Priorities for conserving the world's terrestrial mammals based on over-the-horizon extinction risk

Three major axes of global change put the world's mammal biodiversity at risk: climate change, human population growth, and land-use change.^{1,2,3,4,5,6,7,8,9,10,11,12} In some parts of the world the full effects of these threats on species will only be felt in decades to come, yet conservation emphasizes species currently threatened with extinction, by threats that have already occurred. There have been calls for conservation to become more proactive by anticipating and protecting species that may not yet be threatened, but have a high chance of becoming threatened in the future.^{3,6,8,10,12,13,14} We refer to this as "over-the-horizon" extinction risk, and we identify such species among the world's nonmarine mammals by considering not only the severity of increase in threats faced by each species, but also the way each species' biology confers sensitivity or robustness to threats. We define four future risk factors based on species' biology and projected exposure to severe change in climate, human population, and land use. We regard species with two or more of these risk factors as especially vulnerable to future extinction risk.^{10,15,16,17,18,19} Our models predict that by 2100 up to 1,057 (20%) of nonmarine mammal species will have combinations of two or more future risk factors. These species will be particularly concentrated in two future risk hotspots in sub-Saharan Africa and southern/eastern Australia. Proactively targeting species with over-the-horizon extinction risk could help to future-proof global conservation planning and prevent a new wave of mammal species from becoming threatened with extinction by the end of this century.