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J. Touza, K. Dehnen-Schmutz, and G. Jones
the last unit of prevention equals the additional benefits. This condition
would include the uncertain effects which the measures taken have on the
invasion. Their analysis implies that, when there are a large number of firms,
it is not optimal for any of the firms to undertake biosecurity actions because
chances are that the species will invade anyway. When there is uncertainty,
however, it is optimal to employ more resources in preventing high-damage
events which are considered possible. The key factor is thus the level of surprise
that the decision makers will expect if the invasion takes place, and prevention
expenditures focus on those potential invasions with low levels of
surprise.
From a post-invasion perspective, Olson and Roy (2002) include the uncertainty
of the effects of environmental disturbances on the spread of an
invader when examining the conditions under which it is optimal to eradicate.
They recognise that there may be disturbances which make the invasion
small enough for eradication to be inevitable. They conclude that eradication
is optimal if the marginal costs of controlling a small invasion are less than
the marginal damage (i.e. marginal benefits of controlling), including future
damages due to the expected spread of the invader. Note that because future
damages are included in this condition, eradicating may be optimal even if
the marginal costs of control are larger than the current damages.
Using expert opinions may reduce uncertainty in management decisions.
Eiswerch and van Kooten (2002) identified optimal control management by
consulting experts about the state of the invasion, its spread and its impacts.
Their study of the agricultural weed yellow starthistle (Centaurea solstitialis)
demonstrates that, as the productivity of the land increases, the optimal management
strategy should have higher levels of control activities but, in this
case, it would not be optimal to eradicate the species. Waage et al. (2004) use
Monte Carlo analysis simulation to include uncertainty in a range of biological
and economic parameters (which may reflect a range of expert opinion or
historic evidence). The uncertain parameters include likelihood of entry, the
intrinsic rate of spread, potential yield losses, and export losses. This paper
seeks to develop tools to aid governments in their need to know the identity
and magnitude of future non-native species risks, and uses this information
to anticipate and allocate resources efficiently between IAS.
20.5 Discussion
Given the importance of human interactions in determining the scale and
speed of invasions, economists have become increasingly interested in the
analysis of policies to manage IAS. In this chapter, we surveyed this economic
literature mainly from the perspective of prevention measures. This analysis
shows that, so far, few economic studies have concentrated on the assessment