An invasive species is one that is not native to the area that it comes into. With few or no natural predators, the invasive species can grow or increase in population until it wipes out the native competitors and takes over as the dominant plant or animal. Invasive species of plants and animals are also called “non indigenous species”, “alien species”, or “non native species”.
Human migration and movement can bring plant species invasion. With today’s plantings and landscaping schemes, people want the visual elements that they remember from their homes of origin; or they long for new and exotics species. In other cases, people want the fruits, vegetables, spices and herbs that are essential for their favorite dishes. These species do not have to be very hardy or rapidly reproducing, since it is the repeated plantings that help them to establish.
Commercial interest can bring in invasive plants. In the semi arid western United States, several plant species were brought in because of their ability to withstand drought or to grow rapidly for timber or for livestock feed. The Eucalyptus, goat grass and other species turned out to be more trouble than they were worth.
Nature or accident brings some invasive species. These get into the bilges of boats and ships and are washed out into distant and new waterways, where they take hold. Invasive species also come in on planes, in cargo, and even on the clothes of travelers or the coats and paws of animals.
Invasive plant species must have special characteristics when they arrive by natural means. They must reproduce effectively and rapidly. They must successfully adapt to their new surroundings by changing or mutating very quickly. They can adapt to changing conditions in the new environment. Some disperse their seeds by very effective means and over wide areas. Others have aerial roots and clingers that allow them to climb or to spread as ground cover, smothering and choking out other species of plants or even covering structures.
Some invasive plants burrow underground for a distance, then send up shoots or seedlings, as with dogwood trees.
The USDA has an extensive database of noxious and invasive weeds in the United States. A comprehensive list of 100 of the world’s worst invasive species (plant and animal) is at the Invasive Species Specialist Group database.
An example of an invasive grass.
Goat Grass, Barbed Goat Grass and Jointed Goat Grass was native to southern Europe and parts of Asia. Some believe that the seeds were mixed in with winter wheat seeds from those areas. Now, the weed is designated as a toxic weed and is responsible for over a hundred million dollars annually in lost production or lost wheat sales in the U.S. alone.
An example of an invasive vine.
Kudzu has an amazing story that actually required a lot of help before it took over the southern United States. In fact, the vine now covers over 7 million acres of the deep South. Kudzu was a Japanese favorite that was introduced at the 1876 Centennial Exposition in Philadelphia, Pennsylvania. Landscapers and gardeners loved the beautiful greens and fragrant flowers. Kudzu was widely used to control erosion. The explosion of Kudzu began to choke out the light needed by forests and to choke out other crops. But Kudzu offers so many textile, mineral, food and medicinal benefits that it is a mixed blessing.
An example of an invasive tree.
The California Eucalyptus is a wonderful tree that grows fast, provides excellent wood for burning, but is is a dirty tree that puts tons of highly flammable litter on the forest floor. This proved to be a disastrous combination in the semi arid West, where fires like the October 1991 Berkeley and Oakland hills fires cost 3,000 homes and 24 lives. Since a grove of Eucalyptus will choke out any grasses that might grow and dry out, the flammable litter was the obvious culprit.
Robert Santos of California State University, Stanislaus, has written a wonderful and detailed history of the Eucalyptus of California as it made its complicated and finally boisterous journey from Tasmania, via Australia, to California.
In summary, a good number of the invasive plant species in the United States and the world have come from mankind’s misguided desire to improve on nature, or to enjoy exotic species. Others traveled great distances over water, in animals, on clothing, or on the wind. Even a rushed and horrible human diaspora can transport invasive plant species, either for food, or by accident of nature.
