PROVIDENCE, R.I. [Brown University] — Brown University biologists who set out 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 massive impacts, threatening not only plants that live there, but also the ecosystems they support.

Emily Hollenbeck, who conducted the research while earning her Ph.D. in ecology and evolutionary biology from Brown, made the discoveries through a series of laborious yet informative experiments conducted in the Monteverde mountain region of Costa Rica. Hollenbeck is devoted to learning how climate change affects tropical forests and as part of her dissertation work, spent five years leading research to document the occurrences of plant species called epiphytes on three mountains in Costa Rica and Panama. On one of the mountains, she transplanted plant species among sites that varied in elevation, temperature and aridity, and then observed and quantified the effects on the plants.

According to the study, published in Nature Communications, most epiphyte species struggled to survive outside their native ranges in climate conditions even slightly different from what they typically experience. The researchers concluded that their work strengthened earlier conjecture about risks of widespread extinctions from climate change in tropical mountain ecosystems.

“It's already obvious to people who live here, even those who aren't biologists, that the natural world has been shifting in striking ways over the last 20 or 30 years in response to climate change,” said Hollenbeck, who is now president of the Monteverde Conservation League, a Costa Rican nonprofit focused on conserving and rehabilitating tropical ecosystems and their biodiversity. “It felt really important to conduct a very specific, well-controlled scientific study to provide context and evidence to support what we’re seeing.”

Canaries in a coal mine

While it is widely understood that climate change poses extinction risks for a variety of species, there is limited understanding of these dynamics. That’s especially the case for tropical mountain regions home to some of the world’s most diverse ecosystems, said study co-author Dov Sax, a Brown professor of ecology, evolution and organismal biology who served as Hollenbeck’s dissertation advisor.

Sax said that most estimates of extinction risk from climate change are based on correlative, statistical models that consider what sort of climate conditions species currently experience and whether those conditions are likely to be somewhere nearby in the future. While these approaches work well in the United States and Europe where species’ precise geographic distributions are well studied, Sax said they are poorly suited to tropical settings. There, the distributions of most species are often poorly documented, and it is unclear if species can tolerate conditions that differ from those occurring at their current locations.

“The field doesn’t have a good handle on just how bad the extinction risk to tropical organisms might be under different levels of climate change,” Sax said.

The study directly addresses these limitations for tropical epiphytes in mountains of Costa Rica and Panama, Sax said — and it took an extraordinary amount of work.