Biotic resistance and facilitation of a non-indigenous mussel vary with environmental context

Expansion of invasive species in new host habitats is mediated by a combination of concurrent biotic and abiotic factors. Xenostrobus securis is an invasive mussel species that is spreading in the inner part of the Galician Rias Baixas, particularly in areas of low salinity. It co-occurs with the commercially important blue mussel Mytilus galloprovincialis in mixed patchy aggregations on intertidal shores. We asked whether biotic interactions, i.e. competition, facilitation and predation, could explain patterns of coexistence of the 2 species in an estuarine habitat. We also examined whether the effects of such interactions were dependent upon the environmental context. We manipulated predator access to synthetic aggregations of juvenile mussels at different sites using monospecific aggregations of X. securis and M. galloprovincialis and mixed aggregations at high and low densities. Performance of the species was measured as survivorship, growth and condition index. Predation played an important role in determining abundances of both species, especially in the case of the invader, although the effects on survivorship were context dependent. We did not find evidence of intra-or inter-specific competition. Growth rates of the invader were affected by environmental variability (i.e. significant site effect). Condition index of M. galloprovincialis improved in high density aggregations (monospecific and mixed), but only towards the sea (i.e. the outermost site). Results suggest that facilitative rather than competitive interactions contribute to the abundance patterns of the 2 species, at least during the juvenile stage. The environmental context was an important driver of such patterns. We emphasise the need to address the role of positive interactions regarding marine non-indigenous species.