The interactions between keystone species and ecosystem engineers may have important community-level consequences. We studied the effects of a keystone species, the aphid-tending ant (Formica obscuripes), on the abundance of engineered leaf shelters, the levels of herbivory on leaf tissue and the structure of arthropod communities within leaf shelters on Hooker's willow (Salix hookeriana) in a coastal dune ecosystem in northern California. Leaf shelters on branches with aphid-tending ants had 54% more individuals than shelters on branches without ants, possibly because shelters are used as a refuge from predation. Levels of herbivory were 2× greater on branches without aphid-tending ants than branches with ants. Our study suggests that aphid-tending ants may actually increase the abundance of arthropods at small spatial scales within leaf rolls while simultaneously reducing rates of herbivory at the branch level.