Demographic response of snake-necked turtles correlates with indigenous harvest and feral pig predation in tropical northern Australia

Species that mature late, experience high levels of survival and have long generation times are more vulnerable to chronic increases in mortality than species with higher fecundity and more rapid turnover of generations. Many chelonians have low hatchling survival, slow growth, delayed sexual maturity and high subadult and adult survival. This constrains their ability to respond quickly to increases in adult mortality from harvesting or habitat alteration. In contrast, the northern snake-necked turtle Chelodina rugosa (Ogilby 1890) is fast-growing, early maturing and highly fecund relative to other turtles, and may be resilient to increased mortality. Here we provide correlative evidence spanning six study sites and three field seasons, indicating that C. rugosa is able to compensate demographically to conditions of relatively low subadult and adult survival, caused by pig Sus scrofa (Linnaeus 1758) predation and customary harvesting by humans. Recruitment and age specific fecundity tended to be greater in sites with low adult and subadult survival (and thus reduced densities of large turtles), owing to higher juvenile survival, a smaller size at onset of maturity and faster post-maturity growth. These patterns are consistent with compensatory density-dependent responses, and as such challenge the generality that high subadult and adult survival is crucial for achieving long-term population stability in long-lived vertebrates such as chelonians. We posit that long-lived species with 'fast' recruitment and a capacity for a compensatory demographic response, similar to C. rugosa, may be able to persist in the face of occasional or sustained adult harvest without inevitably threatening population viability.