Nectar and hostplant scarcity limit populations of an endangered Oregon butterfly.

As grassland habitats become degraded, declines in juvenile and adult food resources may limit populations of rare insects. Fender's blue butterfly (Icaricia icarioides fenderi), a species proposed for listing as endangered under the US Endangered Species Act, survives in remnants of upland prairie in western Oregon. We investigated the effects of limited larval hostplants and adult nectar sources on butterfly population size at four sites that encompass a range of resource densities. We used coarse and detailed estimates of resource abundance to test hypotheses relating resource quantity to population size. Coarse estimates of resources (percent cover of hostplant and density of nectar flowers) suggest that butterfly population size is not associated with resource availability. However, more detailed estimates of resources (density of hostplant leaves and quantity of nectar from native nectar sources) suggest that butterfly population size is strongly associated with resource availability. The results of this study suggest that restoring degraded habitat by augmenting adult and larval resources will play an important role in managing populations of this rare butterfly.