Humans are playing an increasingly large role in the expansion of invasive species’ distributions, as we move species both unintentionally and intentionally. These anthropogenic movements of invasives threaten biodiversity and agriculture, modify ecosystems, facilitate the spread of harmful pathogens, and impede conservation efforts. Nevertheless, few (if any) studies have documented the anthropogenic factors associated with the intentional translocation of invasives. Invasive wild pigs (Sus scrofa) have devastating effects to natural systems and their distribution is increasing on all continents except Antarctica. The hunting industry has long been suspected of contributing to the movement of wild pigs, but quantitative evidence of this relationship is lacking. We sought to provide evidence that humans have moved wild pigs and identify anthropogenic factors associated with this practice. We collected wild pig samples from across California and conducted population genetic analyses to evaluate population structure and movement. Hierarchical Bayesian models were used to evaluate the association between movement and anthropogenic factors. We evaluated the effect of several factors on wild pig movement: the number of wild pigs harvested by hunters, the number of game outfitters, the amount of public land, the number of game farms, and the number of domestic pig farms.
Wild pigs in California had high levels of population genetic structure (21 genetic clusters), and generally low rates of migration among clusters, indicating low levels of natural dispersal. However, we found several counties separated by large geographic distances among which movement occurred. Since natural dispersal is low, these large distance migrations confirm that humans have moved wild pigs. When we evaluated the effect of anthropogenic factors on movement, we found that immigration (i.e., movement into counties) was positively associated with domestic pig farms, game farms, and public land, while emigration was positively associated with hunter harvest of wild pigs and the number of game outfitters. Our results support the hypothesis that the hunting industry plays an important role in wild pig movement. Escape from domestic pig farms might also be a source of wild pig populations. Anthropogenic movement of invasives can play a large role in invasive species expansion and population establishment. Therefore, we argue that the propensity of humans to move invasives, and the factors associated with these movements, must be considered in future efforts to manage invasive species.