Is macroinvertebrate habitat use driven by the cascading effects of a native polychaete and a non‐native alga?

Abstract Introductions of novel ecosystem engineers to our estuaries and coasts are often associated with strong impacts on the environment, with some species altering community and behavioral interactions, especially when part of a cascading interaction. In some US Mid‐Atlantic soft‐sediment mudflats adjacent to salt marshes, the native predatory polychaete Diopatra cuprea preferentially decorates its mucus tube with the invasive red alga Gracilaria vermiculophylla . This may be due to a facilitation cascade between these species, possibly increasing the availability of invertebrate prey for D. cuprea . To determine the effects of the facilitation of Gr. vermiculophylla by D. cuprea on invertebrates associated with algae, we compared communities inhabiting Gr. vermiculophylla and Ulva spp. decorations using a field manipulation experiment. Additionally, we tested invertebrate habitat use in the laboratory with a microcosm choice experiment. In the field, the interaction between site, algal species, and worm presence drove macroinvertebrate species richness and abundance, with a pattern of higher species richness on D. cuprea tubes with Gr. vermiculophylla decorations, and a pattern of higher abundance on Ulva spp. decorations when D. cuprea was absent. In the laboratory, the abundant amphipod species Gammarus mucronatus associated with Gr. vermiculophylla over Ulva spp. regardless of D. cuprea presence. Our study indicates that algal species is only one of multiple factors that structure macroinvertebrate community composition, while laboratory trials suggested that specific invertebrates may show a preference for certain algae. This work highlights the complex interactions between native and non‐native species and their associated communities, which may incur facilitation cascades as a result of novel or changing species interactions.