A decade of invasion: changes in the distribution of Didemnum vexillum Kott, 2002 in Narragansett Bay, Rhode Island, USA, between 2005 and 2015.
Didemnum vexillum, an invasive colonial ascidian, has colonized natural and artificial substrates in Narragansett Bay, Rhode Island (USA) since 2000, when it was first discovered in Newport Harbor. A survey of the bay in 2005 found D. vexillum at several coastal sites in the southern portion of the bay, dominating substrata by the end of its reproductive period. The current study examines the near-surface geographic distribution of the ascidian in the bay in 2015 at less than 1 m depth, a decade after the initial survey. According to this study, the ascidian has a more limited distribution in 2015 than in the earlier 2005 survey. Artificial substratum presence, estimated mean salinity, and distance from Providence ports are all positively associated with the 2015 presence of the ascidian in the bay based on results from a linear discriminant function analysis. There are few investigations of long-term changes in invasive species distribution. Based on the findings of this study, further investigation of the causes of long-term change in the geographic distribution and abundance of invasive species, as well as the long-term impacts of such changes on native communities, are recommended.