September 05, 2008
Facts about Invasive Plants
What is an invasive species? Native Plant - a species that reached its location without assistance from humans. Exotic species - a non-native plant or animal introduced into a new location by human activity, either intentionally or by accident. Invasive species - a non-native (adventitious) species that is capable of moving aggressively into a habitat and monopolizing resources such as light, nutrients, water, and space to the detriment of other species. How did they get here? Horticultural activity is responsible for about 60% of invasive species introductions (arboretums, botanic gardens, gardeners). Conservation activities (Soil Conservation Service, Department of Agriculture) introduced about 30% of invasive plants, mostly for screening, windbreak, and erosion control, but also to supply food and cover for wildlife. Accidental introductions make up the remaining 10%. For example, purple loosestrife was first brought to the U.S. in the hold of a ship via ballast water, then later introduced for horticultural purposes). Some species may be native to certain regions of North America where they are not invasive, but arrive in new regions through assisted range expansion or transportation to other parts of the country for ornamental purposes, where they can become invasive (black locust, for example). Why are we concerned? According to the North Carolina Botanical Gardens "Biota of North America" study, at least 4,000 species of non-native plants occur outside cultivation in the United States. Most of these escaped species cause few problems, but 79 species cost the U.S. economy more than 97 billion dollars annually in lost crops, failed recovery efforts for endangered species, and control efforts. Invasive species have contributed to the decline of 42% of U.S. endangered and threatened species, and for 18% of U.S. endangered or threatened species, invasives are the main cause of their decline. Invasive species compete directly with native species for moisture, sunlight, nutrients, and space. Additionally, some studies suggest that the fruit produced by invasives may not be as nutritious for local wildlife, requiring them to eat more frequently. Fruits and seeds of invasive species are the "junk food" of the natural world. Why are invasive species so successful? Most species have predators in their natural range that keep their population numbers in check. When new species are introduced, however, they come without their natural predators. Most invasive species produce copious amounts of seed. This seed is often bird- or wind-dispersed, allowing it to cover great distances in a short period of time. Some invasives have aggressive root systems that can spread long distances from a single plant. These root systems often grow so densely that they smother the root systems of surrounding vegetation. Some species produce chemicals in their leaves or root systems which inhibit the growth of other plants around them. Most invasives cast extremely dense shade beneath which native vegetation can not survive. Most invasives thrive on disturbed soil, such as that around newly developed land, or along highways. As our region becomes more fragmented, through development, local habitats become more vulnerable to invasives. What can we do? Many private organizations and government agencies are beginning to look at this very serious environmental problem, seeking solutions. It appears that areas with intact, highly diverse, or complex systems are more resistant to invasion and dominance by exotic species, so increased protection of areas of natural diversity is one way to defend against species loss to invasives. The first step is to prevent any additional, potentially invasive introductions, because once they gain a foothold they are costly and time-consuming to control. Monitoring and early detection of new infestations is imperative. Invasive species outbreaks are most easily controlled when they are small and the plants are young. If we can not achieve total control or eradication, we must learn to manage invasive species populations to restrict size and spread, and prevent them from establishing new populations. Finally we must take an active role in returning our native vegetation to areas where invasive species have been eradicated. Removing invasives without restoring our native flora only opens the site up for re-colonization by the invading force. In addition to supporting organizations that are trying to understand and control invasives, individuals can consider landscaping with native plants, and inform others of the problem.