Species richness is greatest in the tropics, and much of this diversity is concentrated in mountains. Janzen proposed that reduced seasonal temperature variation selects for narrower thermal tolerances and limited dispersal along tropical elevation gradients [Janzen DH (1967) Am Nat 101:233–249]. These locally adapted traits should, in turn, promote reproductive isolation and higher speciation rates in tropical mountains compared with temperate ones. Here, we show that tropical and temperate montane stream insects have diverged in thermal tolerance and dispersal capacity, two key traits that are drivers of isolation in montane populations. Tropical species in each of three insect clades have markedly narrower thermal tolerances and lower dispersal than temperate species, resulting in significantly greater population divergence, higher cryptic diversity, higher tropical speciation rates, and greater accumulation of species over time. Our study also indicates that tropical montane species, with narrower thermal tolerance and reduced dispersal ability, will be especially vulnerable to rapid climate change.

Janzen's extension of the climate variability hypothesis (CVH) posits that increased seasonal variation at high latitudes should result in greater temperature overlap across elevations, and favour 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 c. 2000 m. 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 CVH 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. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12906/suppinfo is available for this article.

1. Characterizing thermal acclimation is a common goal of eco-physiological studies and has important implications for models of climate change and environmental adaptation. However, quantifying thermal acclimation in biological rate processes is not straightforward because many rates increase with temperature due to the acute effect of thermodynamics on molecular interactions. Disentangling such passive plastic responses from active acclimation responses is critical for describing patterns of thermal acclimation.2. Here, we reviewed published studies and distinguished between different study designs measuring the acute (i.e. passive) and acclimated (i.e. active) effects of temperature on metabolic rate. We then developed a method to quantify and classify acclimation responses by comparing acute and acclimated Q 10 values. Finally, we applied this method using meta-analysis to characterize thermal acclimation in metabolic rates of ectothermic animals.3. We reviewed 258 studies measuring thermal effects on metabolic rates, and found that a majority of these studies (74%) did not allow for quantifying the independent effects of acclimation. Such studies were more common when testing aquatic taxa and continue to be published even in recent years. 4.A meta-analysis of 96 studies where acclimation could be quantified (using 1,072 Q 10 values) revealed that 'partial compensation' was the most common acclimation response (i.e. acclimation tended to offset the passive change in metabolic rate due to acute temperature changes). However, 'no acclimation' and 'inverse compensation', in which acclimation further augmented the acute change in metabolic rate, were also common. 5. Acclimation responses differed among taxa, habitats and with experimental design. Amphibians and other terrestrial taxa tended to show weak acclimation responses, whereas fishes and other aquatic taxa tended to show stronger compensatory responses. Increasing how long the animal was allowed to adjust to a new test temperature increased the acclimation response, but body size did not.Acclimation responses were also stronger with longer acclimation durations.

It has long been recognized that populations and species occupying different environments vary in their thermal tolerance traits. However, far less attention has been given to the impact of different environments on the capacity for plastic adjustments in thermal sensitivity, i.e., acclimation ability. One hypothesis is that environments characterized by greater thermal variability and seasonality should favor the evolution of increased acclimation ability compared with environments that are aseasonal or thermally stable. Additionally, organisms under selection for high heat tolerance may experience a trade-off and lose acclimation ability. Few studies have tested these non-mutually exclusive hypotheses at both broad latitudinal and local elevation scales in phylogenetically paired taxa. Here, we measure short-term acclimation ability of the critical thermal maximum (CTMAX) in closely related temperate and tropical mayflies (Ephemeroptera) and stoneflies (Plecoptera) from mountain streams at different elevations. We found that stream temperature was a good predictor of acclimation ability in mayflies, but not in stoneflies. Specifically, tropical mayflies showed reduced acclimation ability compared with their temperate counterparts. High elevation tropical mayflies had greater acclimation ability than low elevation mayflies, which reflected the wider temperature variation experienced in high elevation streams. In contrast, temperate and tropical stoneflies exhibited similar acclimation responses. We found no evidence for a trade-off between heat tolerance and acclimation ability in either taxonomic order. The acclimation response in stoneflies may reflect their temperate origin or foraging mode. In combination with previous studies showing tropical taxa have narrower thermal breadths, these results demonstrate that many lower elevation tropical aquatic insects are more vulnerable to climate warming than their temperate relatives.