Elevated extinction risk of cacti under climate change.

Cactaceae (cacti), a New World plant family, is one of the most endangered groups of organisms on the planet. Conservation planning is uncertain as it is unclear whether climate and land-use change will positively or negatively impact global cactus diversity. On the one hand, a common perception is that future climates will be favourable to cacti as they have multiple adaptations and specialized physiologies and morphologies for increased heat and drought. On the other hand, the wide diversity of the more than 1,500 cactus species, many of which occur in more mesic and cooler ecosystems, questions the view that most cacti can tolerate warmer and drought conditions. Here we assess the hypothesis that cacti will benefit and expand in potential distribution in a warmer and more drought-prone world. We quantified exposure to climate change through range forecasts and associated diversity maps for 408 cactus species under three Representative Concentration Pathways (2.6, 4.5 and 8.5) for 2050 and 2070. Our analyses show that 60% of species will experience a reduction in favourable climate, with about a quarter of species exposed to environmental conditions outside of the current realized niche in over 25% of their current distribution. These results show low sensitivity to many uncertainties in forecasting, mostly deriving from dispersal ability and model complexity rather than climate scenarios. While current range size and the International Union for Conservation of Nature's Red List category were not statistically significant predictors of predicted future changes in suitable climate area, epiphytes had the greatest exposure to novel climates. Overall, the number of cactus species at risk is projected to increase sharply in the future, especially in current richness hotspots. Land-use change has previously been identified as the second-most-common driver of threat among cacti, affecting many of the ~31% of cacti that are currently threatened. Our results suggest that climate change will become a primary driver of cactus extinction risk with 60-90% of species assessed negatively impacted by climate change and/or other anthropogenic processes, depending on how these threat processes are distributed across cactus species.