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J. Touza, K. Dehnen-Schmutz, and G. Jones
From an economic perspective, it would be efficient to select these strategies
such that the expected marginal benefits of the policy equal its marginal
social costs (Perrings 2005). Therefore, the optimal combination of prevention
and control efforts depends on the conditions of invasion (Jensen 2002;
Finnoff et al. 2005; Leung et al. 2006).
Leung et al. (2006) derive some “rules of the thumb” relating to optimal
expenditure on prevention and control. Their study shows that optimal prevention
expenditure depends on the probability of invasions and it should
decrease as invasions become more unpreventable; optimal control resources
should increase with the value of the invaded habitat and decrease with
uncontrollable damages (i.e. impacts which can not be reduced, regardless of
control efforts). Jensen (2002) include the effect of time to explore the interactions
between current prevention to protect against invasions and future control
to reduce the damages. Similarly to Leung et al. (2006), he shows that prevention
expenditures should be smaller for those invasions more likely to
occur (i.e. higher natural hazard rate). In addition, they are also influenced by
the relative weight of future benefits in present decisions. The lower the rate is
at which future benefits are discounted, the higher is the investment in prevention
because future wellbeing, when the invasion may occur, has a higher
weight in policy decisions. Jensen (2002) also argues that it is optimal to
undertake prevention expenditures if – and only if – the damage costs are
high enough.
Finnoff et al. (2005) include the fact that policies often involve private and
public actions, and explore their possible interactions. They argue that if private
agents’ beliefs over the environment are incomplete (i.e. they behave as if
there is no change), the social planner either free-rides on private investments
or is sole responsible for control actions (limiting the resources on prevention)
and, therefore, the risk and abundance of IAS increase. This means that,
if there is no invasion, private agents will never apply control measures and, if
invasions occur in the future, then the social planner would be forced to
employ greater collective resources in control at the expense of an investment
into prevention policies. This, in turn, would cause an increase in invasions.
By contrast, if the ecosystem is highly invaded, then the private agents’ control
will always be high, the social planner would therefore free-ride on the private
control efforts, public prevention and control would be neglected, and invasions
would also increase. However, if the social planner ignores or is not
aware of the actions of the private agents, then he will either over-prevent and
over-control (if he believes that private agents behave as if there were no invasion)
– and invasions decrease – or he will neglect prevention and control (if
he believes that private agents behave as if there were an invasion) – and invasions
increase. Finnoff et al. (2005) conclude that neglecting these potential
feedbacks can have a strong impact on the policy outcomes.
The cost-effectiveness of prevention and control efforts has also been studied,
and results so far show that pre-invasion strategies are to be preferred.