Coexistence of a native and an invasive mussel species across an environmental gradient: Do interactions matter?

Introductions of invasive species are increasing worldwide, potentially threatening biodiversity. Although invasive species often displace native species by outcompeting them, coexistence between native and non-native species is common in diverse communities. A field experiment investigated the interactions between the native mussel Mytilus galloprovincialis and the invasive black pygmy mussel Xenostrobus securis across an environmental salinity gradient in the Ria de Vigo (NW Spain). The salinity gradient strongly affected the physiological responses of both mussel species as well as the competitive interactions between the two species. Mytilus galloprovincialis had a stronger effect on the invader than vice versa. The competition with M. galloprovincialis and the release from predation in certain estuarine areas may in part explain the dynamics of the invader population in the Ría de Vigo. Furthermore, results suggest that the invader may contribute positively to ecosystem functioning by increasing bentho-pelagic coupling, water-column clearance, and biodeposition of organic material in the innermost parts of estuaries, where it reaches large abundances. These areas are likely to promote and maintain the supply of propagules to the outermost parts, as well as favouring secondary spread along the Galician coast.