Competition, not climate, limits bird species’ ranges in tropical mountains

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 suggest that species interactions have played a much larger part in shaping tropical mountain biodiversity than previously recognized and provide new insights into how tropical mountain species are responding to climate change. Tropical mountains are some of the most biodiverse regions on Earth, often with entirely different sets of species that live only within narrow elevational ranges – a pattern much different than for species in temperate mountains, which tend to have broader elevational ranges. It’s widely thought that this patterning is a result of adaptation to the low-temperature seasonality of tropical climates. While tropical mountain temperatures range from hot in the lowlands to cold in the highlands, they remain relatively stable at any given elevation throughout the year, creating various climatic niches along a mountain slope; physiological adaptations to these myriad niches result in the accumulation of high numbers of species in tropical mountains. However, some researchers have argued that, rather than climate effects, interspecific competition could also limit tropical mountain species’ elevational ranges. To evaluate these two competing hypotheses, Benjamin Freeman and colleagues performed a comparative analysis of forest bird species’ elevational ranges within 31 mountain regions across the globe, using 4.4 million fine-scale locality records from eBird – a global citizen science database of bird distribution and abundance. Contrary to the leading hypothesis, Freeman et al. found that species richness is a better predictor of smaller elevational ranges than temperature seasonality, suggesting that the narrow elevational ranges of tropical birds are driven more by species interactions and competition than climate. However, according to the authors, whether these patterns generalize to other non-bird taxa is a key unanswered question.