Assisted colonisation trials for the western swamp turtle show that juveniles can grow in cooler and wetter climates

Abstract

Species with restricted ranges and long generation times are vulnerable to climate change due to limited opportunity to disperse or adapt. Australia’s rarest reptile, the western swamp turtle Pseudemydura umbrina, persists naturally in only one seasonal swamp that holds water in the Austral winter and spring. A marked reduction in winter rainfall in recent decades has shortened the swamp hydroperiod, restricting when turtles are able to feed, grow and reproduce. To mitigate possible future loss of reproductive capacity in the native habitat, assisted colonisation was trialled in 2016 using 35 captive-bred juveniles. Here, we report the outcomes of this 6 mo trial, which compared the growth of turtles released approximately 300 km south of the species’ indigenous range with growth of turtles released at an existing northern translocation site. We showed that growth rates comparable to those at warmer northern translocation sites can be achieved in the south, even in an atypically cool spring as occurred in 2016. Microclimates available to P. umbrina at 2 southern sites were suitable for foraging and growth in late spring and early summer, but juvenile growth at one southern site was significantly better than at the other, likely due to higher prey biomass when water temperatures were suitable for foraging. These early results suggest that introduction of P. umbrina to seasonal wetlands near the south coast of Western Australia could be considered in the immediate future, but further trials are recommended to assess growth and survivorship over longer periods.

Novel Conditions in Conservation Translocations: A Conservative-Extrapolative Strategic Framework

“…To combat emerging threats, increasingly more extensive ecosystem manipulations are suggested, including assisted colonisation (Brodie et al, 2021), the translocation of individuals outside of their established range as a response to threats such as climate change (IUCN/SSC, 2013). These sorts of translocations, moving organisms outside of both their current and indigenous ranges are inherently extrapolative, as managers are knowingly creating unprecedented conditions and ecological interactions (Rendall et al, 2018;Bouma et al, 2020).…”

“…We urge caution in the application of extrapolative translocation strategies, particularly with regard to assisted colonisation and rewilding projects that seek to create novel ecosystem conditions (Corlett, 2016). In these cases, where there is so much inherent uncertainty, we suggest that decisions should always be informed by what data are available, and that even the most experimental translocations should be based on evidencebased hypotheses regarding the potential success and risks of the translocation (Armstrong and Seddon, 2008;Bouma et al, 2020). This is due to the cautionary tales from the field of invasive species management, in which many species that have been introduced outside of their native range, whether accidentally or intentionally, are a major driver of ecosystem degradation and biodiversity decline, costing conservation management vast resources to reduce and reverse the resulting damage (Brook et al, 2008;O'Donnell et al, 2017).…”

Evaluating candidate wetlands for the assisted colonization of the western swamp turtle Pseudemydura umbrina in a changing climate: Macro‐invertebrate food resources and turtle diet

“…At Moore River and East Augusta this may reflect the shrinking size of wetlands as spring progressed, leading to a concentration of aquatic invertebrates and vertebrates. Meerup, in contrast, receiving higher than average rainfall in 2016 (Bouma et al, 2020), had a higher water level in late spring, and a lower concentration of aquatic animals.…”

“…This difference may have been caused by the development cycle of different frog species: by late spring many tadpoles may already have developed into frogs in the latter wetlands. The higher richness of invertebrates in Moore River Nature Reserve compared with the southern sites might be attributed to warmer shallow water conditions (Bouma et al, 2020) and the relatively wet winter and spring in 2016: in 2016 Moore River received 743.4 mm, the highest annual rainfall since 1999 although the July to December rainfall was slightly below average (Bureau of Meteorology Perth, 2017). Crayfish, some of which can be predators of P. umbrina (Bradsell et al, 2002), were also found in Meerup and East Augusta.…”

“…the impact of turtles on the resident fauna in the translocation sites, the availability of appropriate aestivation sites and the landscape‐scale site characteristics – not just the swamp areas themselves) that need to be addressed to verify the suitability of the trial sites for future assisted colonization. Assisted colonization is a complex and inherently risky strategy, but one that has to be tested as a climate change adaptation tool within conservation management plans for highly threatened species that could soon disappear without human intervention (Bouma et al, 2020; Stanford et al, 2020). The results of this first field trial on food availability for P. umbrina suggest that further assessment of the potential for assisted colonization at one of the two sites evaluated is warranted, with the next trial lasting for a few years and applying robust methods to evaluate prey selection and potential impacts on the recipient community.…”