Invasive species can disrupt food webs by altering the abundance of prey species or integrating into the food web themselves. In the Gulf of Maine, there have been a suite of invasions that have altered the composition of the benthic ecosystem. These novel prey species can potentially benefit native predators depending on their nutritional value and relative abundance. We measured feeding instances of the native blood star, Henricia sanguinolenta, and changes in the seasonal abundances of invasive ascidian prey species. Results indicate that H. sanguinolenta forages optimally, as the blood star will prey on invasive ascidians when in high abundance, but feed on other species during periods of scarcity. Further, our study shows that blood stars prey on a wider variety of species than was previously known, such as small bivalves and barnacles. Additionally, we compared growth and reproduction of sea stars fed different combinations of invasive ascidians (Diplosoma listerianum or Botrylloides violaceus) or a native sponge (Haliclona oculata). Sea stars grew more on the native diet when compared with the invasive ascidian species, and D. listerianum appeared to be a superior quality food source when compared with B. violaceus. By comparing our data with historical data, we determined that there was a dramatic increase in sea star populations between 1980 and 2011, but then populations decreased by almost half from 2011 to 2016–2017. These data suggest that while invasive ascidians may have helped sea star populations at one point, sea stars are declining without their native food source.
Introduced species can have profound impacts on communities by displacing and consuming native species. The intertidal communities in the Gulf of Maine have been radically altered through a suite of invasions including the herbivorous snailLittorina littoreaand the omnivorous crabCarcinus maenasleading to morphological and spatial distribution changes in two native gastropod grazers (Littorina saxatilisandLittorina obtusata). Subsequently, another intertidal omnivorous crabHemigrapsus sanguineushas become abundant in the intertidal, occurring in areas which were once refuges from crab predation. In order to quantify the potential impact ofH. sanguineuson native snails, we conducted bothinandex situexperiments, testing the susceptibility of different sized snails to predation by introduced crabs. Additionally, we measured the distribution, abundance, and morphology of intertidal snails and crabs. Smaller snails of all species were the most susceptible to predation, although susceptibility differed among snail species.Littorina saxatiliswas the most vulnerable to predation, with 73 and 64% of the population susceptible to largeH. sanguineusand smallC. maenas, respectively, while more than 89% of theL. littoreapopulation was resistant to predation.Littorina saxatilishas been relegated to the high intertidal where there is high abiotic stress and poor-quality food, but until the invasion ofH. sanguineus, there was little predation risk. Now, it seems thatL. saxatilisis at the most risk of all three snail species, and may be at threat of local extirpation at locations with high populations ofH. sanguineus.
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