Effects of a nonnative habitat-forming species on mobile and sessile epifaunal communities

Native and exotic sessile invertebrates can act as habitat modifiers and influence the diversity and composition of associated epifaunal communities. Theory and empirical research predict that greater habitat structural complexity will generally result in higher abundance, richness, and diversity of epifaunal species. We evaluated the impact of a recently introduced, structurally complex bryozoan, Watersipora subtorquata, by comparing its associated epifaunal communities with those of primary substrate, other habitat modifiers (tunicates and mussels), and structurally similar mimics. Community composition differed between Watersipora and other treatments for both mobile and sessile epifaunal communities, but species richness and diversity were significantly higher on Watersipora for mobile animals only. Differences in mobile community composition and diversity were driven by polychaete worms, which were found in greater abundance and higher diversity in Watersipora. Crustaceans, although equally abundant, were more evenly distributed across treatments. Sessile epifaunal communities differed little in richness or abundance among treatments, suggesting that while Watersipora provides biogenic surfaces for colonization that adequately compensate for the primary substrate it exploits, its high structural complexity does not correspond to an increase in sessile epifaunal abundance relative to less complex habitat modifiers. Our results suggest that neutral or facilitative effects of exotic species may be common; the strength and direction of the impact of a novel habitat-forming species on epifaunal communities will depend on the quality of the resource it provides, the strength of ecological interactions between habitat-forming species and epifauna, epifaunal habitat preferences, and whether habitat is a limiting resource for a particular species or group.