Battle of the barnacle newcomers: Niche compression in invading species in Kaneohe Bay, Oahu, Hawaii

Abstract

Multiple invasions by ecologically similar species can be viewed as 'natural' addition experiments in which access to key resources might be reduced. Possible outcomes might include: (1) the extirpation of a species already present in the ecosystem, (2) the exclusion of a new invader, or (3) niche compression, with each species using less of the shared resource. Chthamalus proteus, a barnacle that arrived in the Hawaiian Islands ∼30 yr ago, is now the most abundant and widespread non-native barnacle in the intertidal zone on the island of Oahu. In a series of field experiments, I demonstrate that the abundance of an earlier invader-the larger, faster growing barnacle Balanus reticulatus-is reduced via substrate pre-emption in the zone of overlap between the 2 barnacle species. A third barnacle, Balanus amphitrite, which invaded Hawaii earlier than the other two, is now virtually absent from locations where it was once abundant and where C. proteus is now the numerical dominant. B. amphitrite did not settle on plates from which C. proteus was removed, suggesting that the presence of C. proteus is not the proximal cause of its decline. B. amphitrite is still present on Oahu, particularly in lower salinity settings. While successively invading barnacles have reportedly replaced each other in other locations around the world, it appears that invasive barnacles on Oahu are undergoing niche compression rather than complete replacement. © 2009 Inter-Research.