The role of species richness and spatial resources in the invasion success of the colonial ascidian Didemnum vexillum Kott, 2002 in eastern Long Island Sound

Since it was first discovered in the northwest Atlantic in the late 1980s, the non-indigenous colonial ascidian Didemnum vexillum Kott, 2002 has become a common inhabitant of coastal marinas as well as deeper water sites. This species has also been found to have an expanding global distribution leading to questions about its ability to successfully invade into a variety of communities. The goal of the present study was to build on previous knowledge and to examine the possible mechanisms that allow Didemnum to successfully invade fouling communities in eastern Long Island Sound. In separate experiments, we examined whether species richness and spatial resources (i.e. open space) influenced invasion success. Communities at 2 sites, a protected marina and an open coast site, were compared. In the first experiment, small colonies of Didemnum were transplanted into communities that contained different species richness levels (0, 2, 4, 6 spp.) and in the second, small colonies of Didemnum were transplanted into communities that varied in the amount of species cover (0, 60, 50, 100%) that was present. The growth of Didemnum, as well as species initially present and any new species from recruitment was measured each week for approximately 10weeks. Communities with high amounts of initial open space, regardless of species richness, tended to result in increased abundances of Didemnum at the open coast site and initially at the protected marina. At the marina site, growth of recruits and resident species resulted in the rapid loss of space in all treatments in both experiments, which reduced the success of Didemnum. At the open coast site, without increased competition from recruiting species, Didemnum became more abundant and eventually dominated many of the communities in all treatments. Despite being a global invader, Didemnum appears to be a relatively poor competitor for spatial resources. Its dominance seems to depend on the amount of unoccupied space present within communities it's invading. That, coupled with high recruitment from a diverse community, can overwhelm the potential invasion success of this species.