The study took place in Panama’s Soberania National Park, an approximately 100-square-mile area of protected rainforest that is home to more than 500 bird species.
Over the course of the study, researchers caught more than 250 different species in mist nets, but only had enough data to model 20 of the most common.
For 19 of the 20 species sampled, a longer dry season had a negative effect on population size. For six of those species, the effect was especially strong.
Biodiversity in tropical forests is particularly high — and, it appears, particularly sensitive to the impacts of climate change.
Understanding how tropical species will fare in a warming world requires long-term research, which is sorely lacking, especially when it comes to tropical fauna. But the results of a more than three-decade-long study, published in the journal Nature Climate Change last month, finds that tropical birds are highly vulnerable to environmental changes caused by global warming, such as disruptions in rainfall patterns and a longer dry season.
The amount and timing of rainfall is critical to how tropical ecosystems operate, and scientists generally agree that rainfall regimes will change over large areas of the tropics as global temperatures climb.
For the present study, researchers examined dry season length and its effect on the populations of 20 bird species in central Panama over a 33-year period. They found that longer dry seasons decreased the population growth rates and viability of all but one of the species they sampled.
“Because the tropics are relatively stable weather wise, tropical birds aren’t able to handle environmental disturbances as easily, physiologically or behaviorally, as temperate-zone birds,” Jeff Brawn, a University of Illinois ecologist and lead author of the Nature Climate Change paper, said in a statement. “Birds in the [American] Midwest have below-zero winters and 100-degree summers — environmental stress that tropical birds never experience. Consequently, tropical ecosystems and animal populations may be more vulnerable to the effects of climate change.”
Declining bird populations could have a ripple effect across the entire tropical forest ecosystem. Birds provide a variety of vital ecosystem services, such as eating insects and preventing damage to trees, dispersing seeds, and pollinating plants.
The study took place in Panama’s Soberania National Park, an approximately 100-square-mile area of protected rainforest that is home to more than 500 bird species. About 90 percent of the region’s annual rainfall occurs in the wet season, which typically lasts from late April to early January.
Over the course of the study, Brawn and team caught more than 250 different species in mist nets, but only had enough data to model 20 of the most common. “Capture-mark-recapture is the key,” Brawn said. “We let them go and then capture them again. How many we recapture is how we estimate survival rate and the changes in the size of the population.”
For 19 of the 20 species sampled, a longer dry season had a negative effect on population size. For six of those species, the effect was especially strong. The scaly-throated leaftosser (Sclerurus guatemalensis) was the only bird species that tended to increase with drier conditions, the researchers found. “That one seemed to have a favorable reaction to changes in seasonal drought. We don’t know why. It just consistently seemed to do better,” Brawn added.
In addition to looking at the relationship between population growth rates and the length of the dry season during the 33-year study period, Brawn and team projected their results across another 50 years with an average 10 percent annual change in rainfall patterns during the dry season. That 10 percent change only translates to a dry season that is longer by about 12 days after a half century. That’s not be a big difference in terms of time, but, Brawn said, it could have severe implications for Panama’s bird communities.
“And keep in mind that this study looked at just a small slice of the bird community in this forest,” he added. “There are hundreds of bird species who live in the upper canopy, high above the reach of our mist nets. They’re harder to capture so we don’t have data on those species or those that are rarer.”
Then there’s the fact that negative impacts on a large number of avian populations was observed by Brawn and team in a national park. “We worked in a good forest — that is, relatively intact. The study shows that even in a protected park, the large, global effect of climate change could make a lot of habitat unsuitable for a lot of species,” Brawn said.
The fact that these impacts were discovered in the majority of tropical bird species sampled even in protected habitat — i.e., in the absence of habitat loss — should be significant to conservationists. Brawn and team say their findings suggest the need for collaboration between climate scientists and conservation biologists to identify and preserve those areas where future rainfall regimes will be sufficient to maintain wildlife populations.
As was the case with the scaly-throated leaftosser, some species will do well as the tropics heat up, even while other species do poorly. But the bottom line, according to Brawn, is that the tropics will be very different than they are today: “We’re not saying it will be a silent forest but it will sound dramatically different 100 years from now with songs from only those species who persist.”
- Brawn, J. D., Benson, T. J., Stager, M., Sly, N. D., & Tarwater, C. E. (2016). Impacts of changing rainfall regime on the demography of tropical birds. Nature Climate Change. doi:10.1038/nclimate3183