Invasive species may enter new environments through many routes. Some are transported to new places and established intentionally, but with unforeseen consequences. Beach vitex was planted in coastal North Carolina in the 1980's as an ornamental plant for coastal homes.6 However, the plant began to overtake native species after it became established. The plant also does not have the extensive root system that holds sand in place as native plants do. As it spreads, the plants hasten dune erosion by removing plants that secure the sands of the dunes.

Some invasive species were actually brought in as unsuccessful attempts to control other invasive species. In the 1800's, rats that came to the Virgin Islands on ships infested the sugar cane fields on the islands, causing massive crop damage.7 Farmers brought in mongoose as a predatory control for the rats. However, the rats are nocturnal and sleep in trees, whereas the mongoose are diurnal and cannot climb trees, so they were not successful at eradicating the rats. As a result, the islands now have two invasive species to contend with.

Other species are brought to different places intentionally but released accidentally, as happens sometimes with animals in zoos and aquariums.

Invasive species can have a number of negative impacts on the areas that they invade. Perhaps the most significant of these is the widespread loss of habitat. 

The hemlock woolly adelgid is an invasive insect from Asia that rapidly kills infested hemlock trees. In some parts of the Eastern United States, it is estimated that up to 80% of hemlock trees have been killed. These forests represent important habitat for many animals and with crucial habitat gone, species that rely on them may face extinction. Similarly, the health of many forests is threatened by kudzu vines, introduced from Japan in the 19th century as an ornamental plant. This plant was widely distributed across the Southeastern United States as a means of erosion control and as a food source for grazing animals. The vine soon became invasive, however, and can completely overgrow entire forests. In the process, it prevents sunlight from reaching the trees, effectively killing the forest. Additionally, the weight of the thick mats of vines on trees can cause trees to break and fall over. Its ability to quickly overgrow and destroy forests has earned it the nickname “the vine that ate the South.”

Some invaders can physically alter the habitat in addition to destruction. 

Other invasive species may not destroy habitat but can have an impact by killing large numbers of endemic species.

Invasive species can also impact human health. Invasive zebra mussels accumulate toxins in their tissues like PCB's and PAH's. When other organisms prey on these mussels, the toxins are passed up the food chain and can also enter animals consumed by humans.23 Ballast water from ships also sometimes contains harmful bacteria like cholera. Invasive animals can also be vectors for disease.

Many strategies have been developed to stop the damage caused by invasive species and to prevention future invasions. An important component is educating people about the dangers of transporting wildlife to new areas. Many laws and regulations have also been passed to combat the future spread of invasives. Ballast water in tankers is required to be decontaminated before it can be released from the boat.10 Laws have also been passed to restrict the exotic pet trade, such as banning the import of Burmese pythons in the United States.

While many invasive species may not ever by fully eradicated, increased awareness and research offer methods of preventing their spread and controlling the economic and environmental damage they can incur.