Brown University biologists seeking to better understand the effects of climate change on plant species in tropical mountain regions found that even small variations in temperature and moisture can have enormous consequences, threatening not only the plants that live there, but also the ecosystems they support. to support.
Emily Hollenbeck, who conducted the research while pursuing her Ph.D. in ecology and evolutionary biology from Brown, made the discoveries through a series of laborious but informative experiments conducted in Costa Rica’s Monteverde Mountains. Hollenbeck is committed to learning how climate change affects tropical forests and, as part of her dissertation, spent five years leading research to document the occurrence of plant species called epiphytes on three mountains in Costa Rica and Panama. On one of the mountains, she transplanted plant species to locations that varied in altitude, temperature and dryness, then observed and quantified the effects on the plants.
According to the research, published in Nature communication, most epiphyte species struggled to survive outside their native range in climate conditions even slightly different from what they typically experience. The researchers concluded that their work reinforced previous suspicions about the risks of widespread extinction due to climate change in tropical mountain ecosystems.
“It’s already clear to people who live here, even those who aren’t biologists, that the natural world has changed dramatically over the past 20 to 30 years in response to climate change,” said Hollenbeck, who is now president of the Monteverde Conservation League, a Costa Rican nonprofit organization focused on the conservation and restoration of tropical ecosystems and their biodiversity. “It felt very important to conduct a very specific, well-controlled scientific study to provide context and evidence to support what we are seeing.”
Canaries in a coal mine
Although it is widely accepted that climate change poses the risk of extinction of a variety of species, there is limited understanding of these dynamics. That’s especially true for tropical mountain regions that are home to some of the world’s most diverse ecosystems, said co-author Dov Sax, a Brown professor of ecology, evolution and organismal biology who served as Hollenbeck’s dissertation advisor.
Sax said most estimates of extinction risk from climate change are based on correlative, statistical models that consider what kind of climate conditions species are currently experiencing and whether those conditions are likely to occur anywhere nearby in the future. While these approaches work well in the United States and Europe, where the precise geographic distribution of species is well studied, Sax said they are poorly suited to tropical environments. There, the ranges of most species are often poorly documented, and it is unclear whether species can tolerate conditions different from those found in their current locations.
“The field does not have a good understanding of how great the extinction risk for tropical organisms might be under different levels of climate change,” Sax said.
The study directly addresses these limitations for tropical epiphytes in the mountains of Costa Rica and Panama, Sax said — and it took an extraordinary amount of work.
Hollenbeck, with the help of Brown research assistants, carefully examined the distribution of about 70 species of epiphytes across three mountain ranges. The researchers climbed trees and traversed mountainous terrain to transplant about 1,500 individual epiphytes from 15 different species to different altitudes and climatic conditions above and below the altitudes where they normally survive. The researchers monitored the plants every three months for three years.
“We found that these species are really as sensitive to small changes in climate conditions as was widely feared, and that some previous, much smaller-scale experiments in the past had found,” Sax said.
Even the plants with the widest survival range, which were predicted to be the least vulnerable to change, fared worse than the researchers expected.
Furthermore, the findings suggest starkly different outcomes than the temperature conditions expected by 2100 under different climate change scenarios. Under temperatures associated with low emissions scenarios determined by the Intergovernmental Panel on Climate Change (1.5 degrees Celsius above pre-industrial levels), most plant species studied will survive. But under emissions scenarios that are moderately high (3.2 degrees Celsius above pre-industrial levels), 5% to 36% of species studied would become extinct from all mountains in the region and 10% to 55% of species will disappear from the mountain on which they were studied.
“It’s a bit alarming how close we are to a threshold or tipping point for these tropical species,” Sax said, noting that Earth’s surface temperature has already risen by 1 degree Celsius since pre-industrial times. “On the one hand, this gives us hope if we can really slow climate change dramatically, but on the other hand, it shows how little additional warming is needed to lead to the mass extinction events we feared would occur.”
Sax added that with moderate warming, a third of all epiphytes could be lost by the end of the century, which could have far-reaching and unpredictable downstream consequences for the ecosystem and ultimately for the surrounding population and economy.
Hollenbeck, who lives in Costa Rica and teaches science, manages research grants and designs curricula for Avenues the World School, said the epiphytes in the tropical forest are like the canaries in a coal mine.
“Obtaining this level of concrete data required a mind-boggling amount of fieldwork over a long period of time and applies to about 70 plant species,” she said. “But this study only scratches the surface in terms of how climate change affects different species.”
