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Scientists will be able to better identify what amphibian species and habitats will be most impacted by climate change, thanks to a new study by UNSW researchers.

Amphibians are the world's most at-risk vertebrates, with more than 40% of species listed as threatened—and losing entire populations could have catastrophic flow-on effects.

Being ectothermic—regulating their body heat by external sources—amphibians are particularly vulnerable to temperature change in their habitats.

Despite this, the resilience of amphibians to rising temperatures has been poorly understood, with limited data for scientists to draw on.

But now, UNSW researchers have found out how to predict the heat tolerance of 60% of the world's amphibian species—and they've shared their findings in a study in Nature.

Quantifying the resilience of biodiversity to a changing climate is one of the most pressing challenges for contemporary science, says Dr. Patrice Pottier, UNSW post-doctoral researcher and lead author on the paper.

"We wanted to better understand the risk climate change poses to amphibians, and so put together the most comprehensive compilation of heat tolerance limits to date," Dr. Pottier says.

"Heat tolerance limits are the maximum temperature amphibians can tolerate before their physiological systems fail," he explains.

The scientists used 2,661 heat tolerance limit estimates from 524 species to generate data for 5,203 species through data imputation—a statistical method that fills in missing information using patterns from existing data.

"In this case, it predicts heat tolerance limits for species we do not have data for by looking at how heat tolerance is linked to factors like habitat type, environmental temperature, and evolutionary history," says Dr. Pottier.

A novel approach to estimate vulnerability

To assess how vulnerable amphibians are to climate warming, the researchers first estimated the body temperatures amphibians would experience in different microhabitats—terrestrial, arboreal and aquatic.

"We assumed a best-case scenario, where they stay in the shade and keep their skin wet, which could help them survive extreme heat," says Dr. Pottier.

Since extreme heat events are the biggest threat to survival, the researchers then analyzed daily temperature patterns over the past decade to see how often amphibians might face dangerously high temperatures.

Finally, they compared these temperatures to the amphibians' known heat tolerance limits and projected how often these limits might be exceeded under different global warming scenarios (current, +2°C, +4°C) across their geographic range.

Study findings

"We found that 104 out of 5,203 species—2 %—are already exposed to overheating in shaded terrestrial conditions. And a 4°C global temperature increase could push 7.5% of species beyond their physiological limits," Dr. Pottier says.

The study challenges the view of areas most at risk, which has previously often been based on a general latitude gradient to assess overheating risk.

"It has previously often been assumed that species closer to the equator are at greater risk from overheating due to climate change than those in temperate regions," says Dr. Pottier.

"However, our study found that tropical species in the Southern Hemisphere are the most impacted by overheating events, while non-tropical species are more impacted in the Northern Hemisphere.

"Assuming that all tropical species are more vulnerable than temperate species can be misleading. What matters is assessing if the area is going to experience extreme heat events relative to the species' heat tolerance. This requires stepping away from general trends, and identifying specific areas and species at risk."

Mapping the daily temperature fluctuations across regions gives a much clearer picture of how amphibians will be affected by higher temperatures, and highlights the escalating impacts of climate warming.

"Impacts escalate under different climate warming scenarios. There is an increase in impact between the current climate and +2C of warming; but impacts increase disproportionally under +4C of warming," says Dr. Pottier.

"This step-change impact severity shows that going above +2C of global warming can be a tipping point where we may see a lot of local extinctions," he says.

Local extinctions can lead to ecological repercussions, such as reshuffling community compositions, eroding genetic diversity and impacts to the food chain and health of the ecosystem.

"Some amphibian populations may undergo range shifts to more hospitable habitats, but opportunities for this are likely limited due to low dispersal rates and reliance on water bodies," says Dr. Pottier.

"Amphibians are an important part of the ecosystem. For example, the loss of an amphibian population would likely lead to an increase in insect population with carry-over effects on plants and animals. They are prey for many animals and their loss would have knock-on effects on many other species," says Dr. Pottier.

"Besides ecosystem impacts, amphibians are deeply rooted in human cultures and it would be a shame to lose such beautiful and emblematic species."

Next steps

Microhabitat selection is important for amphibians as some species live primarily on the ground, in vegetation, or in water; and some can move between those different habitats.

The study's habitat-specific predictions offer clear management priorities for conservation managers.

"Our analyses made it clear that vegetation and water bodies are critical in buffering amphibians during heat waves," says Dr. Pottier.

"We found that if you provide amphibians with enough water and enough shade, a lot of them can survive extreme heat events. We must protect and restore the environments that allow them to regulate their body temperature," Dr. Pottier says.

"We used very conservative estimates in this study assuming access to cool shaded environments. Therefore, the impacts of global warming will likely exceed our projections," says Dr. Pottier. "So all efforts to limit global warming are needed to protect the world's amphibians."