Data from: Variation in thermal niche of a declining river-breeding frog: from counter-gradient responses to population distribution patterns

When dams or climate change alter the thermal regimes of rivers, conditions can shift outside optimal ranges for aquatic poikilothermic vertebrates. Plasticity in thermal performance and preference, however, may allow temperature-vulnerable fauna to persist under challenging conditions. To determine the effects of thermal regime on Rana boylii (Ranidae), a threatened frog species endemic to rivers of California and Oregon, we quantified tadpole thermal preferences and performance in relation to thermal conditions. We monitored temperature and censused populations across a coastal to inland cline in six catchments where dams have altered thermal environments in close proximity to river reaches with natural conditions. We found geographic variation in population distribution and abundance based on river size combined with water temperature. The large inland rivers that supported breeding frogs, although cooler in spring due to snowmelt, became warmer during the summer than occupied coastal sites. Inland populations were constrained to reaches where the average temperature over the warmest 30 days ranged from 17.6 to 24.2°C, higher than coastal rainfall-driven systems where averages ranged from 15.7 to 22.0°C. Frogs in rivers with hypolimnetic-release dams bred in colder waters than they did in free-flowing rivers. Common-garden and field translocation experiments revealed local adaptations in larval growth and phenotypically plastic thermoregulatory behaviour. Tadpoles from all rivers had a positive linear growth response to temperature, but individuals from inland rivers displayed intrinsically higher growth rates. Consistent with a counter-gradient model of selection in which the response to temperature change is in the opposite direction of the change, individuals from cooler rivers selected warmer temperatures. When reared under common conditions, however, tadpoles showed similar temperature preferences regardless of source river. Our results suggest a role for local growth rate adaptation in structuring the distribution of Rana boylii. Plastic thermoregulatory behaviour by tadpoles may explain how small populations are able to persist where dams release cold water. Management of edgewater habitats to increase the availability of warm micro-sites may ameliorate this impact.

当水坝建设或气候变化改变河流热状况（thermal regimes）时，其环境条件可能会偏离水生变温脊椎动物的适宜生存范围。然而，热性能与热偏好的可塑性，或可让对温度敏感的动物类群在恶劣环境下得以存续。为明确热状况对博氏蛙（Rana boylii，蛙科Ranidae）——一种仅分布于加利福尼亚州与俄勒冈州河流的濒危特有蛙类——的影响，本研究针对其蝌蚪的热偏好与热性能，结合环境热条件开展了定量分析。我们在6个集水区内，沿沿海至内陆的环境梯度开展了温度监测与种群普查：这些集水区内部分河段因水坝工程改变了热环境，且邻近保留有天然状态的河段。研究发现，种群分布与丰度存在地理差异，且该差异与河流规模及水温紧密相关。尽管依托融雪降温，内陆大型河流在春季水温更低，但夏季水温却高于已被蛙类占据的沿海河段。内陆种群仅能存活于最热30天平均水温介于17.6~24.2℃的河段，该水温范围高于降雨驱动型沿海河流系统的15.7~22.0℃。在设有底层泄水坝（hypolimnetic-release dams）的河流中，蛙类的产卵水域水温低于自由流动河流中的产卵水域。同质园（common-garden）实验与野外移置实验结果显示，该物种的幼体生长存在本地适应性，且体温调节行为具有表型可塑性。所有河流的蝌蚪均随温度升高呈现正向线性生长响应，但内陆河流的蝌蚪本征生长速率更高。该结果与反梯度选择模型的预测一致：该模型中，生物对温度变化的响应方向与温度变化本身的方向相反——来自较冷水域的蝌蚪会选择更高的水温。然而，当在同质环境下饲养时，无论来源河流为何，蝌蚪的热偏好均无显著差异。本研究结果表明，本地生长速率适应性在博氏蛙的种群分布格局塑造中发挥了关键作用。蝌蚪的可塑性体温调节行为，或可解释小型种群为何能在水坝泄放冷水的河段存续。通过管理近岸生境以增加温暖微生境的可获得性，或可缓解该类人为活动带来的负面影响。