Reduced fecundity by one invader in the presence of another: A potential mechanism leading to species replacement

As invasive species proliferate and expand their ranges, they often interact either with natives or with other invasives across a broad geographic range. Moreover, because geographic ranges span a diversity of environments, the outcome of interactions between species pairs may vary spatially. The European green crab Carcinus maenas and the Asian shore crab Hemigrapsus sanguineus are both introduced species in North America where they co-occur over a large portion of the Atlantic coast. While interactions between the two crabs in the southern portion of this range within Long Island Sound resulted in the elimination of C. maenas within 2–3 years of H. sanguineus' arrival, species replacement appears to be taking much longer in northern areas within the Gulf of Maine. Previous work implicates predation by H. sanguineus on C. maenas recruits as the mechanism underlying species replacement. Here we explore an alternative or additional mechanism underlying this species replacement that can also account for the observed spatial variation in the timescale of species replacement between northern and southern areas. Specifically, we demonstrate that a previously documented shift in C. maenas diet which occurs in the presence of H. sanguineus can cause a reduction in C. maenas fecundity. This, combined with near-shore current patterns may explain the regional differences in the outcome of this species interaction.

Research highlights
► Interactions between invasive crab species cause the decline of one of these species.
► We document diet shifts as a potentially contributing mechanism for this decline.
► Interactions cause a shift from animal consumption to plant consumption.
► This diet shift subsequently reduces reproductive output.