Data from: Climate variability predicts thermal limits of aquatic insects across elevation and latitude

Janzen's extension of the climate variability hypothesis posits that increased seasonal variation at high latitudes should result in greater temperature overlap across elevations, and favor wider thermal breadths in temperate organisms compared to their tropical counterparts. We tested these predictions by measuring stream temperatures and thermal breadths (i.e. the difference between the critical thermal maximum and minimum) of 62 aquatic insect species from temperate (Colorado, USA) and tropical (Papallacta, Ecuador) streams spanning an elevation gradient of ca. 2000m. Temperate streams exhibited greater seasonal temperature variation and overlap across elevations than tropical streams, and as predicted, temperate aquatic insects exhibited broader thermal breadths than tropical insects. However, elevation had contrasting effects on patterns of thermal breadth. In temperate species, thermal breadth decreased with increasing elevation because CTMAX declined with elevation while CTMIN was similar across elevations. In tropical insects, by contrast, CTMAX declined less sharply than CTMIN with elevation, causing thermal breadth to increase with elevation. These macrophysiological patterns are consistent with the narrower elevation ranges found in other tropical organisms, and they extend Janzen's climate variability hypothesis to freshwater streams. Furthermore, because lowland tropical aquatic insects have the narrowest thermal breadths of any region, they may be particularly vulnerable to short-term extreme changes in stream temperature.詹森提出的气候变率假说（climate variability hypothesis）拓展版本认为，高纬度地区的季节性波动加剧，将导致不同海拔间的温度重叠程度更高，且相较于热带生物，温带生物的热耐受宽度（thermal breadth）更广。 我们通过测量来自温带（美国科罗拉多州）与热带（厄瓜多尔帕帕拉克塔）的62种水生昆虫的溪流水温及其热耐受宽度（即临界最高热耐受温度与临界最低热耐受温度的差值），涵盖了约2000米的海拔梯度，以此检验上述预测。 温带溪流的季节性温度波动及跨海拔温度重叠度均高于热带溪流，且正如预测所言，温带水生昆虫的热耐受宽度显著宽于热带昆虫。不过海拔对热耐受宽度格局的影响却呈现出截然相反的效应：在温带物种中，热耐受宽度随海拔升高而降低，这是由于临界最高热耐受温度（critical thermal maximum, CTMAX）随海拔上升而下降，而临界最低热耐受温度（critical thermal minimum, CTMIN）在不同海拔间保持一致。与之形成鲜明对比的是，在热带昆虫中，临界最高热耐受温度随海拔升高的降幅弱于临界最低热耐受温度，最终导致热耐受宽度随海拔升高而增大。 这些宏观生理格局与其他热带生物所表现出的更狭窄海拔分布范围相符，同时将詹森的气候变率假说拓展至淡水溪流生态系统。此外，由于低地热带水生昆虫的热耐受宽度在所有区域中最为狭窄，它们或许对溪流水温的短期极端变化尤为脆弱。