Epiphytic macroalgae mediate the impact of a non-native alga on associated fauna

Abstract

The introduction of a non-native, habitat-forming macroalga may have impacts on the seaweed-associated faunal community. Codium fragile subsp. fragile is a well-known and widespread algal invader that may compete with native habitat-forming macroalgae and affect ecosystem services. This alga can be abundant at semi-exposed/sheltered sites on the south-west coast of Norway, where its vertical distribution range overlaps with another canopy-forming alga, the native Fucus serratus. Whether these seaweeds support similar or dissimilar associated communities will influence the ecological impact of C. fragile subsp. fragile. Therefore, the richness and composition of fauna associated with these species was assessed by sampling at three localities. While the majority of taxa occurred on both F. serratus and C. fragile subsp. fragile, there were clear differences in community composition between them. Fauna taxa richness and community composition were also related to macroalgal epiphyte abundance, and there were a number of taxa associated to high epiphyte samples. The results suggest that shared macroalgal epiphytes lead to higher similarity between the invertebrate communities associated with C. fragile subsp. fragile and F. serratus, providing correlative evidence that epiphytic macroalgae play a role in shaping the impact of this macroalga on associated fauna.