Competition between a native mussel and a non-indigenous invader for primary space on ­intertidal rocky shores in Chile

Abstract

Non-indigenous marine species have significant effects on rocky intertidal native biota and ecological processes. The tunicate Pyura praeputialis, a recent invader in the Bay of Antofagasta, Chile, has monopolized the low and mid-low rocky intertidal fringe and apparently constrained the native mussel Perumytilus purpuratus to the mid-upper fringe. We performed field experiments to determine interspecific competitive strengths and quantify survival and growth rates between these species at 2 intertidal heights: mid-low and mid-upper intertidal fringes. Our results showed that at the mid-low fringe P. praeputialis had greater competitive strength than P. purpuratus. In fact, the survival and growth rates of P. praeputialis were not significantly affected by the presence of P. pur- puratus. Further, while the survival of P. purpuratus was not significantly affected by the presence of P. praeputialis, its growth rate was affected by the degree of encroachment by the tunicate. Mussels encroached by P. praeputialis grew significantly less than non-encroached ones. At the mid-upper intertidal fringe, the survival of P. praeputialis was significantly decreased by the presence of P. pur- puratus: the tunicate is unable to grow at this intertidal fringe. At the low-intertidal fringe mussel growth rates were significantly greater than at the mid-upper fringe, while survival rates were simi- lar. Our results support the hypothesis that in the Bay of Antofagasta the invading tunicate P. praeputialis is responsible for a major rocky intertidal ecological impact, outcompeting the native mussel from the mid-low fringe and thereby substantially modifying the zonation pattern.