Export of non-native gastropod shells to a coastal lagoon: alteration of habitat structure has negligible effects on infauna

Alteration of physical habitat structure is a fundamental mechanism by which invaders produce ecosystem level effects. We assessed whether, along the east coast of Australia, the impacts of the non-native gastropod Maoricolpus roseus on soft-sediment habitats extend beyond the range of live populations as a result of shell export following death of animals. Sampling over an 18 month period revealed that M. roseus shells were temporally persistent in surface sediments of a coastal lagoon devoid of live populations of the gastropod. The well-preserved shells, of which 92% were entire, did not accumulate above a maximal density of 260 m^{− 2} due to periodic burial. Manipulation of M. roseus shell densities indicated that at densities (140 m^{− 2}) presently experienced within surface sediments of the lagoon, the structure provided by the shells was weakly facilitative of some invertebrate species. Further increasing shell densities to the possible future scenario of 280 m^{− 2}, which may occur under continued expansion of nearby live populations did not, however, strengthen positive effects. To the contrary, plots with higher densities of M. roseus shells contained similar invertebrate assemblages to control plots, without shells. Consistent with the negligible effects of M. roseus shells on infauna, the foraging efficiency of the generalist predator, Carcinus maenas, and the naticid gastropod, Polinices sordidus, were not affected by addition of shells. Surprisingly, even an extreme scenario of 1600 m^{− 2} did not affect their predation. Thus, in this dynamic coastal lagoon, that experiences considerable sediment movement and environmental variability, the structure introduced by import of non-native shells is unimportant in structuring soft-sediment communities. Whether this unrecognised aspect of molluscan invasions impacts the ecology of more stable coastal environments remains unclear and warrants further consideration.