Forecasting at the edge of the niche: Didemnum vexillum in Southeast Alaska

Controlling the spread of marine invasive species is a challenging and costly task. Maps that predict the potential spread of an invader based on known habitat preferences can be extremely valuable for assessing invasion risk and prioritizing management actions for invasion control or prevention. Most maps are developed by using environmental data on the species' known distribution to map the potential niche of the species in a new location. However, this approach is complicated when a species spreads to an area where environmental conditions are much different than in other places it is known to exist. Didemnum vexillum was discovered in Southeast Alaska in 2010, marking the northernmost known range of this species. A self-organizing map (SOM) was used to assess potential habitat for D. vexillum in other parts of Southeast Alaska using summer and winter temperature and salinity as controlling factors. This research highlights the uncertainty of using the species' current distribution to evaluate potential spread to an environment at the edge of a species' environmental tolerances. It also identifies gaps in our knowledge of D. vexillum thermal and salinity tolerances, including potential synergistic and additive effects of both low temperature and low salinity, which limit investigation of mechanistic modeling methods.