The Green Wave: reviewing the environmental impacts of the invasive European green crab (Carcinus maenas) and potential management approaches

The European green crab (Carcinus maenas), native to northwestern Europe and Africa, is among the top 100 most damaging invasive species globally. In some regions, including the Atlantic coast of North America, C. maenas has caused long-term degradation of eelgrass habitats and bivalve, crab, and finfish populations, while other areas are near the beginning of the invasion cycle. Owing to the high persistence and reproductive potential of C. maenas populations, most local and regional mitigation efforts no longer strive for extirpation and instead focus on population control. Long-term monitoring and rapid response protocols can facilitate early detection of introductions that is critical to inform management decisions related to green crab control or extirpation. Once C. maenas are detected, local area managers will need to decide on management actions, including whether and what green crab control measures will be implemented, if local invasion might be prevented or extirpated, and if population reduction to achieve functional eradication is achievable. Owing to the immense operational demands likely required to extirpate C. maenas populations, combined with the limited resources for monitoring and removal, it is unlikely that any single government, conservation, and (or) academic organization would be positioned to adequately control or extirpate populations in local areas, highlighting the importance of collaborative efforts. Community-based monitoring, and emerging methods such as environmental DNA (eDNA) may help to expand the spatial and temporal extent of monitoring, facilitating early detection and removal of C. maenas. While several C. maenas removal programs have succeeded in reducing their populations, to our knowledge, no program has yet successfully extirpated the invader, and the cost of any such program would likely be immense and unsustainable over the long-term. An alternative approach is functional eradication, whereby C. maenas populations are reduced below threshold levels such that ecosystem impacts are minimized. Less funding and effort would likely be required to achieve and maintain compared to extirpation. In either case, continual control efforts will be required because C. maenas populations can quickly increase from low densities and larval re-introductions.