A Long-Term Monitoring Plan for a Threatened Butterfly

Abstract: A long-term monitoring plan using stratified random sampling of population densities of late instar larvae is presented for a large population of the threatened Bay checkerspot butterfly, Euphydryas editha bayensis. A topographic map of the habitat is analyzed for the distribution of slope exposures and a clear sky insolation model is applied to delineate microclimate strata Sixth and seventh instar larval densities are estimated for square-meter quadrats on specific slope exposures, and these density samples are then grouped into the microclimate strata for estimating population size. Additionally, samples of larvae from different slopes are weighed to monitor developmental phenology. The larval population increased from 92,000 in 1985 to 783,000 in 1987, and decreased to 319,000 in 1988. The distribution of larvae changed between years, shifting from cool slopes to warmer slopes as the population grew. These shifts affect developmental phenology and the timing of adult emergence. The procedure produces labor- and cost-effective yearly estimates of population densities of larvae in different microclimates and documents within-habitat responses of the population to a variable climate.