Room for one more? Coexistence of native and non-indigenous barnacle species

Space can be a limiting factor in intertidal communities and competition for this resource plays a role in determining community composition. Invasive species are known to be opportunistic, and are capable of rapidly colonising free space. Since its introduction to Europe, over 70 years ago, the invasive barnacle species, Austrominius (=Elminius) modestus has become widespread on European coasts and is now the dominant barnacle species at numerous locations. However, little is known about its competitive interactions with native barnacle species. During this study, 18 removal plots (25 cm²) were created at each of six sites in south-west Ireland, encompassing a range of exposure levels and including both artificial structures and natural rocky shores. Monthly monitoring of these plots over 19 months revealed that, despite high recruitment of native species seasonally, A. modestus ultimately outnumbered natives in removal plots at four sites. With the provision of free space, the invasive species increased its relative abundance at three sites, but did not entirely displace natives at any site. Continued recruitment of the native species at high abundances on a seasonal basis in addition to the invasive species not entirely colonising all available space, ensure native species can persist and co-exist with A. modestus at the majority of sites at this location.