Adapting Species Risk Assessments to a Changing Climate: The Underestimated Vulnerability of Foundational Trees

Climate change is one of the top threats to biodiversity. However, many species risk assessment frameworks, including the globally authoritative International Union for Conservation of Nature Red List, do not consistently and comprehensively incorporate loss of climatically suitable habitat into listing or threat categorization decisions. Using California's foundational, endemic and near-endemic trees as an exemplary set of high biodiversity and cultural value species (N = 27), we reviewed and synthesized the literature on species' model-projected changes in suitable habitat, and on field-observed spatial and temporal demographic responses to climate change. We demonstrate multiple approaches to using this existing literature to integrate climate threats into species risk assessments, with a focus on the Red List criteria. Over the next century, our focal species are projected to lose between half and three-quarters of their existing suitable habitat, which would lead to substantial impacts on ecosystem structure and function in forests, woodlands and savannas. Overall, species are more vulnerable than is reflected by their current Red List status. Even under conservative estimates of climate change impacts, two-thirds of our focal species could now meet the Red List A3 Criterion for Endangered or Critically Endangered status, because of projected loss of climatically suitable habitat (a six-fold increase). We also found a positive relationship between species' model-projected risk and observed demographic responses to climate change, indicating that in many cases, projected responses are already evident on the ground. To inform conservation prioritization, we visualized the geographic patterns of species' suitable habitat loss using heatmaps. Our results illustrate the need to revise species risk assessments in light of climate change to better align research and conservation priorities with species' vulnerability. They also highlight the conservation needs of currently widespread but high climate-risk species, and the regional threats to ecosystem structure and function from foundational species loss.