Variation in tadpole thermal tolerance and overheat risk across an elevational gradient

Abstract

Global climate change is increasing temperatures and the frequency of extreme weather events, posing growing threats to biodiversity. Thermal tolerance is therefore a key trait for predicting species' vulnerability to climate change. The Climate Variability Hypothesis (CVH) predicts that populations experiencing greater environmental variability should exhibit broader thermal tolerance, yet this prediction has rarely been tested within a single species. We investigated the critical thermal maximum (CTmax) and minimum (CTmin) of Moltrecht's treefrog (Zhangixalus moltrechti) tadpoles across an elevational gradient in Taiwan and related these traits to local microclimatic conditions. Our results showed that high-elevation populations exhibited broader thermal breadth, primarily driven by variation in CTmin, whereas CTmax remained relatively conserved. Cold tolerance also showed greater plasticity than heat tolerance. These findings demonstrate that populations of the same species differ in their vulnerability to climate change, with low-elevation populations facing the greatest risk from future warming.

Keywords: Thermal tolerance; Climate variability hypothesis; Heat-invariant hypothesis; Amphibians.

Methodology and Implementation Steps

This study investigated tadpoles of the endemic Taiwanese Moltrecht’s treefrog (Zhangixalus moltrechti). Between April and September 2024, Gosner stage 25–38 tadpoles were collected from nine sites across Taiwan spanning elevations of 474–2020 m, with at least 35 individuals sampled per site. Tadpoles were captured using dip nets, transported to National Chung Hsing University in insulated containers, acclimated at 23°C for at least 8 h, and then measured for body length, body mass, and developmental stage. At each site, water temperature was recorded every 30 min for 10 consecutive days using HOBO MX2201 data loggers to calculate maximum, minimum, and mean water temperatures, as well as temperature range. A photoperiod index, calculated from the day length on the sampling date, was used to represent seasonal variation. Critical thermal maximum (CTmax) and critical thermal minimum (CTmin) were determined using dynamic assays, with each tadpole subjected to only one treatment. Water temperature was controlled in a thermostatic water bath, with both heating and cooling rates set at 0.25°C/min, and loss of the righting response was used as the endpoint for determining CTmax or CTmin. Principal component analysis was first applied to integrate geographic, individual, and environmental variables, followed by generalized linear models and structural equation modeling to examine their relationships with thermal tolerance. Pearson’s correlation analyses were used to evaluate the associations among elevation, environmental temperature variability, and thermal breadth, as well as to compare the variation in CTmax and CTmin. Finally, Spearman’s correlation analyses were conducted to assess changes in warming tolerance and cooling tolerance along the elevational gradient.

Innovation and Cross-Disciplinary Collaboration

Previous studies in macrophysiology have largely been constrained by species distribution patterns and therefore have primarily relied on interspecific comparisons of physiological traits to investigate the effects of geographic environments on physiological adaptation. Taiwan's exceptional topographic relief allows many species to span broad elevational gradients, making it an ideal natural laboratory for examining intraspecific physiological adaptation. This study not only demonstrates that two classic hypotheses, the Climate Variability Hypothesis and the Heat-Invariant Hypothesis, are applicable to amphibian populations of the same species across different elevational gradients, but also represents the first systematic investigation of this topic on a mountainous island in Asia. Our findings provide an important foundation and valuable reference for future research in physiological ecology and global change biology.

Expected Results and Contributions

1. The first study to investigate the relationship between physiological tolerance and environmental gradients among elevational populations of a single amphibian species:
   This study moves beyond the traditional macrophysiological framework, which has relied primarily on interspecific comparisons, by demonstrating that physiological tolerance can vary among populations of the same species in response to local habitat conditions. It also provides direct evidence that low-elevation populations face greater warming risks, offering an important scientific basis for population-level conservation and management strategies under future climate change.
2. Public Education and Science Communication:
   The findings of this study have been featured by the Environmental Information Center and The Reporter for Advocates, and have also been adapted into popular science content by a scientific illustration studio. These outreach efforts are expected to increase public awareness of global warming and amphibian conservation.
3. International Collaboration and Academic Impact:
   This study was conducted through an international collaboration with researchers at the Doñana Biological Station, Spanish National Research Council. The findings have been presented at several international conferences, including the International Congress for Conservation Biology (ICCB), the International Ecology School, and the Japan–Taiwan Joint Conference on Herpetology. These efforts are expected to enhance the international visibility and academic influence of Taiwan's ecological research.