Analysis of Transect Counts to Monitor Population Size in Endangered Insects The Case of the El Segundo Blue Butterfly, Euphilotes Bernardino Allyni

Abstract

Before, during and after habitat restoration from 1984 to 1994, we monitored population size of the federally listed endangered El Segundo blue butterfly, Euphilotes bernardino allyni (Shields). In the subsequent formalization of a recovery plan for the species, the U.S. Fish and Wildlife Service established several recovery criteria, including a requirement of ‘a scientifically credible monitoring plan’ to track population size annually. To avoid detrimental effects of the extensively used mark-release-recapture method on the delicate El Segundo blue butterfly, which would conflict with protection afforded by the Endangered Species Act, we chose instead to perform transect counts to estimate relative population size. Herein, we analyze the results of our transect counts by three different methods, developed by or modified from Pollard, Watt et al. and Zonneveld. Qualitatively, the three methods, which have different assumptions, produced similar results when applied to the same data. Zonneveld's model estimates death rate in addition to an index of population size, thus providing more information than the other two methods. The El Segundo blue butterfly's sedentary nature and the close relationship of its adult and early stages to one foodplant permits extrapolation of the index of population size based on transect counts, to an estimate of actual population size. Our data document butterfly numbers increasing from 1984 to 1989, but then declining until the end of our observations in 1994. Based on analysis of our El Segundo blue butterfly data, we propose an implementation of a scientifically credible monitoring plan.