Assessing risk of human-assisted spread of invasive forest insects with firewood transport

There has been much recent focus on the potential spread of forest insects with firewood transported by cars and other passenger vehicles. Motivated by certain high-profile invasions, national “don’t move firewood” campaigns have been initiated in the US and Canada, while an assortment of restrictions have been implemented at various jurisdictional levels. Nevertheless, there has been little quantitative assessment of the risks posed by firewood transport. At the same time, a lack of empirical data regarding human-mediated, long-distance dispersal presents an obstacle to realistic modeling of invasions.

We present a study that analyzes the risk of forest insect spread with firewood and develops a related dispersal function for application in geographically explicit invasion models. Our primary data source was the US National Recreation Reservation Service database, which records camper reservations at >2500 locations nationwide. For >5 million individual reservations, we calculated the distance between camper home address and campground location. We then constructed an empirical density kernel from these distance data, and fitted them with various probability density functions. We found the data to be strongly leptokurtic and log-normally distributed. Most campers (59%) traveled less than 100 km, but 8% traveled more than 500 km (and some as far as 4500 km). Additionally, we analyzed the impact of geographic region as well as proximity to major parks and urban centers on the shape of the dispersal kernel. In this presentation, we also discuss the potential for these results to serve as a proxy for other mechanisms of long-distance dispersal.