Genetically-based variation in the functional traits of keystone species can influence the composition of the community within which those species are embedded. Similarly, local adaptation to biotic and abiotic environmental variation can drive population-level variation in such traits. We test the hypothesis that local adaptation of a keystone plant can lead to variation among populations in the community of associated arthropods. Studying a strong environmental cline along the California coast, we test for local adaptation in the perennial shrub Artemisia californica and its consequences for the more than 200 species of associated arthropods.