A Hawaiian limpet facilitates recruitment of a competitively dominant invasive barnacle

At an intertidal site on the island of Oahu, Hawaii, the abundance of the invasive barnacle Chthamalus proteus was negatively correlated with abundance of the native limpet Siphonaria normalis. As the substratum at this site was a conglomerate of materials, we hypothesized that the observed pattern was the result of differences in substratum preferences, and/or negative interactions between the 2 species, or both. We used manipulative field experiments to test these hypotheses, and then generalized our findings with surveys across several sites that differed in substratum type and abundances of barnacles and limpets. The limpet recruited in highest numbers to light-colored substrata. The barnacle settled in highest numbers on rough substrata, with a trend toward higher settlement on dark-colored substrata. Surprisingly, in experimental plots, recruitment of the barnacle was positively correlated with the density of the limpet. In contrast, limpet densities increased in experimental plots from which barnacles had been removed. Thus the pattern at this site appears to result from a combination of substratum preferences and interactions between the 2 species. These experiments and surveys of other intertidal sites on Oahu suggest that (1) limpets and barnacles have little effect on each other at low densities, (2) at higher densities limpets facilitate settlement of barnacles, but barnacles have a negative effect on limpets, and (3) substratum type can mediate the density-dependent interactions between these 2 species by affecting their settlement patterns.