Understanding and Predicting Range Expansion by Alien Forest Pests
Andrew Liebhold, Patrick Tobin, and Kurt Gottschalk
USDA Forest Service Northern Research Station
The process of biological invasion is recognized to be composed of three distinct phases: arrival, establishment and spread. In this review, we concentrate on spread which refers to the expansion of a newly established alien species into suitable portions of the exotic habitat. Understanding the processes that facilitate spread of an alien pest species is critical to the development of strategies for retarding or containing its expansion into new areas. Furthermore, prediction of spread may be of critical importance to development of forest management plans where the pest is expected to invade in the future. We present here a review of the literature on the population ecology of range expansion and provide a discussion, illustrated with examples, of how spread predictions can be integrated with landscape-level forest composition data to aid decision-making.
Most of our understanding of range expansion by alien species has it roots in two papers published in the 1950’s. The first of these was by Skellam who formed a simple reaction-diffusion model that combines exponential population growth with random (diffusive) dispersal. Around the same time, Fisher proposed a model for the spread of an advantageous allele by combining logistic growth with random movement. An elegant property that emerges from both of these models is that the asymptotic rate of radial expansion should be constant and can be predicted as 2 * sqrt(r * D), where r is the intrinsic rate of population growth and D is the diffusion coefficient. While these models greatly simplified many aspects of spread, they have performed remarkably well at capturing the dynamics of spread of many different types of organisms. They also provide an elegant representation of how spread is completely determined by either population growth or by movement; any characteristic of a species or a habitat that affect either of these will affect spread.
One feature of many alien species that has been found to greatly influence rates of spread is the mechanism of dispersal. Some species may be capable of more than one mode of dispersal (e.g., passive movement plus accidental movement by humans) and this feature, termed “stratified diffusion”, has been shown to greatly influence rates of spread. Instead of gradually expanding into contiguous areas, such species may “jump” ahead of the expanding population front and form isolated colonies that expand and coalesce. The movements that cause populations to “jump” ahead are thus of critical importance in determining where and how fast populations will spread. Furthermore, this population behavior provides opportunities for containing the spread of a species; any action that either diminishes jumping or retards the growth of isolated colonies may be an effective approach to limiting range expansion.
Without some understanding of the population biology of an organism, it is impossible to predict spread with any meaningful level of precision. In addition geographical variation in land use and forest composition may affect growth and dispersal of invading populations and thereby affect spread rates, but these effects may be complex and difficult to predict. Landscape-level data on forest composition may thus be of some value in predicting spread but may be even more useful for predicting impacts of pests once they have established in new areas. Bioeconomic models that incorporate geographical variability in forest composition may be of critical value in predicting pest impacts and identifying strategies for minimizing future impacts.
corresponding author:
Andrew Liebhold
USDA Forest Service
Northern Research Station
180 Canfield Street
Morgantown, WV 26505
304-285-1512
aliebhold@fs.fed.us
Encyclopedia ID: p112
