Predation contributes to invasion resistance of benthic communities against the non-indigenous tunicate <Emphasis Type="Italic">Ciona intestinalis</Emphasis>

Marine anthropogenic structures offer novel niches for introduced species but their role in the subsequent invasion to natural habitats remains unknown. Upon arrival in new environments, invaders must overcome biotic resistance from native competitors and predators if they are to establish successfully in natural habitats. We tested the hypotheses that (1) artificial structures (e.g., suspended aquaculture installations) present a niche opportunity for invasive species by providing a refuge from native benthic predators, and (2) native predators in natural benthic habitats suppress successful colonization by invaders. A recruitment experiment showed that the ascidians Pyura chilensis (native) and Ciona intestinalis (invasive) could recruit to both suspended artificial structures and natural benthic habitats. Ciona, however, was only able to establish adult populations on artificial structures. In natural benthic habitats Ciona only recruited and grew in predator-exclusion cages, because without this protection predation prevented its establishment. In predation experiments, native invertebrate and fish predators removed all invasive ascidians (recruits and adults) in benthic habitats, which contrasted with the high adult survival of the native ascidian P. chilensis. The refuge from a number of benthic predators facilitates the establishment of large populations of invasive species on suspended structures. We present a conceptual model of the invasion processes that includes the anthropogenic structures as a transitional stepping-stone that facilitates invasion by enhancing and prolonging propagule supply to surrounding natural communities. Those established invaders might then overcome biotic resistance during time periods when populations of consumers or competitors are weakened by natural or anthropogenic disturbances. Our results suggest that the conservation of natural habitats with a high diversity of native predators can be an effective means to prevent the spread of invasive species growing on suspended structures.