Rangeland, pastureland, national parks, and other natural areas account for about 48% of the total land area in the United States. These lands are critical for livestock production, wildlife habitat, and protecting the integrity of ecological systems. More than 3,300 non-native plant species occur within natural areas in the 48 contiguous United States, and 60 of those are considered to have significant economic or ecological impacts.
Some of the most problematic species include downy brome (cheatgrass), Canada thistle, biennial thistles, leafy spurge, yellow starthistle and knapweeds. The majority of these invasive plants or “weeds” have been introduced from other continents and have few natural enemies to control populations, allowing these species to flourish.
The rate of introduction and spread of invasive plants has increased dramatically over the past 150 years with increases in human activities, trade, and commerce.
Weeds such as the knapweeds, starthistle, leafy spurge, and thistles infest millions of acres, primarily in the western United States (Table 1).
Invasive plants can have a significant impact on humans and the environment. Their negative impacts may be associated with livestock production, native plant and animal communities, ecosystem processes (e.g. hydrologic cycles and fire regimes), land values, and human health.
Invasive non-native plants can alter structure, organization, and function of native plant communities and are a threat to biodiversity. Changes in plant composition also impact wildlife by eliminating food sources or modifying critical habitats.
Invasive plants can also impact soil and water resources. Tap rooted species such as the knapweeds increase surface runoff and sediment yield compared to grass-dominated sites, impacting water quality and lowering the production potential of the land. Yellow starthistle depletes soil moisture reserves and alters the water cycle by utilizing moisture reserves earlier than associated native plants. Riparian plants such as saltcedar impact hydrologic cycles by increasing sediment deposition thus restricting stream channels and increasing severity of subsequent floods.
Invasive plants on rangeland cost billions of dollars annually in the United States based on cost of control, ecosystem losses, and direct impacts to the livestock industry. Weeds impact grazing lands by lowering yield and quality of forage for livestock, impeding access to desirable forage, poisoning animals, increasing costs of managing and producing livestock, and reducing grazing land value.