Climate change could have complex impacts on amphibians – 01/22/2024 – Environment

Over the years, many studies have reaffirmed hypotheses from the 1950s and 1960s that associate physiological and climatic variations. For example, one of them says that species exposed to a greater thermal range in an environment (the difference between minimum and maximum temperatures) would have a greater range of thermal tolerance.

A study supported by Fapesp and published in the journal Integrative Organismal Biology shows that, at least for amphibians in the Atlantic forest, this is not necessarily true.

The work, signed by Brazilian researchers who work in institutions in Brazil, the United States and the United Arab Emirates, points out that some populations living in mountains, where the temperature range is large, do not necessarily have a greater range of tolerance to temperature changes than populations in lower altitude areas.

“This relationship between greater thermal amplitude and greater tolerance to changes only occurred in two of the five species we analyzed in the study”, explains Rafael Bovo, first author of the work, carried out as part of his postdoctoral internship at the Institute of Biosciences from the University of São Paulo (IB-USP) with a scholarship from Fapesp.

Since the same species can be present both a few meters above sea level and high up in mountains, an important part of the researchers’ work was precisely to ensure samples from populations of the same species distributed at different altitudes.

Physiological tests carried out on amphibians indicated that species do not necessarily have a fixed value of thermal tolerance. Therefore, different populations may have greater or lesser tolerance to changes in climate. According to the analyses, greater tolerance to cold in populations living at higher altitudes, where the climate is colder, did not necessarily lead to an increase in the thermal amplitude (window). This window is determined by more than one factor, such as prior exposure (acclimatization) or temperature adaptation.

“If we only analyze thermal tolerance, it is not safe to say that all tropical amphibians are threatened by global climate change. We have shown, in fact, that some populations have greater potential to withstand changes, sometimes even comparable to species from temperate regions that, normally, they have a greater range of thermal tolerance than tropical species”, says Bovo, currently a researcher associated with the University of California, Riverside, in the United States.

Likewise, the study shows that certain populations in a low-altitude area, such as the marsh tree frog (Dendropsophus minutes), are living close to the limit of their thermal tolerance. Therefore, they may not withstand a greater increase in temperature.

Part of the results were obtained during Bovo’s doctorate at the Biosciences Institute of the Universidade Estadual Paulista (IB-Unesp), in Rio Claro, also with a scholarship from Fapesp.

One explanation for why amphibians living in greater thermal ranges have not acquired a greater window of tolerance is the fact that many spend most of the day sheltered, only coming out to hunt and reproduce at night. This saves them from the highest temperatures of the day and avoids major changes in heat tolerance.

On the other hand, night-active amphibians are exposed to colder temperatures, especially high in the mountains, leading to greater cold tolerances. As a result, the change in cold tolerance alone was not enough to significantly alter the tolerance window of some species. This highlights how complex the possibilities of adjusting or adapting to cold and heat are.

Mountains of data

The results are the result of experiments with populations of five species distributed along altitude gradients, which ranged from sea level to 1,600 meters above sea level, in the Atlantic forest.

The 225 amphibians were collected at different points in Serra do Mar and Serra da Mantiqueira, in the States of São Paulo and Minas Gerais. For each species, three to five populations were sampled, considering the geographic region where the animals were found.

For each population, different physiological parameters were measured: dehydration and rehydration rates, minimum and maximum critical temperatures (the coldest and hottest that the individual can withstand without losing basic functions).

Thermal tolerance to cold always increased with altitude, while heat tolerance did not always change, both in Serra do Mar and Mantiqueira. Altitude explained up to 61% of the variation in the species’ minimum critical temperature (up to 4°C lower at higher altitudes), suggesting that the low temperatures characteristic of altitudes lead to greater physiological tolerance to cold.

However, altitude had less relationship with the variation in maximum critical temperature (19% to 36%). In two of the five species, the maximum tolerance reduced as altitude increased, being 0.5° C to 1.8° C lower in highland populations than in lower terrain. The maximum tolerated temperature was higher (around 38.5° C) in larger-bodied species than in smaller ones (around 34° C).

“Given that thermal and water balance parameters are fundamental to the evolutionary history of amphibians, we also measured dehydration and rehydration rates and their relationships with thermal tolerances. Water balance did not show consistent variation with altitude change and correlations between water and thermal parameters were low”, says Bovo.

For researchers, the study brings important warnings about organisms’ responses to climate variation. The authors point out that measuring just one population as a representative of an entire species can bias predictions of vulnerability to climate change.

The results also reinforce the enormous lack of data for tropical species, compared to species from temperate regions. The responses of tropical species to climate change can be complex, as the study shows, making generalizations difficult. The Atlantic forest, with the greatest diversity of amphibians in the world, needs to be better considered.

The data from the work has already been used in mathematical models to predict the impact of climate change on biodiversity. In one of them, for example, the vulnerability of amphibians around the world to increased warming and drought is being quantified.

“These are works in progress, in collaboration with a network of international researchers”, reports Bovo, who established partnerships during a research internship at another unit at the University of California, Santa Cruz. Previously, he had already completed an internship at the same institution, as part of his doctorate at IB-Unesp, in Rio Claro, all with a scholarship from Fapesp.