Multi‐scale relationships in thermal limits within and between two cold‐water frog species uncover different trends in physiological vulnerability

Abstract

1. Critical thermal limits represent an important component of an organism's capacity to cope with future temperature changes. Understanding the drivers of variation in these traits may uncover patterns in physiological vulnerability to climate change. Local temperature extremes have emerged as a major driver of thermal limits, although their effects can be mediated by the exploitation of fine‐scale spatial variation in temperature through behavioural thermoregulation. 2. Here, we investigated thermal limits along elevation gradients within and between two cold‐water frog species (Ascaphus spp.), one with a coastal distribution (A. truei) and the other with a continental range (A. montanus). We quantified thermal limits for over 700 tadpoles, representing multiple populations from each species. We combined local temporal and fine‐scale spatial temperature data to quantify local thermal landscapes (i.e., thermalscapes), including the opportunity for behavioural thermoregulation. 3. Lower thermal limits for either species could not be reached experimentally without the water freezing, suggesting that cold tolerance is <0.3°C. By contrast, upper thermal limits varied among populations, but this variation only reflected local temperature extremes in A. montanus, perhaps as a consequence of the greater variation in stream temperatures across its range. Lastly, we found minimal fine‐scale spatial variability in temperature, suggesting limited opportunity for behavioural thermoregulation and thus increased vulnerability to warming for all populations. 4. By quantifying local thermalscapes, we uncovered different trends in the relative vulnerability of populations across elevation for each species. In A. truei, physiological vulnerability decreased with elevation, whereas in A. montanus, all populations were equally physiologically vulnerable. These results highlight how similar environments can differentially shape physiological tolerance and patterns of vulnerability of species, and in turn impact their vulnerability to future warming.

Linking critical thermal maximum to mortality from thermal stress in a cold-water frog

“…See [41] for full details of CTmax experiments. Briefly, we sampled 10-24 tadpoles (developmental stages 26.5 to 44 [46]; electronic supplementary material, table S1) from each of seven populations of A. montanus in Montana.…”

“…Due to the distance between these streams (7-224 km, electronic supplementary material, figure S1) and limited vagility of the species [47], we refer to these sampled sites (n = 7) as populations. We held tadpoles for 3 days at 8°C, a commonly experienced temperature among populations [41], without food to reduce the effects of natal stream temperature and feeding. For CTmax experiments, tadpoles were moved to an experimental tank and held in individual mesh containers.…”

“…Tadpoles from each population were evenly and randomly assigned to one of five holding temperature treatments: 5°C, 10°C, 15°C, 20°C, 25°C (electronic supplementary material, table S1). Temperatures were decided based on thermal regime data [41]: 5°C and 10°C are commonly experienced temperatures; 15°C and 20°C treatments are near maximum stream temperatures; 25°C is greater than current maximum stream temperatures but is ecologically relevant given current rates of warming [49]. Tadpoles were held in the temperature treatments for 3 days and fed ad libitum by placing rocks collected from their stream in the holding tanks, from which they graze algae.…”

“…A. montanus inhabits cold streams in the northern U.S. Rocky Mountains and extreme southern Rocky Mountains of Canada. This species is already facing population declines that are partly due warming stream temperatures [38], and low CTmax relative to other amphibians [39][40][41] suggests continued risk from warming. Previous research found population variation in CTmax [41], providing an ideal opportunity to investigate if natural population variation in CTmax estimated via the dynamic method and mortality at different ecologically relevant constant temperatures (i.e.…”

“…We expected these two approaches to be related, such that populations with higher CTmax experienced less mortality. We also tested the strength of CTmax as a predictor of mortality from thermal stress by comparing its performance against local thermal conditions, which are commonly related to physiological traits [15,[42][43][44], including in this system [41].…”