Hydrothermal physiology and climate vulnerability in amphibians

Abstract

Concerns over the consequences of global climate change for biodiversity have spurred a renewed interest in organismal thermal physiology. However, temperature is only one of many environmental axes poised to change in the future. In particular, hydrologic regimes are also expected to shift concurrently with temperature in many regions, yet our understanding of how thermal and hydration physiology jointly affect performance and fitness is still limited for most taxonomic groups. Here, we investigated the relationship between functional performance, hydration state and temperature in three ecologically distinct amphibians, and compare how temperature and water loss can concurrently limit activity under current climate conditions. We found that performance was maintained across a broad range of hydration states in all three species, but then declines abruptly after a threshold of 20-30% mass loss. This rapid performance decline was accelerated when individuals were exposed to warmer temperatures. Combining our empirical hydrothermal performance curves with species-specific biophysical models, we estimated that dehydration can increase restrictions on species' activity by up to 60% compared to restriction by temperature alone. These results illustrate the importance of integrating species' hydration physiology into forecasts of climate vulnerability, as omitting this axis may significantly underestimate the effects of future climate change on Earth's biological diversity.

Keywords: dehydration; desiccation; ecophysiology; frogs; global change; thermal performance.

Figure 1.

Bioclimatic affiliations of each study species (insets, left to right: Ascaphus truei, Pseudacris regilla, Spea intermontana) based on sampling occurrence sites varying in (a) mean annual temperature and (b) total annual rainfall across their range. (Online version in colour.)

Figure 2.

Relative jump performance (scaled to maximum individual performance) as a function of both dehydration (proportion of initial mass lost) and environmental temperature for (a) Ascaphus truei, (b) Pseudacris regilla and (c) Spea intermontana. Absolute performance curves are available in electronic supplementary material, figure S2. (Online version in colour.)

Figure 3.

Hours of restriction estimated for an average day in July across 100 sampled occurrence sites from species-specific biophysical models for (a) Ascaphus truei, (b) Pseudacris regilla and (c) Spea intermontana. Hours of restriction are estimated at different body sizes for each species (x-axis), and for different physiological limits based on just temperature (red) or both temperature and dehydration (blue). Each point represents an average estimate of physiological restriction at a site, with lines to illustrate the per-site effect of incorporating dehydration limits, the large points and lines represent the average effect across sites for each model organism size. (Online version in colour.)