Taking stock of climate-driven risks to mountain biodiversity

Mountain biodiversity is rapidly reorganizing as species migrate upslope to track climate warming. Despite the potential threats of mountaintop extirpation, range shift gaps, and lowland biodiversity attrition, empirical evidence of these risks remains scarce. We analyzed 8,800 records of historical and modern elevational range limits for 440 animals and 1,629 plant species and found that the risk of mountaintop extirpation did not exceed random expectations. Upper limits expanded for species with narrow ranges or lowland affinities, but lower limits showed little contraction, implying scant risk to date of these threats, even in the tropics. Yet the predominance of upslope expansion combined with delayed mountaintop extirpations points to a biotic homogenization, which may profoundly alter biotic interactions in mountain ecosystems with increasing warming.