Magnitude-duration relationships of physiological sensitivity and environmental exposure improve climate change vulnerability assessments

Integrating thermal physiology with environmental temperature is essential
to understanding distributions of species and vulnerability to climate
change. Warming tolerance—the difference between an organism’s maximum
thermal tolerance (Tmax) and maximum habitat temperature (Thab)—is
frequently used to integrate organismal sensitivity and environmental
exposure. Traditionally, applications of warming tolerance define Tmax and
Thab as invariable magnitudes, yet tolerance magnitude depends on exposure
duration and diel temperature cycles expose organisms to a range of
temperature magnitudes and durations. How traditional (i.e., acute)
estimates of warming tolerance compare to estimates from prolonged
exposures remains poorly understood. In this study, magnitude-duration
curves for tolerances of one cold-water, two cool-water, and one
warm-water species of freshwater fish were compiled from the literature
and compared to magnitude-duration exposures from 66 streams across the
eastern United States. Warming tolerances were estimated for exposure
durations spanning 0.01 to 24 hours. Current acute (0.01 hours) warming
tolerances ranged from median 6.30°C for the cold-water species to 9.68°C
for the warm-water species. The lowest warming tolerances corresponded to
prolonged exposures lasting median 3.85 to 5.30 hours among species and
were 2.51 to 4.38°C lower than acute estimates. Although acute estimates
remained positive in historically occupied and unoccupied streams (6.30°C
versus 2.33°C), estimates based on prolonged exposure were positive at
occupied streams of the cold-water species but transitioned to negative in
unoccupied streams (2.19°C versus -1.12°C). Acute warming tolerances for
the cold-water species also remained positive under future climate (6.29
to 4.23°C) but approached zero at prolonged durations (2.19 to 0.09°C) and
transitioned to negative for 47.2% of streams. Results demonstrate that
acute measures of Tmax and Thab overestimate warming tolerances and
therefore underestimate climate change vulnerability. Integrating
magnitude-duration relationships into warming tolerance estimates can
elucidate physiological mechanisms underlying species distributions and
can improve accuracy of climate change vulnerability assessments.

将热生理学与环境温度数据整合，是解析物种分布格局与评估气候变化脆弱性的关键前提。
热耐受升温裕度（warming tolerance），即生物体的最大热耐受温度（Tmax）与生境最高温度（Thab）之间的差值，常被用于整合生物体的热敏感性与环境暴露情况。
传统的热耐受升温裕度应用均将Tmax与Thab视为固定不变的数值，但实际上热耐受程度取决于暴露时长，且昼夜温度循环会使生物体处于一系列不同温度幅值与暴露时长的组合环境中。
目前学界对传统热耐受升温裕度估算（即急性暴露估算）与长期暴露估算之间的差异仍缺乏深入认知。
本研究从已发表文献中整理了1种冷水性、2种凉水性及1种暖水性淡水鱼类的热耐受幅值-时长曲线，并将其与美国东部66条溪流的温度幅值-时长暴露数据进行对比分析。
研究针对0.01至24小时的暴露时长范围估算了热耐受升温裕度。
当前急性暴露（0.01小时）下的热耐受升温裕度中位数范围为：冷水性鱼类6.30℃，暖水性鱼类9.68℃。
各类物种的最低热耐受升温裕度对应于中位时长3.85至5.30小时的长期暴露，其数值较急性暴露估算结果低2.51至4.38℃。
尽管在历史分布与未分布的溪流中，急性暴露估算的热耐受升温裕度均为正值（分别为6.30℃与2.33℃），但基于长期暴露的估算结果则呈现差异：冷水性鱼类在历史分布溪流中该值仍为正值，而在未分布溪流中则转为负值（分别为2.19℃与-1.12℃）。
在未来气候情景下，冷水性鱼类的急性热耐受升温裕度仍为正值（6.29℃至4.23℃），但在长期暴露条件下则趋近于零（2.19℃至0.09℃），且有47.2%的溪流对应的估算值转为负值。
研究结果表明，基于急性暴露的Tmax与Thab测算会高估热耐受升温裕度，进而低估物种的气候变化脆弱性。
将热耐受幅值-时长关系纳入热耐受升温裕度估算，可解析物种分布格局背后的生理机制，并提升气候变化脆弱性评估的准确性。