Re-evaluating eradication of nuisance species: invasion of the tunicate, Ciona intestinalis

www.frontiersinecology.org © The Ecological Society of America M non-native tunicate species have recently invaded the coastal waters of North America and pose serious threats to marine ecosystems (McKindsey et al. 2007). The tunicates occur in such massive numbers that the substratum itself may be entirely obscured (Figure 1). In particular, an invasion by the tunicate Ciona intestinalis (henceforth “Ciona”) is considered to be at a “crisis level”, and is a major marine invasive species issue for the Department of Fisheries and Oceans Canada (DFO; T Landry pers comm). This species has a remarkably high fecundity and reproductive rate (Gray and Christie 1983), with several generations often present in a single season (Svane and Young 1989). Ciona is thought to have been introduced to the province of Prince Edward Island (PEI), Canada, by the bivalve aquaculture trade (Lambert and Lambert 1998). It is a “broadcast spawner”, freely releasing gametes into the water column; however, its transfer between bays seems relatively low (about one new bay per year), compared to the massive growth and spread within bays. Around PEI, Ciona has spread from first detection in 2004 to smothering densities in several bays in 2008. Ciona blankets aquaculture crops, putting 77% of Canada’s mussel farms in danger (DFO 2006). Mussel farmers who have spent decades developing their farms are losing their livelihoods (T Landry pers comm). Despite its social relevance, current managerial guidance is insufficient to control Ciona. This may be because information on population biology rarely exists early in an invasion, and this has been used to justify inaction (Simberloff 2003). As a result, an invader will often have major impacts before action is taken (Mack et al. 2000). To minimize such impacts, managers need to speed up their rate of response, despite uncertainty and limited information (ie the precautionary principle; Kriebel et al. 2001). This may be possible, as only a subset of data is typically required to assess the feasibility of a given management option (Roe 1998; Simberloff 2003). Here, we identify the information required to assess the possibility for eradication of invasive species. Eradication is an important concept in invasion biology and is often an explicit goal of government efforts. However, there are few examples of successful eradications (see Mack [2000] for a review), especially in marine systems (but see Culver and Kuris 2000; Bax et al. 2001; Miller et al. 2004; Wotton et al. 2004; Anderson 2005). The few marine successes have typically occurred in the early stages of invasions; after establishment and initial spread, eradication is usually no longer considered a management option. For Ciona, in particular, eradication has been considered virtually impossible, given the species’ CONCEPTS AND QUESTIONS