Environmental Engineering Reference
In-Depth Information
regions, and the potentially subjective nature of assessments of future risk, the scope for
protection-motivated discrimination may be great.
Research gaps
The biggest gap in our understanding of the relationship between trade and invasive
species is clearly empirical. Few studies have tested the strength of the relationship
between trade volumes, patterns and biological invasions. Moreover, despite the prepon-
derance of anecdotal evidence suggesting that marginal invasion risk (MIR) varies across
export partners, mode of transport and commodity type, virtually no empirical studies to
date have allowed MIR to vary by type. Costello et al. (2007) is one exception,
nding that
MIR varies widely across trading regions, however, these authors restrict their attention
to a single importing region, a single transport mode (ocean vessel), and a generic measure
of imports (tonnage).
Another topic needing further research concerns stepping-stone invasions: a species
native to region A that invades region B becomes a risk to B's trade partner C even if coun-
tries A and C refrain from trading with one another. Drake and Lodge (2004) use shipping
data to estimate a uniform rate of port-to-port infections. Among other things, they con-
clude that some of the ports that are
observed
to be most highly infected, San Francisco Bay
and the North American Great Lakes in particular, may in fact be relatively unimportant
sources of secondary infections; they explain this discrepancy by pointing to the variations
in search intensity (for NIS) across ports. Even if an NIS is unable to establish itself in a
stepping-stone location, it may nevertheless be able to contaminate outgoing tra
fi
c, as
documented by Apte et al. (2000) for Mytilus galloprovincialis in Pearl Harbor. The role of
secondary infections and the role of policy, in particular preferential trade zones and hub-
spoke shipping arrangements, need further empirical and theoretical exploration.
Incorporating an importer's ecological history into policy is another area requiring
further study. Most extant trade and invasives research maintains the assumption that
damages from a particular exotic species are a function only of the size of the introduced
population. However, ecological research suggests that damages depend critically on
interactions with other species present, both native and invasive. Species interactions have
been studied in the context of reactive policy (e.g. Simberlo
et al., 1997) but not for pre-
emptive policy. Given that trade policies can themselves induce substitutions across com-
modities, trade partners and transport modeds, the general equilibrium consequences of
type-speci
ff
c policies need to be better understood.
As we stressed earlier in this review, we believe that a critical aspect of trade and inva-
sives interactions lies with uncertainty. Researchers often presume an exogenous degree
of uncertainty, e.g. per individual damages are a random variable with known distribu-
tion. However, in practice the degree of uncertainty borne by policy-makers is a choice.
Policy-makers can assign resources to learning more about an invasives problem via
observation, adaptive management and restricted trials. In short, governments can invest
funds to reduce uncertainty. Investment is costly because it reallocates resources away
from prevention e
fi
ff
orts such as inspections and enforcing quarantine. The optimal
tradeo
between investment and search/enforcement will ultimately depend on the rela-
tive costs of type I and II errors. Liberal entry policy will allow too much contaminated
trade, prompting ecological damage. Restrictive treatment of imports will unnecessarily
raise prices in the importing country, generating consumption and production distortions.
ff
