A new study contributes to the understanding of why tropical mountain birds inhabit such narrow elevation ranges, a mystery that has perplexed scientists for centuries. While many people assumed that temperature was to blame for these restricted distributions, new research suggests that competition from other species plays a larger role in shaping bird ranges.
Researchers from the University of British Columbia and the Cornell Lab of Ornithology conducted the study, which included 4.4 million citizen science observations of 2,879 bird species from around the world. The findings were published in the journal Science.
Competition with other species, not climate, drives the distribution of bird biodiversity in tropical mountains, one of Earth’s hottest biodiversity hotspots, according to a new study of 31 mountain regions across the globe.
The findings imply that species interactions have played a much larger role in shaping tropical mountain biodiversity than previously thought, and they provide new insights into how tropical mountain species are responding to climate change. Tropical mountains are among the most biodiverse areas on the planet, with entirely different sets of species living only within narrow elevational ranges – a pattern that differs greatly from that of temperate mountains, which have broader elevational ranges.
You have this incredible biodiversity in mountain ranges, especially in the tropics. From one vista point in the Andes, you can see a mountain slope that’s home to as many species as there are in the entirety of North America. We wanted to know, how does that work?
Benjamin Freeman
“You have this incredible biodiversity in mountain ranges, especially in the tropics. From one vista point in the Andes, you can see a mountain slope that’s home to as many species as there are in the entirety of North America,” said lead author Benjamin Freeman, a postdoctoral fellow at the University of British Columbia. “We wanted to know, how does that work?”
The range data was obtained by Freeman and his colleagues by analyzing records from eBird, a citizen science project run by the Cornell Lab that contains sightings from hundreds of thousands of birdwatchers around the world. This allowed them to examine the ranges of more than a quarter of the world’s bird species spread across five continents, on a scale previously unimaginable to researchers.
“Aside from eBird, you just have really coarse range maps, especially on a global scale,” said Cornell Lab co-author Eliot Miller. “The eBird database is unique in both space and time, providing us with more insight into bird distribution around the world than any other organism.”
The researchers looked for connections between elevational range size and two factors: temperature stability throughout the year, and range overlap with other species. If range size was correlated with a consistent climate, this would indicate that birds are limited by their own biology — they have become so accustomed to a particular temperature that they cannot survive elsewhere. If birds occupied smaller ranges where they overlapped many other species, then increased competition for resources may be restricting them.
The findings showed that a stable climate did not predict a smaller range size. However, there was strong evidence that when there was more overlap among the ranges of a greater number of species, the ranges were smaller.
Despite his suspicion that competition played a role, Freeman says he was taken aback by the clarity of the results. “I had noticed some patterns that suggested competition might be important in bird distribution,” he said, “but I was still surprised to find such a strong signal in this study.”
In many cases, closely related species also restricted one another’s distributions. For example, the Ornate Hawk-Eagle, a massive raptor of tropical forests, occupied a larger span of elevations in areas where it did not coincide with the similar Black-and-chestnut Eagle. When the two inhabited the same mountain range, they avoided one another, with the Ornate Hawk-Eagle passing up elevations it would normally favor elsewhere in its range.
However, some researchers argue that, in addition to climate effects, interspecific competition may limit the elevational ranges of tropical mountain species. To assess these two competing hypotheses, Benjamin Freeman and colleagues used 4.4 million fine-scale locality records from eBird, a global citizen science database of bird distribution and abundance, to conduct a comparative analysis of forest bird species’ elevational ranges within 31 mountain regions around the world.
While this study sheds light on one aspect of mountain species’ ranges, the authors suggest that much more research is needed. Many details about how other aspects of a bird’s ecosystem influence its distribution remain unknown.
