SPATIAL DISPERSAL OF EXOTIC PESTS - THE IMPORTANCE OF EXTREME VALUES

In modelling the incursion of an exotic pest (screwworm fly) into Australia, results from a deterministic approximation of the population's overall dispersal were verified against a stochastic model which directly simulated the movements of individual flies. Whilst agreeing reasonably well overall, the major discrepancies occurred at the fringes of the expanding infestation. Here, the deterministic model consistently under-predicted population densities, despite being based on the same assumptions. These differences are attributed to the far movers, a small proportion of the population which invade virgin territory and establish populations which, over time, seed further, more distant populations. The averaging of dispersal mechanisms in the deterministic approximation fails to reproduce this realistically expected pattern. These results raise important implications both for managers of a screwworm fly invasion, and for modellers of systems which encompass extreme events and distributions.