Climate change threatens treefrogs

Researchers analyze potential effects of climate change on distribution of four species occurring in most of South America expected for 2050.

By Peter Moon  |  Agência FAPESP – Climate change in the twenty-first century is a threat to biodiversity, placing many species of plants and animals at risk across the planet.

Biologists Tiago Vasconcelos and Bruno do Nascimento, affiliated with the Department of Biological Sciences at São Paulo State University (UNESP) in Bauru, São Paulo, Brazil, have just published a study of the potential effects of climate change on the distribution of four species of treefrog found in many parts of South America expected by 2050.

The article, “Potential Climate-Driven Impacts on the Distribution of Generalist Treefrogs in South America”, published in the journal Herpetologica, describes some of the findings of a broader project that aims to predict the impact of climate change on the distribution of 350 species of amphibians native to the Atlantic Rainforest and 150 species occurring in the Cerrado to determine priority areas for conservation of these species. The project is entitled “Macroecology of anuran amphibians from Cerrado and Atlantic Forest” and is supported by FAPESP.

The article highlights four treefrog species with generalist habits, meaning that they are not specialist feeders and that they are highly dispersed and are able to make use of different resources offered by the environment. The four species in question occur in both open and forest areas.

Dendropsophus minutus and D. nanus are among the widest-ranging South American frogs, occurring in almost all South American tropical and subtropical habitats east of the Andes.

Scinax fuscomarginatus is found in a diverse array of habitats, from the northern border of the Amazon basin to northwestern Argentina, while S. fuscovarius has a smaller geographic range than the others but occurs in diverse habitat types between central Brazil and central Argentina. The latter is one of the most common frogs in South America and is often found in anthropized environments.

The species’ ecological niches were modeled as a function of six climate variables: the annual mean temperature, the maximum temperature of the warmest month, the minimum temperature of the coldest month, the amount of annual precipitation, precipitation seasonality, and the amount of precipitation during the warmest three-month period of each year.

Present-day climate variables were obtained from the WorldClim database, while variables for the future scenario (2050) were gathered from four different Atmosphere-Ocean Global Circulation Models (AOGCMs) available via the General Circulation Model portal.

All variables were selected to determine the climatically suitable areas for survival of the four species, both now and in 2050.

According to the modeling algorithms, the predicted areas of occurrence varied from 3.3 million km² to 11.2 million km² in the case of D. minutus, from 2.1 million to 12 million in that of D. nanus, from 2.3 million to 13.6 million in that of S. fuscomarginatus, and from 2.6 million to 14.5 million in that of S. fuscovarius.

According to the researchers, the findings displayed an apparent disparity, predicting a smaller area of occurrence for the treefrog species that are most widely distributed today in terms of latitude. The explanation is that the use of different modeling algorithms inherently generates different predictions regarding potential areas of occurrence.

For this reason, the researchers decided to minimize the variability of the predictions generated by the different methodologies. The solution was to produce a consensus map of predicted distributions by omitting isolated areas that were over 400 km from any known occurrence of a species and combining those that remained.

Based on the climate change scenarios for 2050, all four species were predicted to experience a decrease in their geographic range, some very significantly. The species potentially most affected was D. minutus, with a 52% predicted loss of suitable habitat. Compared to their current distribution, S. fuscomarginatus and S. fuscovarius were predicted to lose 43% and 31%, respectively. The least affected species was D. nanus, with a 14% predicted decrease in suitable habitat by 2050.

Roughly speaking, the potential areas of occurrence for all four species overlapped. As a result, in the context of the climate change scenarios used, it was only to be expected that the species with the widest distribution would lose the most in terms of climatically suitable area. The clearest example is the 52% loss of climatically favorable area for D. minutus.

The reduction in total potential habitat area does not point directly to extinction. “We’re not saying these species will disappear,” Vasconcelos stressed. “If a species manages to respond to climate change with behavioral and physiological alterations, it should have a strong chance of surviving in its present-day habitat.”

Species that cannot respond sufficiently will tend to become locally extinct. In other words, populations may disappear from areas in which the climate is different for the species as a whole while surviving in areas where climate change is less significant.

“Organisms that inhabit the northernmost tropical regions already face a warmer environment than organisms living farther south. They’re already surviving at higher temperatures,” Vasconcelos said.

“Physiological studies show the upper limit for temperature tolerance in amphibians varying between 40°C and 45°C, so the northernmost treefrog populations are already living in a scenario close to their maximum thermal tolerance.”

If temperatures in these areas continue to rise as indicated by the models, there is a real chance that the species will not succeed in making the adaptations necessary to survive in a warmer environment, so they may face local extinction.

“As these species adapt to a warmer environment, there may be a change in the time of year when their level of activity peaks, which typically occurs during reproduction. Sometimes, they already have this capacity to adapt but don’t display it under present conditions in their current habitats because they don’t need to,” Vasconcelos said.

“Another way to ensure survival would be to move to a similar environment, but that depends on their capacity to disperse.”

Conservation strategies

The importance of this study, according to Vasconcelos, is that it shows what can be expected for generalist species. Most research of this kind focuses on specialist or endangered species.

One of the most worrisome findings is that these generalist frogs are not expected to find favorable areas in the Pantanal floodplain in 2050. “This is disquieting because it means there are poorer prospects for survival of specialist species in the area and even for other generalists in different groups of animals and plants,” Vasconcelos said.

The researchers also aim to propose conservation strategies. “We want to determine where new reserves and conservation areas should be created on the basis of our predictive models,” Vasconcelos said.

The article “Potential Climate-Driven Impacts on the Distribution of Generalist Treefrogs in South America” (doi: http://dx.doi.org/10.1655/HERPETOLOGICA-D-14-00064), by Tiago S. Vasconcelos and Bruno T.M. do Nascimento and published in Herpetologica, can be read at www.bioone.org/doi/10.1655/HERPETOLOGICA-D-14-00064.