Consistent Extinction Risk Assessment under the US Endangered Species Act

Identifying species at risk of extinction is essential for effective conservation priority-setting in the face of accelerating biodiversity loss. However, the levels of risk that lead to endangered or threatened listing decisions under the United States Endangered Species Act (ESA) are not well defined. We used a Bayesian population modeling approach to estimate levels of risk consistently for 14 marine species previously assessed under the ESA. For each species, we assessed the risks of declining below various abundance thresholds over various time horizons. We found that high risks of declining below 250 mature individuals within five generations matched well with ESA endangered status, while number of populations was useful for distinguishing between threatened and "not warranted" species. The risk assessment framework developed here could enable more consistent, predictable, and transparent ESA status assessments in the future.