Worldwide, herpetofauna are in decline and are increasingly of concern for conservation organizations as climate change, habitat loss, emerging infectious diseases, invasive species, and human exploitation all impact populations. Turtles in particular are an imperilled group, with more than half of all living species being considered vulnerable or worse by the International Union for the Conservation of Nature (IUCN). Turtles are thus considered high priority for conservation efforts around the globe. In Australia, many of the >20 species of native turtles are under threat. Several Australian species are endemic to small ranges, with some restricted to single river catchments, adding urgency to the necessity of their protection. Bell's turtle (Myuchelys bellii), "yiwaang" in Nganyaywana or "yiwanga" in Dhanggati, is a large freshwater turtle species endemic to some rivers in the New England Tablelands of New South Wales and Queensland. Like many Australian turtle species, the Bell's turtle is under threat, with nest raiding by invasive red foxes (Vulpes vulpes) of chief concern due to its negative impacts on recruitment. Other threats include potential competition with the Murray River turtle (Emydura macquarii), which is expanding its range into Bell's turtle habitat, and cataract-like eye abnormalities in some Bell's turtle populations. These threats, causing particularly low recruitment and limited species' range, have prompted the IUCN to list the Bell's turtle as endangered.
The aim of this thesis was to develop and test new conservation methods for protecting turtle nests against fox depredation, identify the species of turtle eggshells by eggshell microstructure, and to investigate the potential threats posed by interspecific competition and disease. Two nest protection methods were trialled in an attempt to curtail nest depredation rates: the use of large nesting refuge structures, and ultrasonic animal repellent devices. Results were unfortunately inconclusive. While nesting female turtles showed some apparent interest in the refuge structures, favoured nesting habitat is within the flood zones of streams for this species, and refuge structures were frequently inundated and damaged by flooding. Surprisingly, no foxes were recorded on the sites chosen to test the ultrasonic repellent devices, both before or after activation of the devices, so their utility at repelling foxes specifically remains uncertain. However, other mammal species that can hear in the ultrasonic range did not show any detectable aversion to the repellent devices, inferring that this method would be an ineffective deterrent to foxes. Neither nest protection method were therefore considered to be successful.
Turtle nests are often raided by predators before they hatch, and determining the turtle species that laid the nest can be challenging if there are multiple species nesting in a location. To test whether ootaxonomy (diagnosis of the provenance of eggs) was possible using microstructural features of eggshells, eggshell fragments from four native turtle species (Bell's turtle, Murray River turtle, eastern long-necked turtle (Chelodina longicollis), and Bellinger River turtle (M. georgesi) were scanned with a scanning electron microscope (SEM), and microstructural features of the eggshells measured and compared. Central plaque size emerged as the most diagnostic feature, with Eastern long-necked turtle not having any visible pores, Murray River turtle having small pores, and both Myuchelys species having larger pores. Further refinement of sampling protocols are required, but these initial results suggest that SEM is a promising tool for ootaxonomy in turtles. A dichotomous key for the three species found in New England is proposed.
The potential for competition between Bell's turtles and Murray River turtles was analysed by comparing the size and condition between Bell's turtles that were sympatric or allopatric with Murray River turtles. Sympatric adult Bell's turtles, particularly females, were on-average significantly smaller and had lower body mass than allopatric turtles. As mature, breeding females are of vital importance to population persistence in turtles, further investigations into the interactions of Bell's turtles and Murray River turtles are recommended.
Finally, a landscape level analysis of the cataract-like eye disease was conducted. Abiotic factors were modelled and the most-predictive models identified through model selection. Mean annual solar radiation emerged as the strongest predictor, albeit with low effect size: turtles captured in higher-radiation areas had a higher probability of developing cataracts than turtles in lower-radiation areas. Further avenues to establish the link between eye abnormalities and radiation are suggested. Most concerning, while females with clinical signs were recaptured in later years with status either unchanged or without abnormalities (implying that females recover from the condition), no males captured with clinical signs were ever re-captured. While a skewed sex ratio in the area where the clinical signs are most prevalent could explain these data, more worryingly sex-biased mortality might also be occurring.
The research outlined in this dissertation provides valuable information for establishing policy and best practices for Bell's turtle conservation. These results were achieved in challenging conditions as the study area went through major drought and bushfire periods, followed by research restrictions related to the global pandemic that impacted the entire study. Nonetheless, many of the lessons learned here are likely applicable to other freshwater turtle species. Conservation programs have limited resources, and the null results of the tested nest protection methods would caution organizations from diverting valuable resources towards ineffective strategies. By contrast, the promising results of SEM ootaxonomy show that the method shows promise worth pursuing. Finally, the large-scale modelling of competition and eye disease presence provides avenues for future research efforts to proceed with clear objectives, namely to better study the interspecific interactions between Bell's turtles and Murray River turtles, and to investigate the link between local radiation levels and eye abnormalities in turtles.