Behavioral differences underlie different community impacts of two similar marine invaders

Since its introduction in the early 1800s, the European green crab Carcinus maenas has been a dominant predator on New England shores. However, it is currently being displaced from one of its primary habitats, the rocky intertidal zone, by the advancing invasion of another predator, the Asian shore crab Hemigrapsus sanguineus. While these two crab species are similar in habitat use and diet, they differ greatly in the strength of both lethal and nonlethal aggressive behavior. Specifically, cannibalism and intraguild predation both cause higher mortality in Carcinus than in Hemigrapsus. Similarly, interference from conspecifics and heterospecifics has a much greater negative impact on Carcinus predation than on Hemigrapsus predation. This difference likely contributes to the ongoing species shift, to the drastically different population densities of the two species, and to the resulting impacts on the native community. Adding insult to injury, prey do not behaviorally respond to Hemigrapsus to the same extent as Carcinus—this not only heightens Hemigrapsus’ impact on prey densities but also eliminates most behaviorally mediated positive indirect effects on lower trophic level species. Thus, in areas where Hemigrapsus has replaced Carcinus, it will have large impacts on the invaded community due to a combination of weak predator interference that facilitates high population density, and the absence of a moderating influence on the impacts of other intertidal consumers.