Bioeconomics of managing the spread of exotic pest species with barrier zones.

Exotic pests are serious threats to North American ecosystems; thus, economic analysis of decisions about eradication, stopping, or slowing their spread may be critical to ecosystem management. The proposed bioeconomic model assumes that the rate of population expansion can be reduced (even to negative values in a case of eradication) if certain management actions are taken along the population front. The area of management can be viewed as a dynamic barrier zone that moves together with the population front. The lower is the target rate of spread, the higher would be both benefits and costs of the project. The problem is to find the optimal target rate of spread at which the present value of net benefits from managing population spread reaches its maximum value. If a population spreads along an infinite habitat strip, the target rate of spread is optimal if the slope of the cost function versus the rate of spread is equal to the ratio of the average pest-related damage per unit time and unit area to the discount rate. In a more complex model where the potential area of expansion is limited, two local maxima of net benefits may exist: one for eradication and another for slowing the spread. If both maxima are present, their heights are compared and the strategy that corresponds to a higher value of net benefits is selected. The optimal strategy changes from eradication to slowing the spread and finally to doing nothing as the area occupied by the species increases. The model shows that slowing the spread of pest species generates economic benefits even if a relatively small area remains uninfested. The cost of slowing the spread can be estimated from a model of population expansion via establishment of isolated colonies beyond the moving front. The model is applied to managing the spread of the gypsy moth (Lymantria dispar) populations in the United States.