Invasive species are those plants that are able to rapidly move into and dominate or disrupt native communities. Most are nonnative, meaning that they are not naturally present in an area, but were introduced by humans. Other names include "non-indigenous," "alien," "exotic pest plant," or "weed." Free from the vast and complex array of natural controls present in their native lands--including competition, herbivores, parasites, and diseases--exotic plants may experience rapid and unrestricted growth in new environments. Invasiveness is characterized by any or all of the following: strong vegetative growth, abundant seed production, high seed germination rate, long-lived seeds, and rapid maturation to a sexually reproductive (seed-producing) stage or the ability to quickly reproduce vegetatively (clonally). Their phenomenal growth allows them to overwhelm and displace existing vegetation and form dense stands.
In the mid-1800s, exotic weeds began arriving with immigrants from overseas. Many were intentionally brought to California by settlers for use as garden and ornamental plants, or in hopes of making profit through agriculture or timber harvesting. Others have been unintentionally introduced in our highly mobile global society. Seeds or other reproductive plant parts can "hitchhike" in bedding, livestock feed, firewood, tires, gravel, ship cargo, on animals, and even on people's clothing. Not all exotics are disruptive, but those weeds that have no natural competitors or predators to control their numbers in new habitats can expand quickly.
The introduction of harmful exotic species is a global problem. A 1998 study published in BioScience concluded that invasive species contribute to the endangerment of 49 percent of all species (and 57% of plant species) designated as threatened and endangered under the Endangered Species Act (Wilcove, 1998). A 2005 Cornell University study estimated that invasive plants and animals cost the US economy $120 billion annually (Pimentel, 2005). Non-native plants are generally present in areas that have been disturbed (usually by human use), such as roadsides, grazed pastures, old homesteads, campgrounds and trails, but the most invasive species eventually move into pristine natural communities as well. Today, around 5% of National Park Service lands are dominated by invasive plants and 70% of documented invasive species on park lands are plants (NPS 2009). Visit the National Park Service's Invasive Species Management site to learn more. Visit our Integrated Pest Management page to learn how Point Reyes National Seashore staff are attempting to protect native flora and fauna by reducing invasive plant species.
Continue reading: Changing our Natural Ecosystems