Higher abundance and diversity in faunal assemblages with the invasion of Codium fragile ssp. fragile in eelgrass meadows

The present study examined how species-specific attributes of the invasive alga Codium fragile ssp. fragile (hereafter Codium) and eelgrass Zostera marina influenced faunal assemblages associated with eelgrass ecosystems in îles de la Madeleine, eastern Canada. Direct association of species with Codium (<1 m) was evaluated and compared to eelgrass faunal assemblages from invaded beds. Potential neighborhood effects (ca. 10 m) were assessed by comparing invertebrates associated with eelgrass in invaded and non-invaded beds. The influence of Codium on more mobile species was evaluated by comparing fish assemblages associated with macrophytes in invaded and non-invaded beds. In addition, species–area relationships for low mobility species associated with Codium were contrasted between beds where Codium or eelgrass was the dominant structuring macrophyte. Codium had distinct invertebrate species assemblages, as there was a greater abundance and diversity of invertebrates associated with Codium than with eelgrass, and multivariate assemblage structure differed between macrophyte types. In contrast, no neighborhood patterns were observed as invertebrate assemblages associated with eelgrass did not differ between invaded and non-invaded beds, suggesting that the mechanisms that account for differences between the invertebrate assemblages are a function of the macrophyte’s nature and operate at small spatial scales. Fish community structure differed between invaded and non-invaded beds, largely due to a greater abundance of 2 fishes, Apeltes quadracus and Tautogolabrus adspersus, in invaded beds. Epifaunal abundance and species richness were positively correlated with Codium thallus biomass, and the abundance relative to biomass correlation was greatest where Codium was the dominant macrophyte. Experimental manipulation of thallus structure to create straight or branched Codium fronds with similar surface area showed that associated invertebrate and gastropod abundance was not influenced by this factor and indicated that factors other than macrostructure complexity influenced the presence of epifauna on the introduced alga. Overall, the present study suggests that the invasion of eelgrass beds by Codium increases faunal density and diversity. However, longer-term effects of Codium invasion in this and other eelgrass ecosystems require further assessment, as the severity of the invasion effects on faunal communities likely depends on the interaction between Codium and eelgrass.