Longer periods of drought threaten Brazilian amphibians

Brazil is home to the world’s greatest diversity of amphibians: Of the more than 8,000 species known worldwide, some 1,200 are found in the country, mainly in the Amazon and Atlantic Forest biomes. But their future is at risk due to longer dry periods in their habitats.

Drought and amphibians are not a viable combination: These animals depend on water and humidity to survive; without that, they may dehydrate in a few hours and die.

“Amphibians have thin, permeable skin, unlike other land animals such as humans, birds, reptiles and insects. Their moist skin allows water and other substances to pass through, making it more sensitive to dry air. This is why you don’t see frogs exposed to the sun for long periods of time — they dry out much faster than other animals,” explains biologist Nicholas Wu, a researcher at Australia’s Western Sydney University. With the help of other scientists, Wu mapped areas of the planet where the group of water-sensitive vertebrates known as anuran amphibians — frogs, toads and tree frogs — will face drier and more arid conditions in the future, which could lead to a greater risk of population decline and even extinction.

“We found that many parts of the world where anurans live are at risk of becoming more arid and suffering from increased droughts. We also found that the combination of higher temperatures and drier environments will have major impacts on their activity window; that is, the hours when the weather does not prevent anurans from foraging and breeding,” the researcher says.

Wu’s study, published in Nature Climate Change, indicates that up to 33% of anuran habitats could become more arid by 2100. In a global warming scenario, if the climate becomes 4º Celsius (7.2° Fahrenheit) warmer in the future, 36% of these habitats will experience longer-lasting droughts, with an increase of up to four months per year.

Also according to the analysis, stronger droughts are expected to double the rates of water loss through the skin of these animals. In the future, the combination of drought and warming is expected to halve the time in which anurans would be active.

And this reality is already being seen by biologists. “Most species are not active all year round. In Brazil, it was common to notice greater activity by many anurans during the first rains in September,” says Rafael Bovo, a researcher at the University of California, Riverside and one of the co-authors of the study. “However, in recent years, when we carried out sampling work in the field, we often observed delays in both the rainy season and the period when these animals are more active.”

The Brazilian scientist points out that by becoming active later, these amphibians may have less time for biological development — for example, of their sexual organs — to carry out the so-called reproductive courtship, to find food in order to have energy and, finally, to find partners.

Concern about southern Amazon & Atlantic Forest frogs

The analysis, based on several climate scenarios and the distribution of more than 5,000 species of anurans, indicates that several regions of the planet with large concentrations of these animals will be affected by aridification in the coming decades, but a greater impact will be seen in parts of Africa and particularly South America.

What really concerns those involved in the study is that there are many species of amphibians in the forests of the South American continent, and many are endemic, that is, they do not exist anywhere else, as is the case of the Guinle treefrog (Aplastodiscus leucopygius) and the red pumpkin toadlet (Brachycephalus pitanga), which only occur in the Atlantic Forest.

“The biome is home to more than 700 species of anuran amphibians known in Brazil, and more than 50% of them are endemic,” Bovo emphasizes. “When you think about a scenario in the Atlantic Forest, with the loss of almost 90% of its original vegetation, and now with even faster climate deterioration, many of these species could disappear.”

In the Amazon, the alert falls on the area known as the Arc of Deforestation, in the south of the biome, where there is greater expansion of agricultural activities. “This is the area on our map with critical patches where there will be an increase in droughts, and this region has been suffering from a historical process of deforestation since the 1970s, including the construction of roads, which destroy not only the forests but also other important habitats for anurans,” Bovo points out.

Adaptation and evolution: an unknown

Amphibians are essential to their ecosystems. The many environmental services they provide include regulating insect populations such as mosquitoes, known disease vectors, cycling nutrients and sustaining various food webs.

In a warmer and drier climate, the question is whether there will be time for these animals to adapt or evolve over generations to survive these new conditions.

“It is possible that amphibians will adapt to a warmer world, but they need time to do so. Currently, the rate of warming exceeds their ability to adapt, which is why many frogs are at risk of extinction,” Wu says. “If we provide some environmental protection, for example by restoring forests, this will provide suitable microclimates for amphibians to adapt to our rapidly changing world.”

The researchers stress that studies like this are important to help develop public conservation policies while these species still exist, before it is too late and they have already gone extinct.

The next step is to better understand how some animals are more resilient than others. As examples, Bovo cites species with specific adaptations such as those that produce wax on their skin and can withstand adverse situations without dehydrating too quickly and others that bury themselves, creating a cocoon with accumulated dead skin as a physical barrier to reduce water loss.

“Many of the studies conducted in recent decades on climate change have focused only on temperature changes and not on the separate or combined effects of heat and drought, as our study does. Furthermore, we are learning that it is not just the average that matters; we also need to take into account temperature and humidity variations and the more frequent occurrence of extreme events,” the biologist emphasizes. “That is why it is not easy to answer the simple question of whether all or only some groups of animals that depend on water, such as amphibians, will survive climate change,” he concludes.

Banner image: The lesser treefrog (Dendropsophus minutus), a species that inhabits the forests of South America. Image by Reuber Brandão via Wikimedia Commons (CC BY 4.0).

Study shows dire outlook for amphibians: 40% threatened with extinction

Citation:

Wu, N. C., Bovo, R. P., Enriquez-Urzelai, U., Clusella-Trullas, S., Kearney, M. R., Navas, C. A., & Kong, J. D. (2024). Global exposure risk of frogs to increasing environmental dryness. Nature Climate Change, 14(12), 1314-1322. doi:10.1038/s41558-024-02167-z

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Roberto Cataldo
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