively more stable than E. macquarii, primarily because there were proportionally more juveniles in the population. Management options focusing on reducing adult mortality in both species will be most effective.