Multi-year occupancy of the hops blue butterfly (Celastrina humulus): habitat patch colonization and extinction

Abstract

The hops azure (Celastrina humulus) is a locally-abundant, rare butterfly in Colorado and Montana, that uses wild hops (Humulus lupulus) as a host plant. Because of the patchy distribution of wild hops and current land use changes, the butterfly is a species of conservation concern. The hops azure is abundant along the riparian systems of the U.S. Air Force Academy (Academy) because wild hops is readily available and most land-use impacts have not reached much of the Academy. However, the riparian systems of the Academy are experiencing increased flooding from off-base, hard-surface development, making it harder for riparian vegetation, like wild hops, to thrive. To describe the prevalence and persistence of the hops azure, we conducted multi-year occupancy sampling to understand habitat-patch occupancy changes, such as patch extinction and colonization, and to identify factors that impact detectability. Wind speed, the area of wild hops, and the amount of cloud cover and solar exposure influenced probability of hops azure detection. Patch occupancy and extinction are influenced by the area of wild hops, suggesting that as host plant patch sizes get smaller, patch extinction increases and occupancy decreases. Detectability and occupancy were higher than expected, and the probability of patch extinction and colonization were extremely low. Management efforts to expand wild hops growth within the hops azure’s range, increase continuity of wild hops expanses, and retain the hydrology that supports wild hops should increase the stability of azure populations.