Experimental evidence for positive effects of invasive seaweed on native invertebrates via habitat-formation in a seagrass bed

Relatively few studies have aimed to test if invasive species have positive effects on native species, for example via formation of habitat or by amelioration of environmental stress-conditions. The red macroalga Graciloria vermiculophylla, from the West Pacific, is invading estuarine mudflat and scagrass habitats along East Pacific and East and West Atlantic coastlines. I tested if low (55-110 g WW m(-2)) and high (220-440 g WW m(-2)) densities of G. vermiculophylla have positive or negative effects on the macroinvertebrates (> 2 mm) that inhabit Zostera marina seagrass beds. The experiment was conducted over 34 days at both 0.5 and 2 m depth at Snaptun Harbor, Denmark. I found positive effects of Gracilaria on most invertebrates, with statistically significant results for "all invertebrates", "gastropods", and "bivalves", and a near-significant result for "crustaceans". Both quantitative and qualitative habitat-resource models may explain these positive effects; i.e. "more habitats" exist in the presence of Gracilaria and/or the "habitat differs" between Gracilaria and Zostera vegetation. Future studies should test these two general explanatory models and quantify (a) if density thresholds exists were effects shift from positive to negative, (b) specific mechanism whereby positive effects occur, (c) if Gracilaria provide a novel or substitute drift algal habitat, and (d) the larger-scale ecosystem implications of this invasion.