Community-wide effects of nonindigenous species on temperate rocky reefs

Ecological interactions among invading species are common and may often be important in facilitating invasions. Indeed, the presence of one nonindigenous species can act as an agent of disturbance that facilitates the invasion of a second species. However, most studies of nonindigenous species are anecdotal and do not provide substantive evidence that interactions among nonindigenous species have any community-level effects. Here, using a combination of field experiments and observations we examine interactions among introduced species in New England kelp forests and ask whether these interactions have altered paradigms describing subtidal communities in the Gulf of Maine. The green alga Codium fragile was observed at the Isles of Shoals, Maine, USA, in 1983 and has since replaced the native kelp as the dominant seaweed on leeward shores. Experiments manipulating kelp and Codium reveal that Codium does not directly inhibit growth or survival of kelp. Codium does, however, successfully recruit to gaps in the kelp bed and, once established, inhibits recruitment of kelp. A second nonindigenous species, Membranipora membranacea, grows epiphytically on kelp, and experiments reveal that the presence of Membranipora reduces growth and survival of kelp, resulting in defoliation of kelp plants and gap formation in kelp beds. In the absence of Codium, kelp recolonizes these gaps, but when present, Codium colonizes and prevents kelp recolonization. Manipulations of herbivores demonstrate that herbivory will reinforce Codium dominance. Thus, the demise of New England kelp beds appears to result from one invasive species facilitating the spread of a second nonindigenous species.