Conservation implications of turtle declines in Australia’s Murray River system

Abstract

Conservation requires rapid action to be effective, which is often difficult because of funding limitations, political constraints, and limited data. Turtles are among the world’s most endangered vertebrate taxa, with almost half of 356 species threatened with extinction. In Australia’s Murray River, nest predation by invasive foxes (Vulpes vulpes) was predicted to drive turtle declines in the 1980s. We assessed populations of the broad-shelled turtle (Chelodina expansa), eastern long-necked turtle (C. longicollis), and Murray River turtle (Emydura macquarii) in the Murray River and some of its associated waterways. Our results suggest that the predicted decline is occurring. All three species are rare in the lower Murray River region, and were undetected in many locations in South Australia. Moreover, E. macquarii had considerable population aging almost everywhere, possibly due to comprehensive nest destruction by foxes. Chelodina longicollis also had population aging at some sites. Sustained low recruitment has potential to lead to collapses as turtles age, which is particularly worrying because it was predicted over 30 years ago and may have already occurred in South Australia. Our results show that turtle declines were not mitigated since that prediction. If the crash continues, a vertebrate guild responsible for considerable nutrient cycling in the aquatic ecosystem will disappear. Our results highlight a worst-case outcome when species declines are predicted, but insufficiently mitigated.

Prey-switching does not protect a generalist turtle from bioenergetic consequences when its preferred food is scarce

“…Our results have implications for turtle and freshwater conservation. Emydura macquarii and C. longicollis are in decline across their range, while C. expansa is listed as endangered [9,46,47]. Much of the Murray-Darling river system is degraded [35], and many wetlands have been lost due to flow regulation by dams [21], or drought [24].…”

“…We calculated Scaled Mass Indices (SMI) following Peig and Green [28]. We calculated separate SMI for each sex because males and females often exhibit different body condition relationships, but were unable to separate sex in C. longicollis because we could not distinguish between sexes [47]. In all calculations, we used straight carapace length as our estimate of "length".…”

“…In all calculations, we used straight carapace length as our estimate of "length". To estimate the mean body lengths of males and females, and the relationships between body mass and length used to calculate SMI [28], we included a broader database of turtle body sizes from the entire southern Murray River basin (27 C. expansa, 247 C. longicollis, 729 E. macquarii; [47]). We then compared SMI across all four wetlands using separate ANOVAs for females and males, within each species (PROC GLM).…”

Scavenging by threatened turtles regulates freshwater ecosystem health during fish kills

“…Turtles have drastically declined in the Murray–Darling Basin 18 , 19 . Up to 100,000 tonnes of turtles were historically present 14 , but in some locations they are now so rare that they are not captured 19 . As turtle populations continue declining, we predict that water quality will worsen, with implications for both the ecosystem and humans.…”

“…In the 1980s, India recognised this issue and reintroduced about 40,000 turtles into the Ganges River to address its pollution problem of decomposing human remains 17 . In the Murray–Darling Basin of south-eastern Australia, in the 1970–1980s turtles were estimated to eat up to 430 tonnes of carrion per day 14 , but all three endemic species are currently threatened 18 , 19 . Since 1976, in parts of their range Chelodina longicollis has declined by > 90%, and Emydura macquarii by 69% 18 .…”