Drier and hotter conditions caused by climate change threaten species that exist close to their physiological limits, as well as those with limited ability to move. Habitat specialists may also be particularly vulnerable if they have specific abiotic requirements. Here we assess whether thermal and hydric constraints can explain the highly restricted and declining distributions of the critically endangered terrestrial-breeding frog, Geocrinia alba. We also evaluate the species’ vulnerability to climate change based on the similarity of current microclimatic conditions to their physiological limits. We found that G. alba had low thresholds of thermal and desiccation tolerance relative to other anuran species. The estimated thermal optimum (Topt) and critical thermal maxima (CTmax) were 23.3°C and 29.6°C, respectively, and adult frogs had an absorption threshold (AT, the lowest water potential at which water can be absorbed from a substrate) of −50 kPa, the lowest recorded for an amphibian. Comparing environmental conditions and water loss in the field using agar models showed that riparian habitats where frogs occur provide a unique microclimate in the landscape, offering significantly lower desiccation risk during extreme summer conditions compared to immediately adjacent riparian and terrestrial habitats. Monitoring of microclimate conditions within occupied frog habitats over 2 years showed that in extreme dry and hot years the AT was exceeded at six of eight sites, and Topt was exceeded at two of eight sites. Given their specific physiological limits, the apparent rarity of suitable microclimates and a regional drying–warming trend, we suggest that G. alba occupies a potentially disappearing niche and may be indicative of other habitat specialists that rely on ephemeral drainages. More broadly, this study highlights that desiccation thresholds may tightly constrain amphibian distributions and need to be considered along with thermal tolerance thresholds when predicting the impacts of climate change.

中文翻译：

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低干燥和耐热性将陆地两栖动物限制在稀有且正在消失的小气候生态位

气候变化导致的更干燥和更热的条件威胁着接近其生理极限的物种，以及那些移动能力有限的物种。如果栖息地专家有特定的非生物需求，他们也可能特别脆弱。在这里，我们评估热和氢约束是否可以解释极度濒危的陆地繁殖青蛙 Geocrinia alba 的高度受限和下降的分布。我们还根据当前小气候条件与其生理极限的相似性来评估该物种对气候变化的脆弱性。我们发现 G. alba 相对于其他无尾目物种具有较低的耐热和耐干燥阈值。估计的最佳热 (Topt) 和临界热最大值 (CTmax) 分别为 23.3°C 和 29.6°C，成年青蛙的吸收阈值（AT，可以从底物吸收水的最低水势）为-50 kPa，这是两栖动物的最低记录。使用琼脂模型比较田间的环境条件和水分流失表明，青蛙出现的河岸栖息地在景观中提供了独特的小气候，与紧邻的河岸和陆地栖息地相比，在极端夏季条件下提供显着降低的干燥风险。对 2 年内占用的青蛙栖息地内的小气候条件的监测表明，在极端干燥和炎热的年份，八个地点中有六个超过了 AT，八个地点中有两个超过了 Topt。鉴于它们特定的生理限制、合适的小气候明显稀有以及区域性的干燥变暖趋势，我们建议 G. alba 占据了一个可能正在消失的生态位，可能表明其他栖息地专家依赖于短暂的排水系统。更广泛地说，这项研究强调干燥阈值可能会严格限制两栖动物的分布，并且在预测气候变化的影响时需要与耐热阈值一起考虑。