The global distribution patterns of alien vertebrate richness in mountains

The diverse biotas of the world's mountains face a challenging future due to increasing threats like climate change, land-use change, and biological invasions, the last being particularly understudied in these regions. Here we compile occurrence records for 717 alien vertebrate species distributed in 2984 mountains worldwide. We analyze their distribution, biogeographic origin, presence in protected areas, and the drivers' explaining alien vertebrate richness in mountains. We find that the alien vertebrates most frequently recorded are birds (318 species) and mammals (161 species) reported in 2595 and 1518 mountains globally, respectively. The Palearctic, Nearctic, and Australasian realms are the most common recipients; the Nearctic, Indo-Malay, and Afrotropic realms are the most frequent donors. Almost 50% of the alien species studied also occur in protected areas. Proxies of anthropogenic impacts (e.g., higher road density or lower biodiversity intactness) and mountains' physical characteristics (e.g., elevation range and roughness) explain the distribution of alien vertebrates in mountains. Importantly, the magnitude of invasions in tropical mountains could be underestimated due to sampling bias towards the Northern Hemisphere and Australia. Our large-scale assessment reveals the advance of alien vertebrates in mountains worldwide and urges attention to minimize the impacts of biological invasions on the exceptional mountain biotas.