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September 28, 2022

Interspecific Competition Draws Lines For the Elevations of Tropical Mountain Birds

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Halfway down the east slope of the Andes in Ecuador, sightings of the luminous yellow beaks of the chestnut-mandibled toucan are abruptly replaced by the colorful beaks of the white-throated toucan. Narrow elevation residences of tropical mountain birds like these have baffled scientists for centuries and have been assumed to be attributed to changes in temperature. 

However, a new study out of the Cornell Lab of Ornithology finds that these limited altitude residencies are due to interspecific competition where individuals of different species compete for the same resources. The study, published in Science, was conducted using eBird data from 4.4 million citizen science observations of 2,879 bird species around the world.

The tropical mountains are home to incredible biodiversity with unique and distinct species that inhabit very narrow elevation ranges. This is unlike temperature mountains, areas of mild temperature, where species commonly inhabit more broad elevation ranges.

Scientists have long assumed that the distinct distribution in the tropical mountains was attributed to changes in temperatures with elevation. The differences in climate, from the hot lowlands to the cold highlands, were believed to create a variety of ecological niches and a myriad of unique species. 

Now, the research team from Cornell and University of British Columbia show that interspecific competition plays a much greater role in shaping the biodiversity of tropical mountains than previously thought. The eBird data used revealed a strong correlation between interspecific competition and bird elevation inhabitation. 

For example, different species of birds fall into different elevation boundaries. At the elevation boundary, these birds strongly interact with one another which suggests that competition is important, according to co-author Eliot Miller. “If one [bird species] was removed, the other would expand into its range,” Miller said. Such data, collected across 31 montane regions supports the idea that competition, not climate, is the main factor that drives bird species into very narrow elevation ranges. 

These findings provide insight into the impact of climate change on tropical ecosystems. As global warming continues, birds in the tropical mountains will move up slope towards cooler climates. This upwards migration may start to directly affect competition between species, possibly resulting in a loss of certain species. 

“It’s more evidence that the web of interactions in these tropical ecosystems is really important in structuring how communities respond [to climate change],” Miller said.

In the future, Miller looks forward to investigating how speciation occurs between close relatives and why one group of individuals may be more dominant than the other, allowing them to occupy their ancestral climate while pushing others to another habitat. Others in the Cornell Lab of Ornithology, like Ben Van Doren, are interested in how tropical mountain ecosystems are shifting with climate change. 

Large datasets for research like this, couldn’t have been collected without contributions from citizen scientists, allowing researchers like Miller and others to continue monitoring bird populations around the world. 

“It’s really powerful that we have this many people in the world who are interested in nature because it means they’re willing to help protect it,” said Miller.