Preventing Freshwater Turtle Extinctions

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DISCUSSION Here we show that headstarting could be an effective tool for managing freshwater turtles against multiple threats that affect multiple life history stages. In our models, headstarting from harvest populations is the only tool that eliminates all risks of extinction, while maintaining small increments in population growth. Similar to previous studies (e.g., Heppell et al. 1996; Heppell 1998), small increments in adult mortality have greatest effect on population growth and extinction risk, but if other threats simultaneously affect other life history stages (e.g., recruitment), eliminating harvest pressures on adult females alone will not eliminate the risk of population extinction. Our study clearly shows that headstarting can effectively halt or reverse declines of C. longicollis populations in the face of multiple threats, especially in cases where external threats affect multiple life history stages of freshwater turtles. In our case study, factors that have impacted C. longicollis will never dissipate until populations are extinct, or technology to reduce threats from invasive predators becomes more effective. Headstarting should be the primary conservation tool for managing C. longicollis in decline and similar analyses should be conducted to evaluate its value for each species requiring active management. Management of a population or a species under threat often focuses directly on reducing impacts on the life history stage(s) affected. In doing so, focus inevitably is directed to the threat, rather than on the impacts on the affected population. Plant biologists and conservationists have long criticised classical biocontrol for lacking quantitative assessments of effectiveness, especially post-release (McEvoy & Coombs 1999), yet invasive vertebrate pest management primarily focuses on reducing densities of invasive predators or herbivores. The core components of conservation policy to manage their impacts is to reduce predator numbers in an area using lethal methods, such as poison baiting, shooting, and trapping. The actual efficacy (e.g., reduced impact on target species or increases in biodiversity) of such programs are rarely assessed (Walsh et al. 2012) and success is determined by the number of carcasses, reduced activity of the target species or the number of baits taken. Efficacy of these programs is vital given the limited resources available for most conservation programs and the high costs associated with lethal control. AU$21.3m was spent on labor costs alone for red fox control in Australia in 1998–2003, but the benefits to native prey are largely not known (Reddiex et al. 2006). Management often assumes that there is a linear relationship between impact and invasive species densities; hence successes in reducing invasive species are also lauded as conservation outcomes. Yet, rarely are the relationships between densities and impact linear, and in some rare instances, reductions in invasive animals may result in increased impacts on native species (Spencer et al. 2016). Clearly, impacts on turtle nests are not a direct function of increased activity or densities of foxes in south-eastern Australia (Fig. 7), and management focusing on reducing foxes must achieve nothing short of complete eradication from an area to truly reduce nest predation rates (Fig. 7); an achievement that appears impossible for any extended period of time (Table 3). Conservation outcomes and management decisions should be based on the relationship between the level of threat and impact on the affected species. The impacts of introduced species are a product of their density and functional response to prey densities. Functional responses are the changes in the rate of prey consumption by individual predators, relative to prey abundance (Holling, 1959). Numerical responses are the aggregative rates of prey consumption by all predators relative to prey density, which 10 Spencer R-J et al. 2017 Critically Evaluating Best Management Practices for Preventing Freshwater Turtle Extinctions. Conservation Biology. In Press.
Fig. 8. Locations of turtle mortality sightings in south-eastern Australia from September 2014-September 2016 in TurtleSAT (http://TurtleSAT.org.au). Photo of a dead C. longicollis from Victoria uploaded to TurtleSAT. change with predator density via reproduction or migration, in response to changes in prey density (Holling, 1959). Traditional management of introduced predators focuses on reducing predator populations, and thus focuses on numerical responses, which fails to manage for functional responses, and may not eliminate impacts of highly-efficient individual predators (Spencer et al. 2016). For conservation, functional responses become critical when introduced species densities are declining, as functional responses essentially define the levels (numerical reductions) that management must achieve if resources are directed into removing introduced species. For example, if one predator in a system can achieve extremely high levels of predation, only management options that completely eliminate that individual from the system will be effective. In open systems at a landscape level, this level of management is impossible to achieve, or not cost effective (Doherty and Ritchie 2016). In a review of the literature to determine efficacy of standard lethal control methods in Australia, we found that few studies were able to reduce fox numbers for extended periods, and none were able to eradicate them (Table 3). Thus, freshwater turtle populations are unlikely to get any relief from standard fox control programs (assuming functional responses remain constant after a reduction of foxes) and alternative management techniques should be considered to manage turtle populations (or other species) under threat by predation from invasive predators like foxes. Likewise, reducing adult road mortality for turtle species that are highly mobile across terrestrial habitats, like C. longicollis, has been historically difficult. Fences are required to funnel turtles to suitable crossing locations (ecopassages) and prevent them from crossing the road, but may themselves cause mortality (Ferronato 2014). In addition, some turtles can climb fences, which limits their effectiveness (Baxter-Gilbert et al. 2015). If the fence is effective, turtles are then funnelled to cross underneath the road via an ecopassage, but there is conflicting evidence whether they are always willing to do so (Woltz et al. 2008; Baxter-Gilbert et al. 2015), and some evidence that the ecopassage itself may act as a prey trap (Little et al. 2002). Furthermore, constructing suitable road crossings for wildlife at suitable densities on a roadway is expensive (Mata et al. 2008; Baxter-Gilbert et al. 2015). Thus, there are substantial feasibility challenges in reducing or preventing adult turtle 11 Spencer R-J et al. 2017 Critically Evaluating Best Management Practices for Preventing Freshwater Turtle mortality on roadways. Extinctions. Conservation Biology. In Press.
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Preventing Freshwater Turtle Extinctions

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