Point-count methods to monitor butterfly populations when traditional methods fail: a case study with Miami blue butterfly

Abstract

Established butterfly monitoring methods are designed for open habitats such as grasslands. Not all rare species occupy habitats that are easy to see across and navigate, in which cases a new approach to monitoring is necessary. We present a novel use of point transect distance sampling to monitor the Miami blue, a highly endangered butterfly that occupies dense shrub habitat. To monitor Miami blue density, we developed surveys consisting of butterfly counts in semi-circular plots. We examined the rate at which an observer detects new butterflies to determine the survey duration that meets the key assumption that butterflies are detected at their initial location. As a related secondary goal, we identified the determinants of adult flight phenology to target monitoring efforts during periods of high adult abundance. We observed peak Miami blue densities in April and July/August 2012, and July/August 2013. We estimated density using detections from a 10-sec survey, our most defensible and conservative estimate. Peak daily density estimates ranged from 592 to 680 butterflies per hectare. Adult density was related to precipitation patterns, with high densities occurring 4–6 weeks after particularly wet 4-week intervals. For butterfly species that exist in high enough densities, we recommend using point transect distance sampling in habitats where traditional methods are impossible to implement.