Tropical mountains host some of our planet's hottest biodiversity hotspots. Ascending just a few hundred metres up an equatorial slope can reveal canopies teeming with distinct bird populations, such as hummingbirds, toucans and owls at each narrow slice of mountain side you cover. These differences yield stunning biodiversity, but we don't yet fully understand why tropical birds seem confined to such slim slivers of elevation.
Historically, people have pinned climate as the culprit. Tropical lowlands are typically hot while the highlands run much cooler and temperatures tend to remain seasonally stable, which creates narrow and distinct climatic niches along a mountain's slope. It's reasonable to predict that birds would prefer to live at temperatures that are just right for them – their ‘Goldilocks’ zone. Competition between different species of birds, however, can also constrain elevational ranges. For example, research has documented narrow elevation ranges of animals that live on mountain regions rich in species but broader ranges on mountains with fewer species. Benjamin Freeman and colleagues from University of British Columbia, Canada, and the Cornell Lab of Ornithology, USA, set out to measure the relative importance of these two hypotheses – climate and competition – for elevational range breadth using a global comparative approach of tropical bird species across 31 montane regions, from the Northern Rockies in the USA to the Southern Alps in New Zealand.
The researchers, taking advantage of the arsenal of data available on eBird – a global citizen science project that logs avian sightings – accessed 4.4 million detailed records to define the elevation range within which each species lives. They found that the number of species present in a region (that is the species richness or range overlap with other species) is a stronger predictor of elevational range than is temperature seasonality (or stability throughout the year). For example, consider two regions on opposite slopes of the Andes at the equator: the west and the east. These regions host similar temperature seasonality and mountain heights. The eastern slope, however, hosts 120 more species of birds but each species is spread over a significantly narrower elevational range, which is consistent with the competition hypothesis.
Next, the authors examined the specific mechanisms by which competition could lead to the birds’ narrower elevational ranges in more biodiverse locations. To do this, they analysed the territorial push and pull within pairs of closely related species that inhabit different elevational zones. They found in nearly half the cases (23 of 52) – and mostly in cases where birds show territorial behaviours – that related but competing species limit each other's ranges. For example, when two closely related species of Campephilus woodpeckers inhabit the same range, they avoid each other, and Campephilus haematogaster pass up the opportunity to inhabit elevations they would normally occupy if residing there alone. Clearly, rivalries between tropical bird species have a major impact on where they choose to live.
Despite over a century of investigation, the role of biological factors versus nonbiological factors in setting species ranges remains contentious, but competition certainly plays a role. This study also provides insights into how and why tropical birds (and possibly other tropical animals, from mammals to reptiles and amphibians) are responding to climate change. As a consequence of global warming, we've witnessed many dramatic species-wide shifts upslope to higher and cooler elevations. Most mountains, however, are shaped like pyramids with less and less suitable land at higher elevations, leading to increased competition. Thus, upslope range shifts generally lead to progressive declines in population size – a so-called ‘escalator to extinction’. The degree to which the prevailing predictions of widespread extirpations of tropical species will come to pass depends on our ability to understand both biological and nonbiological influences on community structure, such as climate and competition.