Abstract
Non-indigenous species are often identified as threats to native species and communities. Yet, the mechanisms that enable many of these invaders to thrive and alter their newly invaded habitats are still not fully understood. This applies to habitats such as widespread sedimentary shorelines characterized by the presence of scattered biogenic clumps of blue mussels (Mytilus edulis) structurally more complex than bare sediments. In Atlantic Canada, some of these shorelines are numerically dominated by native mud crabs (Dyspanopeus sayi) but have been gradually invaded by the European green crab (Carcinus maenas). This study describes between-habitat (mussel clump vs. bare sediment) differences in density and diversity of invertebrates. It also tests the impact of juvenile green crabs in comparison to native mud crabs using two approaches: First, measuring habitat-related differences in these crabs’ feeding rates on a common prey (soft-shell clams, Mya arenaria). Second, measuring their influence on invertebrate communities associated with mussel clumps. The results show that mussel clumps hold higher invertebrate density and diversity than surrounding sedimentary bottoms. In the laboratory, the feeding rates of native mud crabs were dependent on the type of habitat (sand flat > mussel clump), whereas those of green crabs were significantly higher and unrelated to the habitat in which predation occurred. In field experiments, juvenile green crabs were also the only predators that changed community structure in the mussel clump habitat. These results indicate that green crabs can cause a significant impact on native species and communities. Moreover, they suggest that the ability of this species to overcome the refuge provided by complex biogenic habitats for prey may represent an unexplored mechanism to explain this invader’s expansion here and elsewhere.
Highlights
The spread of marine non-indigenous species has been a recurrent topic in the ecological literature [1,2,3,4]
The average number of species was three times higher in sediments associated with mussel clumps than in those associated with surrounding bare sands (7.75 and 2.5 species quadrat-1, respectively; paired t-test p = 0.001; Fig 2; Table 1)
The nMDS plot (Fig 3) identified a strong difference in composition and abundance of communities associated with mussel clumps and bare sediments
Summary
The spread of marine non-indigenous species has been a recurrent topic in the ecological literature [1,2,3,4]. A growing number of studies have shown that non-indigenous species have community-wide and food web effects that should not be disregarded, when these invaders are predators [4, 15, 16]. In habitats such as marine soft-bottoms, epibenthic predators can have an unparalleled impact in regulating community structure and function [17,18,19,20]. When these relatively smooth and featureless sediments are associated with structurally complex biogenic habitats such as eelgrass, mussel or oyster beds [21], the impact of predators becomes less clear [22,23,24,25]
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