Rocky intertidal ecosystems are characterized by strong interactions between species, notably with regard to competition and predation. What happens when a non-indigenous species inserts itself into the mix? According to invasion theory, species interactions should prevent the proliferation of exotics. In a field experiment by Jennifer Ruesink, however, both biotic resistance and facilitation occurred. Introduced Pacific oysters Crassostrea gigas were eaten by native predators, but native neighbors conferred protection to the alien species. Worldwide, native species have had numerous opportunities to interact with non-indigenous oysters, which have been introduced to more than 70 coastal regions.