Contrasting vulnerability profiles for insular assemblages of birds and mammals facing global change

Island biota is currently threatened by global anthropogenic pressures. The limited ability of island species to track suitable conditions and their inherent sensitivity to anthropogenic changes make insular biota highly vulnerable to global threats. Although vulnerability assessments of species to single threats individually have been conducted, the vulnerability of island faunas to multiple global change drivers has not. We evaluated the relative vulnerability of 266 insular bird and mammal species from 45 islands in six well-studied archipelagos to multiple threats. We first quantified the current exposure of islands to climate change, land-use change, and biological invasions. We then calculated insular assemblage sensitivity based on species' ecological characteristics, such as diet and habitat specialization, generation length, and geographical rarity. To assess the adaptive capacity of insular species in response to these threats, we examined biotic and abiotic features, such as species' dispersal ability, terrain heterogeneity, and proportion of protected area. Exposure and adaptive capacity markers varied greatly among the six archipelagos, but the mean sensitivity of assemblages was similar across islands. Hawai'i, the Azores, and the Mascarenes had high vulnerability scores. Climate change and biological invasions were dominant threats in Hawai'i, and land-use change was the dominant threat in the Mascarenes, Azores, and Canaries. Assemblages from the Galapagos and Tristan da Cunha had lower vulnerability to current threats. The differences in vulnerability among island assemblages, mostly arising from differences in exposure and adaptive capacity, mean these islands' conservation needs differ. Our results delineate the mechanisms behind the vulnerability of insular biota under global change, a necessary step to effectively preserve island biodiversity and its associated human benefits.