Invasion dynamics: interactions between the European Green Crab Carcinus maenas and the Asian Shore Crab Hemigrapsus sanguineus

Abstract

As ecosystems are increasingly inhabited by multiple invasive species, interactions among exotic species have the potential to negatively impact native communities and habitats, often in counterintuitive ways. In rocky intertidal habitats along southern New England, the recently introduced Asian Shore Crab, Hemigrapsus sanguineus, has been implicated in the displacement of the more established invasive European Green Crab, Carcinus maenas. We coupled faunal sampling with two field experiments to examine whether H. sanguineus is also displacing C. maenas in the Gulf of Maine. First, we investigated natural recruitment of the two species across an elevation gradient, using cage exclusion units to manipulate predator access. Excluding H. sanguineus adults significantly increased the density of C. maenas recruits, but not conspecific recruit density. Next, to simulate the transition that occurred from pre-invasion to the establishment of H. sanguineus, we conducted a predator inclusion experiment manipulating the density of adult H. sanguineus to examine the effects on C. maenas and conspecific recruit densities. In contrast to the previous experiment, we found that the density of H. sanguineus included in cages negatively affected conspecific recruit density but had no effect on C. maenas recruit density. We hypothesize that juxtaposing results from the two experiments may be a consequence of the considerably greater abundance of C. maenas recruits relative to H. sanguineus recruits during the first experiment. Our work highlights the importance of considering the multitude of factors that influence competitive and intraguild predation interactions, including density-dependent effects, ontogeny and life history characteristics.