MEPS 236:137-154 (2002)  -  doi:10.3354/meps236137

ABSTRACT: Increasing awareness of the ecological and economic problems caused by marine introductions has brought calls for the development of predictive models of their impacts on other species. We examined the effects of an introduced marine polychaete, Sabella spallanzanii (Gmelin, 1791), on the development of subtidal marine epifaunal assemblages. As an invader of hard subtidal substrata, S. spallanzanii alters the structure of the habitat by forming a canopy of filamentous feeding structures suspended above the substratum on long, slender tubes. As resident species (including S. spallanzanii) can strongly affect the recruitment of sessile taxa into these assemblages, we predicted that one of the effects of this introduction would be to change the relative abundances of understorey species and hence the assemblage structure. In this study we aimed to determine whether the effects of S. spallanzanii on sessile assemblages would be apparent after periods of 2 or 6 mo, and to evaluate the mechanisms underlying any of such observed effects. Further experiments on individual taxa were done to assess the influence of this exotic species on growth rate and mortality of sessile invertebrates as possible reasons for observed effects. The abundances of 9 taxa were affected by the presence of S. spallanzanii after 10 wk of assemblage development on cleared and canopy-covered areas. Multivariate analyses showed that assemblages beneath canopies were significantly different from those outside canopies. After 6 mo, however, there were few differences between canopy-covered and cleared areas, and assemblages did not differ. We found little evidence that S. spallanzanii canopies could affect already established assemblages. There were significant effects of the S. spallanzanii canopy on survival and growth rates of epifaunal taxa, although the results were not consistent among experiments, suggesting temporal variability in the processes that cause such changes. There are several implications of these results for the assessment of the impacts of exotic species, including the appropriate choice of timescale over which to conduct experiments and the generality of such experiments.

KEY WORDS: Introduced species · Impact · Epifauna · Sabella spallanzanii · Recruitment · Assemblage development