Positive effects of a dominant invader on introduced and native mudflat species

Abstract

Many introduced species have negative impacts on native species, but some develop positive interactions with both native species and other invaders. Facilitation between invaders may lead to an overall acceleration in invasion success and impacts. Mechanisms of facilitation include habitat alteration, or ecosystem engineering, and trophic interactions. In marine systems, only a handful of positive effects have been reported for invading species. In an unusual NE Pacific marine assemblage dominated by 5 conspicuous invaders and 2 native species, we identified positive effects of the most abundant invader, the Asian hornsnail Batillaria attramentaria, on all other species. B. attramentaria reached densities >1400 m -2 , providing an average of 600 cm of hard substrate per m 2 on this mudflat. Its shells were used as habitat almost exclusively by the introduced Atlantic slipper shell Crepidula convexa, the introduced Asian anemone Diadumene lineata, and 2 native hermit crabs Pagurus hirsutiusculus and P. granosimanus. In addition, manipulative experiments showed that the abundance of the mudsnail Nassarius fraterculus and percentage cover of the eelgrass Zostera japonica, both introduced from the NW Pacific, increased significantly in the presence of B. attramentaria. The most likely mechanisms for these facilitations are indirect grazing effects and bioturbation, respectively. Since the precise arrival dates of all these invaders are unknown, the role of B. attramentaria's positive interactions in their initial invasion success is unknown. Nevertheless, by providing habitat for 2 non-native epibionts and 2 native species, and by facilitating 2 other invaders, the non-native B. attramentaria enhances the level of invasion by all 6 species.