Population parameters and life-table analysis of two coexisting freshwater turtles: are the Bellinger River turtle populations threatened?

Abstract

Two species of freshwater turtle coexist in the Bellinger River: Elseya georgesi is common but limited to the Bellinger River, whereas Emydura macquarii is widespread but rare in the Bellinger River. The Bellinger River population of E. macquarii has been proposed as a distinct subspecies, so it may be endangered. Survivorship, fecundity, growth, size and age were determined for El. georgesi and the finite rate of increase (λ) was estimated by a life-table analysis using mark–recapture data from surveys between 1988 and 2004. These parameters were compared with those of well studied populations of E. macquarii to assess whether modelling the demographic parameters of El. georgesi could serve as a surrogate for estimating the influences of these demographic parameters on λ in the Bellinger River population of E. macquarii. We estimated that ~4500 El. georgesi inhabit the study area and, despite a size distribution strongly biased towards large individuals, the population is increasing (λ = 1.15) in the best-case scenario, or slightly decreasing (λ = 0.96) in the worst-case scenario. Comparing El. georgesi with E. macquarii from the Bellinger River and elsewhere suggests that E. macquarii grows faster, attains greater maximum size, has a greater clutch size and a higher fecundity than El. georgesi. Hence, El. georgesi does not serve as a good surrogate to determine demographic influences on λ in E. macquarii.