Trade-associated pathways of alien forest insect entries in Canada
Long-distance introductions of new invasive species have often been driven by socioeconomic factors, such that traditional ‘‘biological’’ invasion models may not be capable of estimating spread fully and reliably. In this study we present a new methodology to characterize and predict pathways of human-assisted entries of alien forest insects. We have developed a stochastic quantitative model of how these species may be moved with commodity flow through a network of international marine ports and major transportation corridors in Canada. The study makes use of a Canadian roadside survey database and data on Canadian marine imports, complemented with geo-referenced information on ports of entry, populated places and empirical observations of historical spread rates for invasive pests. The model is formulated as a probabilistic pathway matrix, and allows for quantitative characterization of likelihoods and vectors of new pest introductions from already or likely-to-be infested locations. We applied the pathway model to estimate the rates of human-assisted entry of alien forest insect species across Canada as well as cross-border transport to locations in the US. Results suggest a relatively low nationwide entry rate for Canada when compared to the US (0.338 new forest insect species per year vs. 1.89). Among Canadian urban areas, Greater Toronto and Greater Vancouver appear to have the highest alien forest insect entry potential, exhibiting species entry rates that are comparable with estimated rates at mid-size US urban metropolises.