Liking the pattern to the mechanism: How wild boar (Sus scrofa) promote plant invasion

Background/Question/Methods

Old world wild boars (Sus scrofa) have traditionally been associated with promoting exotic plants invasions; however, to date there is little evidence on the mechanisms behind this pattern. Feral pigs may facilitate exotic plant establishment directly by dispersing their seeds and/or indirectly by enhancing their establishment in disturbed areas. Although previous research has shown that feral pigs effectively disperse seeds, no information is available about their relative importance compared to natural dispersal rates or their interaction with soil disturbance (rooted patches). The aim of this study is to understand the role of feral pigs in promoting plant invasion by identifying the degrees to which seed dispersal and/or soil disturbance are driving plant invasions. To study the role of feral pigs in facilitating exotic plant seed dispersal, we collected ~150 fresh feral pig droppings in Isla Victoria, Patagonia, Argentina during the fruiting season 2010, and as a control we collected a soil sample with the seed bank adjacent to the each dropping. The numbers of viable seeds were determined and compared by greenhouse germination. To test if soil disturbance by feral pigs promotes plant invasion, we used a long-term exclosure experiment in which we planted 20 seeds of each of seven exotic invasive plant species inside (unrooted) and outside (rooted) 30 exclosures as well as in artificially rooted patches.

Results/Conclusions

We found that the numbers of native and exotic seedlings that germinated per gram of dropping are less than the numbers found per gram of the soil sample, and similar in terms of species richness. In contrast, we found that seedling emergence of exotic species was 3.5 times higher in rooted areas compared to unrooted patches. Additionally, exotic species establishment in artificially rooted patches was significantly higher than in unrooted areas but less than in naturally rooted patches. These experiments show that soil disturbance is the main mechanism by which feral pigs facilitate the establishment of exotic plants and suggest that feral pigs are causing other changes in soil properties than just the mixing the soil horizons.