Probability of Nonindigenous Seafood Species Becoming Established

In a recent article, Chapman et al. (2003) assessed the potential risk of establishment by marine bivalve species that have arrived in the United States through trade in live seafood. Here we draw attention to a poorly documented aspect of their methods and their misuse of base-rate probabilities. Although our criticisms cast doubt on Chapman et al.’s calculations of the risk for establishment of species in trade, the most general message of the paper remains unscathed: trade in live seafood is a significant source of invasive species and should be subject to greater risk management. Our criticisms concern how the risk posed by the species in trade should be estimated. These estimations are central in the ongoing development of quantitative methods to assess the risk of establishment of nonindigenous taxa of all kinds in many pathways (National Research Council 2002). Chapman et al.’s basic premise is that numerous nonindigenous bivalve species have become established in the northeastern Pacific through the live food trade and that future establishments via this pathway can be expected. Their specific aim is to estimate how many of the species currently in trade, but not yet established, will eventually become established. Through a literature survey and their own investigations they identify 24 nonindigenous bivalve species for sale along the northeastern Pacific coast, some of which are already established in the region. These 24 species are the object of subsequent analyses, about which we have two important concerns. First, Chapman et al. do not offer sufficient information for readers to evaluate the accuracy of their list of species in trade. Instead of purchasing and identifying species, they relied on “publications . , personal observations, and Internet sources” (p.1387). This statement of methods is inadequate to instill confidence in the identifications and prompts a number of questions. How much time did the authors spend observing, and where did they make these observations? How exactly were species identified from Internet sources? How many species were identified by each method? Given that many bivalves are difficult to identify, especially when the biogeographic region of origin cannot be assumed (Robinson 1999), the accuracy of identifications from casual observation of specimens in markets, photographs on the Internet, or names provided by vendors should be treated with much more caution than Chapman et al. applied. Results from other studies (e.g., Crossman & Cudmore 1999; Kay & Hoyle 2001; Maki & Galatowitsch 2004) demonstrate that vendorprovided species identifications of live aquatic organisms in trade are often incorrect. For example, Maki and Galatowitsch (2004) found that 15% of plants sold were misidentified by the vendor, and our own experience (unpublished data) indicates that misidentification rates by vendors of freshwater and wetland biota are higher than 40%. The number and identity of bivalves in Chapman et al.’s analyses, then, must be viewed with some skepticism. Second, the authors’ use of base rates of establishment (sensu Smith et al. 1999) violates the assumptions implicit in their base-rate calculations. They calculate base-rate probabilities of establishment (the ratio of species established:introduced) for each of three geographic regions of introduction. (Chapman et al. use introduction, whereas most biologists who study species invasions use establishment, the term we use here.) The three base rates, respectively, infer that bivalves from the live seafood trade become established in the northeastern Pacific at (1) the rate that the 24 nonindigenous species in trade on the West Coast of the United States have established anywhere in the world (15/24); (2) the rate in the northeastern Pacific (this rate is given three times as 11/24 [p.1386], 10/22 [p.1390], and 10/24 [p. 1390]); and (3) the rate everywhere but the northeastern Pacific (3/24). Our criticism does not concern the differences among these three alternatives; instead, we criticize the inconsistency between the logic implicit in the calculation of all three base rates and their subsequent application. Chapman et al. construct their base rates on the implicit assumption that the introduced but not established species will never establish. This logic seems reasonable to us (ignoring possible concerns about, for example, time lags): species that have been in trade for some time without establishing demonstrate a tendency not to establish. The problem arises when Chapman et al. then use these base rates to determine how many of the species in trade but not established in the northeastern Pacific will establish there in the future. If they had used their calculated base rates to estimate the probabilities of establishment of an independent set