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Submodule 3: Invasive Plants
General Characteristics of Invasive Plants
Invasive plants are plants that grow outside of their native habitat and cause harm to their new environment. They are highly competitive and persistent. It is estimated that there are nearly 1,500 species of plants known as “weeds” of foreign origin currently found in the US; there are other plants that are invasive and native to this country. Different researchers have estimated that there are from 1,000 to over 22,000 species of potentially invasive plants that have yet to be introduced into the US. This illustrates the need to be vigilant and to step-up efforts to prevent invasive plant introductions into this country. Most of those responsible for fighting plant infestations realize that it is less expensive to prevent invasive plant introductions than to try to control or eradicate plants once an infestation has occurred.
Boundaries of species habitats, including those of plant species, have been continually changing over the course of history. There are many plant species found in the US and, in particular, in certain regions of the US, that were not thought to have originally evolved in the area (thousands of years ago). Most discussion of invasive plants relate to plant introductions to the US since European settlement of the country began in the early 1500s.
Most invasive plants are observed to invade areas with an environment similar to their native site, although this is not a guarantee for being able to predict invasions. While it is nearly impossible to predict those plants that will become invasive and those that will not, there are some characteristics of invasive plants and susceptible environments that have been described based on ecosystems where invasions have already occurred. It appears that the best predictor of the invasive nature of a plant in a particular area is past infestations of the same plant in a similar environment.
Native plant communities respond differently to non-native plant invasions. While it is frequently difficult to determine exact cause and effect, it is commonly thought that invasive plants decrease biodiversity of native communities by “choking out” plants that exhibit less competition for resources (such as water, nutrients, light, and space) when compared to invaders. Major structural changes can also occur in plant communities due to invasion by an aggressive plant that is taller than native species; the tall plants tend to shade the site causing plants that require direct sun, now in the under story, to die out.
Musk thistle (Carduus nutans) crowds out native species and forage for livestock.
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