Will tropical biodiversity run dry under climate change? Two visions for the future

But Jeff Brawn, co-lead author of the Global Change Biology study, says birds are only part of the picture.

"Compared to a more optimistic future precipitation scenario, we predict the 'business-as-usual' scenario will be potentially catastrophic for resident forest birds in the Neotropics. But really, we're just using birds as an illustration in this study. We will likely see similar threats for mammals, reptiles, amphibians, arthropods, fungi, and plants. And the implications for agriculture are not insignificant either," said Brawn, professor emeritus in the Department of Natural Resources and Environmental Sciences in the College of Agricultural, Consumer and Environmental Sciences at Illinois.

Brawn and his colleagues previously analyzed the impact of longer dry seasons on 20 bird species in Panama, finding significantly lower population growth rates in several species. Although that study leveraged a robust 30-year dataset, he knew 20 species were only a drop in the bucket. Would the pattern hold for all 3,000 resident forest-dwelling bird species in the Neotropics?

Brawn teamed up with David Luther, associate professor in the College of Science at George Mason University, and others, including UCLA's Rong Fu, to forecast how precipitation will change by the year 2100 in the Neotropics under two climate scenarios. The business-as-usual scenario, known as SSP-8.5, represents a pessimistic vision of the future with no carbon mitigation. The team also modeled the impacts of SSP-2.6, which reflects aggressive mitigation action and a rapid transition to a low-carbon economy.

Because birds already experience greater physiological and behavioral stress during dry periods -- more difficulty finding food, weight loss, delayed reproduction, and greater mortality -- the team focused specifically on how SSP-8.5 and SSP-2.6 might lengthen or shorten that season throughout the region.

"Precipitation changes could have a huge impact on these systems, especially with regard to biodiversity, but temperature has dominated the climate change landscape until recently," Luther said. "Precipitation has been neglected for too long."

Mapping predicted changes in dry season length across the Neotropical region -- comprising Central America, the Caribbean, and South America -- the team then layered on the distribution of some 3,000 forest bird species to understand how their ranges might overlap with these changes.

"We were able to say where the dry season is going to get longer or shorter, and to what extent, and how many species currently living in those places will be affected. This included species that might be disproportionately affected because they don't live anywhere else," Luther said. "As a last measure, we overlaid the global layer of world protected areas and looked at whether they are going to get a lot drier or wetter."

The SSP-8.5 analysis projected that the dry season will lengthen by at least 5% across three-quarters of Neotropical lowland forests. Those conditions will impact nearly 2,000 species of resident birds, according to the results. In stark contrast, only 10% of lowland forests will get drier under the more optimistic scenario, with just 90 bird species exposed to longer dry seasons.

Longer dry seasons could make it harder for birds to access food resources, reproduce, and survive, but fewer rainy days could also have major implications for tropical ecosystems and species distributions on a more fundamental level.

"Think of a closed canopy rainforest. If it gets too dry, there's going to be a threshold where it'll open up and become a savannah," Brawn said. "A lot of the forest birds won't do well in savannahs, but the savannah birds will do better. And it's possible some closed-canopy forest species may do better in an open forest. We don't know how it will all play out."

The maps highlight which areas will be hardest hit under both scenarios, but Luther says there's another way to interpret the maps.

"An equally important way to look at this is to focus on places that are more stable into the future. If we're going to plan future conservation efforts, we should put extra effort into those areas because they're less likely to dry out. This will be especially important for prioritizing existing protected areas and potentially establishing new ones," he said. "Hopefully conservation organizations will pay attention to this."

The researchers hope the minimal predicted impacts under SSP-2.6 will motivate and expedite action by policymakers, but climate policy is only one piece of the puzzle.

"If anything, we're painting a rosy picture in this paper because we're not accounting for deforestation at all. If there's fewer trees, it's hotter and drier," Brawn said. "Unfortunately, thousands of hectares of tropical forest disappear every year, so the situation is actually more worrisome."

Luther adds, "The good news is these are all things we can solve, if we choose to. We can do the right thing."