Plant sex alters Allee effects in aggregating plant parasites

Species interactions are central to our understanding of population dynamics. While density typically strengthens competition, reducing absolute fitness, Allee effects can reverse this pattern, increasing fitness with density. Allee effects emerge in host-parasite systems when higher parasite densities dilute immune responses or increase resource-mobilization. The optimal density of individuals in these systems should be influenced by how host quality alters the rates at which facilitative and competitive effects change across densities. We tested these ideas using sumac (Rhus typhina) and a gall-forming parasite (Melaphis rhois) that attacks sumac leaves. Fitness peaked at intermediate densities , indicating an Allee effect, but the fitness peak depended on host sex. Patterns of abundance mirrored fitness patterns, with galls clustered on leaves and female hosts supporting greater numbers of galls. Within leaves, galls near the stem were more fit, and gall-makers preferentially ovipos...