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Invasive species are plants and animals that spread quickly, displace other species, and produce a significant change in the natural environment. They are not native to the ecosystem in which they are found; they have been introduced by humans either deliberately or accidentally. They cause vast ecological and economic damage and are a growing problem in almost every ecosystem in the United States. The U.S. Fish and Wildlife Service has made prevention and control of invasive species a high priority.
Invading plants and animals are often very successful when introduced to a new environment because they have no natural enemies that keep the population in check. They out-compete other species for food and space. They usually mature quickly, reproduce often, and can survive in extreme conditions.
Purple loosestrife
More than 135 non-native species have been introduced to the Mississippi River Basin during the past 100 years. Not all non-native species are invasive. Some do not spread uncontrollably, but many others have severe negative effects on the environment. Some species such as the starling, house sparrow, and common carp have been here so long that we forget they are not native to the United States. Other species (including zebra mussels, purple loosestrife, and bighead carp) have been here a shorter period of time but are no less detrimental to native plants and animals.
It may take many years to wipe out invasive species in even a small area. Once they become well-established, total elimination is usually impossible. Control methods for plants include cutting, tilling, burning, herbicides, and "biological control." Biological control involves the use of insects, parasites, or pathogens that specifically destroy the target species. One size does not fit all; usually a combination of methods is best, based on the individual situation.
Missouri, Illinois, and Iowa each have noxious weed laws that require land managers to control specific plants including musk thistle, Canada thistle, Johnson grass, field bindweed, and purple loosestrife. Many units of Mark Twain NWR Complex have such weeds that must be controlled.
Silver carp, bighead carp, and grass carp were brought to the U.S. in the 1970's to control unwanted algae and vegetation in southern fish ponds. They eventually escaped into the Mississippi River and have been rapidly spreading upstream. Grass carp have been known to clean entire lakes of all aquatic plants. Bighead and silver carp compete directly with native fish for food by eating algae, zooplankton, and insects. Today many commercial fishermen are abandoning fishing sites because they are unable to lift nets that are always "loaded" with these Asian carp. No effective control measures have been found yet and these species continue to grow, reproduce, and spread at an alarming rate.
Grass carp
These are some of the invasive plants and animals that are most severely impacting habitat on the refuges of the Mark Twain Complex:
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For more information, check out the 
invasive species website.
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