Avian Conservation and Ecology (Jun 2022)
Assessing extinction risk, conservation reliance, and down-listing potential for two endangered Hawaiian waterbirds
Abstract
Conservation reliant species complicate the idealized view of endangered species recovery by depending on perpetual management to maintain viable populations, making the goal of de-listing via threat removal potentially unattainable. The Hawaiian Stilt (Himantopus mexicanus knudseni) and Hawaiian Coot (Fulica alai) are conservation reliant, endangered waterbirds endemic to the Hawaiian Islands. The former is currently under consideration for down-listing to “threatened” under the United States Endangered Species Act. Down-listing criteria for both species include a low probability of extinction according to population viability analysis and a sustained population of over 2000 individuals. We investigated the population viability, equilibrium population size, and degree of conservation reliance of these two species using stochastic individual-based population models and Bayesian state-space analyses on abundance time series. For each species, we evaluated the probability of extinction (one sex remaining) and quasi-extinction (N 2000 birds. We found a low probability of extinction (0% and 11% for stilts and coots, respectively) or quasi-extinction (0% and 16%, respectively). However, sensitivity analysis of simulated populations showed that many vital rates in both species are close to threshold values, across which extinction risk increases dramatically. Cessation of management in major population strongholds would negatively affect several of these parameters, likely resulting in declines. Additionally, the posterior probability estimates of equilibrium population size for the top-performing population models of both species showed that the probability of meeting the population size down-listing criterion was 0.03 for Hawaiian stilts and 0.06 for coots. Our findings suggest that these two species are conservation reliant and not candidates for down-listing.
