Activity of a freshwater turtle varies across a latitudinal gradient: implications for the success of assisted colonisation

The value of assisted colonisation as a response to climate change can only be realised if focal species are well suited to their new habitats. For ectotherms, new habitats must offer microclimates that promote crucial behaviours such as thermoregulation and foraging. The Western Swamp Turtle (Pseudemydura umbrina), a Critically Endangered species from southwestern Australia, serves as a global case-study of assisted colonisation in action. Initial trials where juvenile P. umbrina were released into wetter and cooler climates found that individuals spent considerable time at body temperatures that apparently limited their growth. Using high-resolution biologging data (temperature and depth), here we tested if turtle activity is thermally constrained in cooler latitudes by releasing 48 juveniles into seasonal swamps at three sites. One site was core natural habitat, and the other sites were wetlands 380 km apart that offered either warmer or cooler microclimates. Generalised additive mixed models were used to evaluate behaviours and time spent at optimal temperatures for approximately one month following release, and growth rates were measured and analysed after release until the end of the hydroperiod 4-5 months later. We found that turtles released into the most poleward (southern) wetland spent significantly less time active and basking and grew significantly less compared to turtles released further north. When analysed together, behavioural and growth datasets showed that activity was positively correlated with growth rates. We conclude that poor growth of turtles in the southern wetland was likely a result of lower body temperatures, stemming from a reduced ability to thermoregulate in water. Consequently, for assisted colonisation of P. umbrina to be successful, recipient wetlands must offer aquatic microclimates that are sufficiently warm to promote foraging activity that leads to growth, and ultimately to maturation.

作为应对气候变化的辅助定植（assisted colonisation）手段，其价值唯有在目标物种能够良好适应新生境的前提下才能得以实现。对于变温动物（ectotherms）而言，新生境必须提供能够促进体温调节（thermoregulation）、觅食等关键行为的微气候（microclimates）。产自澳大利亚西南部的极危（Critically Endangered）物种西澳沼泽龟（Western Swamp Turtle，Pseudemydura umbrina），现已成为全球辅助定植实践的经典研究案例。早期试验将幼年西澳沼泽龟放归至更为湿润凉爽的区域后发现，受试个体有大量时间处于会显著抑制其生长的体温区间内。本研究依托高分辨率生物记录（biologging）数据（涵盖温度与水深），通过将48只幼年西澳沼泽龟放生于3个监测点的季节性沼泽中，验证了凉爽纬度区域内该龟类的活动是否会受到热约束。其中1个监测点为该物种的核心天然栖息地，另外2个监测点的湿地彼此相距380公里，分别提供偏暖或偏凉的微气候环境。研究采用广义可加混合模型（Generalised Additive Mixed Models, GAMM），对放生后约1个月内的龟类行为以及处于最优体温区间的时长进行评估；并在放生后至水文周期（hydroperiod）结束（4至5个月后）的时间段内，对龟类的生长速率开展测量与分析。结果显示，与放归至更北侧湿地的个体相比，被放生于最南端（极向）湿地的龟类，其活动与晒背时长均显著更少，生长速率也明显更低。将行为与生长数据集整合分析后可见，龟类活动时长与生长速率呈显著正相关关系。本研究推断，南部湿地中龟类生长迟缓的原因，大概率是其在水中的体温调节能力受限，进而导致体温偏低。因此，若要实现西澳沼泽龟辅助定植的成功，接收湿地必须提供足够温暖的水生微气候，以促进能够推动生长乃至最终成熟的觅食行为。