UC Riverside

Abstract: This study analysed threats to federally and State‐listed endangered and threatened animal taxa in California, United States, and how threats varied by taxa, habitat use, spatial extent, severity, geographical region and endemic status using threat categories from the International Union for Conservation of Nature (IUCN) Red List Threats Classification Scheme and information from scientific literature and reports. A majority of the taxa evaluated were associated with freshwater habitats and were endemic to California. The most threatened taxonomic groups were fish, followed by mammals and birds. The number of threats was mostly evenly distributed across the State's three geographical regions (i.e., North, Central and South), and no single region had a disproportionately high number of endangered animal taxa. Freshwater taxa were the most affected in nearly every threat category, suggesting that freshwater taxa are more threatened than their terrestrial and marine counterparts. In descending order, the most prominent threats across all taxa were habitat loss, invasive species, climate change and altered hydrology. Threats identified as high severity also tended to have a high spatial extent and vice versa. This study shows that the numerous freshwater faunas in California are disproportionately affected by threats also found in other freshwater systems and Mediterranean‐climate regions, highlighting the scope of the freshwater biodiversity crisis in California. Managing priorities to target the most pervasive threats to endangered freshwater taxa documented in this study will help safeguard freshwater biodiversity against human threats in California and beyond.