Using Population Viability Analysis to Develop Recovery Criteria for Endangered Insects: Case Study of the Fender's Blue Butterfly

Abstract

The U. S. Endangered Species Act (ESA) requires that recovery plans establish "objective, measurable" criteria on which to base listing decisions. Recovery plans are more effective if these criteria are clearly linked to the biology of species of interest. We reviewed recovery plans for 27 listed insect species. Recovery criteria for threatened and endangered insects were poorly linked to species biology. We used population viability analysis to develop quantitative recovery criteria for insects whose population sizes can be estimated, and applied this framework in the context of a recently listed butterfly, the Fender's blue (Icaricia icarioides fenderi). We used a simple diffusion-approximation approach developed to estimate extinction risk from count-based census data. The method assumes that future trends can be predicted based on current conditions and that observer error is minimal. Of 12 sites we surveyed for at least 8 years, only one population had a high likelihood of persistence. Given observed variation in population growth rate (σ 2 = 0.79) and an initial population size of 300 butterflies, a minimum average growth rate (λ) of 1.83 would be needed for there to be a 95% probability that a population would survive 100 years. As the number of independent sites increased, the minimum λ required to have a 95% probability that at least a population at one site would survive 100 years declined. For downlisting, we recommend that three independent sites be maintained or restored in each region of the Fender's blue, with minimum average growth rates of at least λ = 1.55 over 10 years. Given that 41 insect species are listed under the ESA, development of quantitative recovery criteria would be useful and feasible for at-risk insect species whose population sizes can be estimated.