Invasive plant transfer risk scores

Following the methods of Rockefeller (2016), we computed a transfer risk potential per invasive species per trailhead. For each trailhead, we computed the total number of visitors per town by summing the group size in origin data by town and trail register. Using these data, we computed a trail-town exposure score: the amount of potential exposure per species to each trailhead per town. We computed this score by multiplying the species’ total number of observations per town and the number of visitors per town. From these values, we computed a raw total exposure potential per species per trailhead by summing the trail-town exposure score across all towns. We log-transformed the raw total exposure score due to its skewed distribution and then normalized these values based on a pooled distribution of the scores across all trailheads. This resulted in transfer risk potential score per species per trailhead. It can be interpreted as the relative risk of that invasive plant species being inadvertently transported to that location.