Evaluating the status and habitat preferences of the threatened skipper, Euphyes vestris harbisoni, in southern California, USA

The Harbison’s dun skipper subspecies (Euphyes vestris harbisoni) has a geographically limited distribution in southern California, USA and northern Mexico, isolated from the other subspecies. This skipper is a specialist, primarily found in riparian oak woodlands and only feeds on the San Diego sedge as a larva. Several extirpations have been documented, generating concern for its long-term persistence. We conducted visual surveys in 2021 and 2022, and a marking study in 2022, to assess population sizes. Habitat preferences were explored by collecting habitat covariates and analyzing GIS available environmental data. A significant decline in skipper counts occurred during a drought, with abundances remaining small following the drought. Population size estimates from a marking study had low accuracy due to the small population sizes. For this reason, we used the annual maximum count (Max Count) as a robust and efficient metric for monitoring this skipper. No difference was found between used and unused sedge patches using vegetation-based habitat covariates. However, a GIS approach described the historic distribution as woodlands at intermediate elevations, warmer summer temperatures, and intermediate levels of summer evapotranspiration. Implications for insect conservation: The decline in abundance and number of populations justify future conservation efforts, including identifying areas that should be prioritized for future surveys and restoration efforts. Most importantly, this study demonstrates that these woodlands should be managed/protected in their entirety to allow for annual movement of sedge plants and skippers.