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Influence of Habitat and Predation on Population Dynamics of the Freshwater Turtle Myuchelys Georgesi

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Influence of Habitat and Predation on Population Dynamics of the Freshwater Turtle Myuchelys Georgesi Herpetologica, 69(1), 2013, 46–57 Ó 2013 by The Herpetologists’ League, Inc. INFLUENCE OF HABITAT AND PREDATION ON POPULATION DYNAMICS OF THE FRESHWATER TURTLE MYUCHELYS GEORGESI 1,2,4 1,3 SEAN J. BLAMIRES AND RICKY-JOHN SPENCER 1School of Biological Sciences A08, University of Sydney, NSW 2006, Australia. 2Department of Life Science, Tunghai University, 181 Section 3, Taichung-kan Road, Taichung City, Taiwan 407-04, R.O.C. 3School of Science and Health, Wildlife and Aquatic Ecology Group (Native and Pest Animal Unit), University of Western Sydney, Locked Bag 1797, Penrith NSW 2751, Australia ABSTRACT: Demographic models identify whether animals are vulnerable to local extirpation, but including all ecological parameters across life history stages may be impeded by practical difficulties. When processes acting on certain life stages cannot be measured, extrapolations are often made. A previous study documented that the range of the turtle Myuchelys georgesi is restricted to the Bellinger River, New South Wales, Australia, and its population is stable. We assessed whether M. georgesi selects certain habitats by comparing their distribution among different water holes. We assessed the threat of catfish predation by examining the stomach contents of catfish specimens. We then evaluated whether threats to M. georgesi were likely to have been underestimated by extending our previous demographic model. We did this by revising the previous estimates of adult, juvenile, and hatchling survivorship under hypothetical variations in water hole use and in the presence or absence of catfish predators. We found that M. georgesi preferentially uses moderate to deep water holes. We also found that although catfish 250À400 mm consume hatchling or juvenile turtles, those . 400 mm do to a greater extent. By making observations of catfish in the Bellinger River and incorporating their presence into our model, we found catfish presence to influence juvenile, but not adult, water hole use. Our reassessment of k suggests that it may have been previously underestimated and that the threat to M. georgesi may be greater than we thought as the population is sensitive to variations in water hole depth and exposure of juveniles to predators. Events that alter key habitats and expose turtles to fish predators across the river should, accordingly, be evaluated further so they can be accounted for when managing the river. Key words: Catfish; Demographic modeling; Habitat selection; Hatchling predation; Myuchelys georgesi; River turtle THE HABITATS that animals choose to occupy habitats, however, are rarely incorporated into provide a variety of resources. For example, population viability analyses (but see Lomoli- animals may use specific habitats because they no and Creighton, 1996; Fryxell, 2001; Pringle provide refuges from predators (Morris, 2003; et al., 2003). Baker and Sheaves, 2009). Other factors such The widespread degradation of freshwater as the availability of food, nest sites, and inter- ecosystems renders their management a high and intraspecific interactions may also influ- priority. Freshwater turtles are considered ence how animals respond to changes in the indicators of the quality of freshwater ecosys- environment and the type of habitats they use tems, but many populations of freshwater (Riechert and Gillespie, 1986; Manly et al., turtle are currently threatened with extinction 2002; Morris, 2003, 2005). Understanding due to habitat loss or introduced predators habitat use by rare animals enables biologists (Mitchell and Klemmens, 2000; Spencer and and managers to assess the vulnerability of Thompson, 2005; Browne and Hecnar, 2007). animal populations at specific locations (Pri- Stage-based population models are useful for mack, 1998; Browne and Hecnar, 2007; Edge identifying the influences of threatening et al., 2009), and to identify features in the processes across turtle life-history stages ecosystem integral to their survival (Clements (Heppell et al., 1996; Chaloupka, 2002; et al., 2006; Newton and Herman, 2009). Spencer and Thompson, 2005; Peckham et Specific information on habitat selection and al., 2011), although they have limitations specific vital rates within chosen, or other, (Taylor, 1995). One limitation is that robust analysis depends on information about the 4 CORRESPONDENCE: e-mail, [email protected] survivorship of a population being equally 46 March 2013] HERPETOLOGICA 47 available across all life-history stages (Cha- decline over 20 generations but its restricted loupka, 2002; Spencer and Thompson, 2005). range renders it vulnerable to factors that alter This may not be possible if certain stages are its demographic structure, such as the intro- more difficult to sample than others. To duction of foxes (Blamires et al., 2005). In mitigate such limitations, population modelers making this assessment, however, few free- may use extrapolative methods (Taylor, 1995; swimming hatchling turtles were captured, so Boyce, 2002). Detailed examinations of the hatchling capture data were incorporated into consequences of using extrapolative methods estimates of juvenile survivorship. Additional- are, however, seldom done. ly, the influence of habitat use across life Population models usually predict that the stages was not considered. Thus, it is possible stability of turtle populations is sensitive to that the assessment underestimated the ex- changes in the survivorship of adults (Cha- tinction threat and may have misidentified loupka, 2002; Blamires et al., 2005; Spencer threatening processes. Accordingly, we used and Thompson, 2005; Enneson and Litzgus, previously published information (Blamires et 2008). Information about survivorship across al., 2005; Spencer et al., 2007; Georges et al., all life stages, however, is often not available 2011) to reassess the population dynamics of (Chaloupka, 2002; Enneson and Litzgus, M. georgesi in the Bellinger River taking into 2008). Survivorship at the hatchling or juve- account its habitat use, which we assessed nile stage is notoriously difficult to assess in using capture–mark–recapture data at water turtles because of extremely low encounter holes with classified depths. We examined the rates. Researchers may thus apply one of five stomachs of catfish specimens to establish if extrapolative techniques when hatchling or they are potentially a significant predator of juvenile survivorship cannot be directly de- turtles. We also counted the number and termined: (1) incorporation of the hatchling or estimatedthesizeofcatfishseenwhile juvenile stage with the egg stage (Blamires et sampling turtles in the Bellinger River to al., 2005); (2) estimation of survivorship at the confirm that they are found in the same hatchling or juvenile stage from other demo- locations as turtles. graphic parameters, such as adult survival or We used the above information to first growth (Pike et al., 2008; Mogollones et al., determine whether water hole position or 2010); (3) estimation of hatchling or juvenile depth, used as a proxy of habitat selection, and survivorship on the basis of that of a predator presence influenced survivorship of phylogenetically or ecologically analogous turtles (stratified by life stage). We then surrogate (Mitchell, 1988; Blamires et al., developed a revised population viability model 2005; Pike et al., 2008); (4) estimation of for M. georgesi to further explore the conse- hatchling or juvenile survivorship on the basis quences of habitat selection and predator of assessment of the ecological factors that presence on population stability. influence survivorship, e.g., the presence of predators; or (5) extrapolation of survivorship MATERIALS AND METHODS from a measurable population growth param- We collected M. georgesi by snorkel diving eter (Enneson and Litzgus, 2008). or hoop trapping along a 30-km stretch of the The freshwater turtle Myuchelys georgesi Bellinger River upstream from Thora, New (previously called Elseya georgesi by Cann, South Wales (Fig. 1), during annual or 1998 and Blamires et al., 2005, and Elseya biannual trips lasting 5À21 d to the site latisternum by King and Heatwole, 1994a,b) between 1988–1991 and 2000–2008 (see King has a range confined entirely to a 30-km and Heatwole, 1994a,b; Blamires et al., 2005; stretch of the Bellinger River and a few Spencer et al., 2007; Georges et al., 2011). isolated pockets of the nearby Orara River Individual turtles were marked by a combi- (Cann, 1998) in New South Wales, Australia nation of three notches in the marginal scutes (1528470E, 308260Sto1528300E, 308270S, to be readily identified when recaptured datum ¼ AMG66; Fig. 1). A recent population (Blamires et al., 2005). Straight-line carapace assessment using life table analysis found that and plastron lengths and widths were mea- the M. georgesi population is not likely to sured on each turtle caught using calipers. 48 HERPETOLOGICA [Vol. 69, No. 1 FIG. 1.—Location of the Bellinger River, showing the 30-km stretch along which Myuchelys georgesi is found and the divisions between the regions we designated as upstream, midstream, and downstream in our analyses. The numbers (1– 16) represent the 16 water holes where . 95% of the marked M. georgesi were captured. Redrawn from www. environment.gov.au/ieo/bellinger/maplg.htm Each turtle was weighed (61g)onan 2005). The capture location for each turtle electronic balance and sex identified (males was recorded as belonging to one of 21 water identified as turtles . 115 mm plastron length holes that were identified on a topographic with obviously
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