Habitat disruption by a coastal invader: local community change in Atlantic Canada sedimentary habitats

While digging and foraging, the non-indigenous green crab (Carcinus maenas) creates a landscape of distinctive pits or depressions in the sediment. Despite their visibility and widespread occurrence in Atlantic Canada and elsewhere, the community influence and persistence of this disturbance remain undocumented. This study addressed this gap in our knowledge using two approaches. First, in both sandy and muddy habitats, we monitored fresh feeding pits (disturbed sediments) for up to 9–11 days after their formation, recording their sediment properties and diversity and density of invertebrate fauna, and comparing these characteristics to those of ambient (undisturbed) sediments in similar habitat. Second, we quantified local-scale invertebrate diversity and density in feeding pits and ambient sediments in muddy habitat only, at three other sites within a Marine Protected Area (MPA). Grain size did not differ between disturbed and ambient sediments and did not change over time within habitats. We also found no significant differences in invertebrate diversity and density between disturbed and undisturbed sandy sediments. In contrast, the invertebrate fauna differed significantly between disturbed and ambient muddy sediments, particularly during the first 4 days after disturbance. Feeding pits in muddy sediments also took twice longer to fill up than pits in sandy sediments. These results were consistent with the comparison of disturbed and undisturbed muddy sediments in the MPA: at least at the local scale, the foraging by this invader significantly altered community structure. Ambient sediments had a higher number of species and nearly twice as many invertebrates compared to disturbed sediments. Overall, our results suggest that visual evidence of green crab feeding pits in muddy sediments can be used as a fairly reliable predictor of local-scale changes in invertebrate communities. The persistence of these local-scale changes depends on the type of habitat in which the disturbance takes place.