Controlling invasive species is difficult because it only takes a small initial population to introduce a species. With the amount of planes and ships that travel the world on a daily basis, preventing species introductions is nearly impossible. Perrings et al. (2005) suggest imposing tariffs that would factor species introduction into the cost of exporting goods. Other solutions involve managing introduced populations before they become established. The Working for Water program in South Africa employs people to remove invasive plants along rivers, and it has been very successful (Van Wilgren 1998). In addition to managing the ecological consequences of invasive species, this program provides employment and protects water resources in areas where both are scarce.