Invasive Species in the United States:
Human migrations over thousands of years have dramatically altered the earth's landscapes. One way that this has been keenly felt is through the transportation of exotic invasive species across geographic borders. This movement of plants, animals, fungi, bacteria, and viruses has had an enormous economic impact on humans and has caused large-scale disruptions to ecosystems, including loss of both habitats and species. While land managers have long struggled with the impact of invasive species, controlling the problem has come into national focus only in the past few years as the large scope of economic and environmental issues has come more into public view. Solutions to invasions have been tried with limited success. However, continued efforts are yielding some measure of control and are therefore worth pursuing.
In order to understand how and why species invasions come about, it is necessary to view the historical transport of species in parallel with human migrations. As humans migrate, they bring more with them than just luggage they consciously select. They also inadvertently carry along other organisms. For example, they bring human diseases within their own bodies. Their luggage and food stores for the journey may contain seeds from local plants. If they bring animals with them, such as cattle and horses, these animals may harbor seeds in their hoofs. Especially if animal feed is brought along (hay or grain), there will be many local seeds, fungi, and bacteria along for the ride. Spotted knapweed is one such example (Alper). A spiny weed that invades pasturelands, it likely originally came with American-bound ships along with other seeds used for pasture or agriculture. Neither cattle nor horses will eat the plant, and it displaces other grass plants that the animals would normally eat, thus reducing the land's productivity. By December 2004, spotted knapweed covered approximately 4...