Recent research from the University of Illinois Urbana-Champaign and George Mason University reveals that shifting rainfall patterns in the Neotropics, one of the planet’s most diverse ecosystems, could endanger two-thirds of its bird species by 2100, should climate change continue unabated. This is particularly alarming as the area is home to 30% of the world’s bird species.
Jeff Brawn, a co-lead author of the study published in Global Change Biology, emphasizes that birds are just one aspect of a much larger issue.
“When comparing a more hopeful rainfall scenario with the ‘business-as-usual’ outlook, we anticipate a potentially disastrous outcome for forest-dwelling birds in the Neotropics. However, birds serve merely as an example in our research. Similar threats are likely to impact mammals, reptiles, amphibians, arthropods, fungi, and plants. Additionally, the repercussions for agriculture could be significant,” remarked Brawn, a professor emeritus in the Department of Natural Resources and Environmental Sciences at Illinois.
Previously, Brawn and his team studied the effects of prolonged dry seasons on 20 bird species in Panama, discovering markedly reduced population growth rates in several of them. Although they used a robust 30-year dataset, Brawn realized that 20 species were only a small sample. Would the trend apply to all 3,000 forest-resident bird species within the Neotropics?
To gain more insight, Brawn collaborated with David Luther, an associate professor in George Mason University’s College of Science, along with other researchers including UCLA’s Rong Fu, to project changes in rainfall patterns by 2100 in the Neotropics under two different climate scenarios. The ‘business-as-usual’ scenario, termed SSP-8.5, outlines a bleak future without any carbon reduction efforts, while SSP-2.6 illustrates the positive outcomes of vigorous mitigation actions and a swift shift towards a low-carbon economy.
The team examined how these scenarios could either extend or shorten the duration of the dry season, as birds already face various stressors during dry spells, including challenges in securing food, weight loss, delayed breeding, and heightened mortality rates.
“Changes in precipitation could greatly affect ecological systems, particularly regarding biodiversity, which has often been overlooked in favor of temperature impacts,” Luther stated. “Precipitation has been neglected for too long.”
By mapping anticipated shifts in dry season lengths across the Neotropical region—which includes Central America, the Caribbean, and South America—the team superimposed the distributions of approximately 3,000 forest bird species to assess how their habitats might coincide with these changes.
“We determined where the dry season is expected to lengthen or shorten, to what degree, and how many species currently residing in those areas would be affected. This included species that may be uniquely vulnerable because they have no alternative habitats,” Luther noted. “In a final step, we analyzed global protected areas to see how much they would dry out or become wetter.”
According to the SSP-8.5 projections, the dry season is anticipated to lengthen by at least 5% across three-quarters of Neotropical lowland forests, affecting nearly 2,000 resident bird species. In stark contrast, under the more hopeful scenario, only 10% of lowland forests would experience drier conditions, impacting just 90 bird species.
Longer dry seasons can hinder birds’ access to food, their reproductive success, and survival rates, but fewer rainy days could also fundamentally disrupt tropical ecosystems and species distributions.
“Consider a closed canopy rainforest. If it becomes too dry, it’ll reach a point where it opens up and turns into a savannah,” Brawn explained. “Many forest birds won’t thrive in savannahs, while savannah birds might prosper instead. It’s possible that certain closed-canopy forest species could adapt better to more open environments. The outcomes remain uncertain.”
The maps reveal which regions will be most adversely affected under both scenarios, but Luther suggests another perspective on the information.
“Another crucial way to interpret these maps is to identify locations that will remain more stable in the future. For effective conservation planning, we should focus additional resources on these areas, as they’re less likely to experience drying. This is vital for prioritizing existing protected regions and establishing new ones,” he advised. “We hope conservation groups will take this into account.”
The researchers aim for the minimal predicted impacts associated with SSP-2.6 to inspire policymakers to take swift action, although climate legislation is just one piece of the larger puzzle.
“In fact, this paper might be overly optimistic because we haven’t considered deforestation at all. If the number of trees decreases, it will become hotter and drier,” Brawn cautioned. “Regrettably, thousands of hectares of tropical forest vanish each year, making the situation even more concerning.”
Luther adds, “The silver lining is that these are all problems we can address if we choose to. We have the power to make the right choices.”
