Uncertainty In Population Estimates For Endangered Animals

This article examines initial population estimates in recovery plans for species protected under the Endangered Species Act (ESA), which may lead to delisting species too soon if they are not accurate. The authors reviewed 200 recovery plans for terrestrial vertebrate species listed under the ESA through April 2013. They determined that most recovery plans did not specify a margin of error in the initial population estimates, which sets the stage for questionable recovery criteria. The authors propose re-surveying listed species to establish more accurate baseline population estimates, and present a method of calculation that will better reflect margins of error when establishing population recovery goals. As there are insufficient federal resources available to accomplish this, they call for academic institutions and others to collaborate more actively with government agencies towards this goal.

“United States recovery plans contain biological information for a species listed under the Endangered Species Act and specify recovery criteria to provide basis for species recovery. The objective of our study was to evaluate whether recovery plans provide uncertainty (e.g., variance) with estimates of population size. We reviewed all finalized recovery plans for listed terrestrial vertebrate species to record the following data: (1) if a current population size was given, (2) if a measure of uncertainty or variance was associated with current estimates of population size and (3) if population size was stipulated for recovery. We found that 59% of completed recovery plans specified a current population size, 14.5% specified a variance for the current population size estimate and 43% specified population size as a recovery criterion. More recent recovery plans reported more estimates of current population size, uncertainty and population size as a recovery criterion. Also, bird and mammal recovery plans reported more estimates of population size and uncertainty compared to reptiles and amphibians. We suggest the use of calculating minimum detectable differences to improve confidence when delisting endangered animals and we identified incentives for individuals to get involved in recovery planning to improve access to quantitative data.”