Saving a Species – Conservation Ecology of the Endangered Western Saw-shelled Turtle (Myuchelys Bellii)

Abstract

<p>The Anthropocene era has ushered in unprecedented biodiversity loss and ecological disruption, placing numerous species, including many turtles, at risk. Turtles, ancient reptiles with a fossil record dating back over 220 million years, now face an existential crisis due to anthropogenic pressures. Conservation efforts have struggled to reverse the decline of turtle populations, with over half recognised as threatened. Turtles occupy vital ecosystem niches, and their decline poses threats to biodiversity and ecosystem stability across the globe. In Australia, the introduced European red fox (<i>Vulpes vulpes</i>) pose a significant threat to the persistence of freshwater turtles. Foxes raid high proportions of nests and are likely limiting juvenile recruitment. Traditional lethal fox control methods have shown limited effectiveness at reducing nest predation rates and can pose risks to non-target species. Effective alternative conservation strategies are urgently needed to bolster juvenile numbers in Australian freshwater turtle populations. One species of conservation concern is the western saw-shelled turtle (<i>Myuchelys bellii</i>), an endangered freshwater species with a limited distribution. Populations are characterised by a predominance of mature individuals and sustained low levels of juvenile recruitment. The predation of nests by invasive foxes has been identified as a key threat to <i>M. bellii</i> populations and my PhD research focuses on the implementation and evaluation of strategies to mitigate this threat.</p> <p>I evaluated the effectiveness of customised electric fences, in combination with individual nest protection, for shielding <i>M. bellii</i> nests from fox predation (Chapter 2), I compared the numbers of raided and intact turtle nests found in paired fenced treatment and unfenced control areas. I also individually protected all intact nests found in both area types with wire mesh or steel cages. The total numbers of nests found in treatment and control areas did not significantly differ, but significantly more intact nests were found in treatment areas and significantly more raided nests in control areas. The fences were occasionally damaged by livestock, wildlife, and flooding, rendering them inoperative for varying periods of time. Despite these breaks in functionality, nests inside the fences were raided on only a few occasions. My study demonstrates that electric fences can provide an effective method of protecting entire nesting areas from depredation by foxes.</p> <p>Despite the high efficacy of the in-situ nest protection strategies, logistical challenges in safeguarding sufficient nests, alongside threats from other predators and environmental extremes, can compromise the hatching success of eggs and reduce the number of <i>M. bellii</i> hatchlings that enter the waterways. Artificial incubation of eggs and the release of hatchlings into the wild is a common ex-situ strategy used to try and boost endangered turtle populations. I investigated a range of incubation temperatures to establish an optimal temperature for maximum hatching success of <i>M. bellii</i> eggs (Chapter 3). Eggs were kept at constant temperatures (27°C, 28°C, and 29°C) to assess the impact on incubation period, hatchling dimensions, and residual yolk. Incubation time decreased with higher temperatures. </p> <p>I examined egg and hatchling morphology, finding a correlation with maternal size and mass. A constant 27°C incubation temperature produced the highest hatching success and smallest external residual yolk. In this study I also developed an incubation protocol which resulted in 97% hatching success for <i>M. bellii</i>. This research outcome optimises artificial incubation for <i>M. bellii</i> and serves as a crucial guide for freshwater turtle conservation efforts. </p> <p>Little is known about the movement, behaviour, or survival of hatchling <i>M. bellii</i> in the wild. To address this gap and inform conservation actions, I monitored 39 <i>M. bellii</i> hatchlings using VHF microtransmitters during their first two weeks in the wild (Chapter 4). On release, the hatchlings dispersed in both upstream and downstream directions and daily movements averaged between 47–62 metres. Over the course of the study, the maximum cumulative distance moved by an individual hatchling was 2008 m. The hatchlings used grassy rivulets to traverse between pools and their movements were influenced by water temperature and water level, with higher water levels correlating with increased downstream movements. Hatchlings were predominantly located among vegetation at the water's edge, accounting for 99% of observations. Hatchlings showed a preference for shallow shoals and sections of stream with a mid-stream depth < 2 m. Furthermore, hatchlings were more often found in areas dominated by sedge compared to its proportional availability, while avoiding bare banks and open water. The hatchlings demonstrated a diurnal movement pattern and survival rates exceeded 90% during tracking. The soft-release of hatchlings had no discernible benefit in comparison to the hardrelease of hatchlings. My findings suggest that releasing hatchlings into preferred microhabitats could bolster conservation efforts for <i>M. bellii</i>. Additionally, my study emphasises the importance of vegetative cover along stream edges in providing shelter for hatchling <i>M. bellii</i>. </p> <p>There are critical gaps in our knowledge of <i>M. bellii</i> growth rates, age at size, and longevity. This information is pivotal for effective design and evaluation of conservation management actions. I analysed data from a long-term <i>M. bellii</i> capture-recapture study spanning more than 19 years and incorporating growth increments from 465 individual turtles with recapture intervals of at least one year (Chapter 5). I boosted the representation of juveniles in our analysis by including growth increment data retrospectively derived from scute annuli measurements and by including juveniles of unknown sex in models for both females and males. I used the Fabens modification of the von Bertalanffy growth model to refine estimates of age at maturity for <i>M. bellii</i> and show that the species matures at a younger age than previously estimated —approximately 11 years for females and 8 years for males. My study confirms rapid growth in <i>M. bellii</i> juveniles, with growth rate declining post-maturation. The refined growth models from this study will aid in population viability assessments, planning for future conservation actions, and the evaluation of past conservation efforts for <i>M. bellii</i>.</p> <p>In response to the urgent need for effective conservation strategies, my study focuses on evidencebased actions to reinforce juvenile numbers in <i>M. bellii</i> populations. This body of work provides novel insights into <i>M. bellii</i> ecology and practical recommendations for conserving this endangered species.</p>