Invasive nonnative plants can displace native plant species, including some endemic to the park’s geothermal habitats, change the nature of vegetation communities and affect fire frequency and the distribution, foraging activity, and abundance of wildlife. These changes can profoundly affect the entire ecosystem. For example, nonnatives that are unpalatable to wildlife may replace preferred native plants, leading to changes in grazing activity. In turn, this stresses plants not adapted to grazing.
Invasive plants have altered views of the park’s cultural landscapes and historic districts. Seeds may be spread by people and their vehicles, wild and domestic animals, and sand and gravel used for construction and maintenance work. The most vulnerable areas have been disturbed by human use: along the roads, trails, and rivers; though they are spreading from developed areas to the backcountry. Restoring native plants in an area that has become infested is extremely difficult.
In addition to about 1,386 native plant species, 225 nonnative species have been documented in the park through ongoing survey efforts. Not all of these nonnative species are still present in the park, but most of them are.
Managing Invasive Species
Controlling all the invasive plants—some well-established—is unrealistic. Staff prioritize treatments based on the threat they pose to native plant communities and the likelihood for successful control. Some infestations can be eradicated if the species is treated when the outbreak is still small; other species, such as spotted knapweed (Centaurea maculosa), are so common that stopping them from spreading is the primary goal. This strategy has helped prevent high priority invasive species from moving into wilderness areas where control is more difficult.
Nonnative vegetation was found on 7,189 acres of the 7,914 acres inventoried in 2015. Sometimes exotic plants are found in pure populations with few native plants, but are most often mixed within native plant communities. Based on program priorities for species, the equivalent of 120 acres of pure non-native populations were treated during 2015. Physical removal is the preferred method of control when feasible, but pulling or cutting of some of the perennial species serves to stimulate new growth, and the use of herbicides becomes necessary to control aggressive species over large areas. Plants were physically pulled or clipped on 20 acres while the remaining 100 acres were treated with herbicides. Most of the 36 species targeted for treatment are listed by the states of Idaho, Montana, or Wyoming as “noxious weeds,” which means they are detrimental to agriculture, fish and wildlife, aquatic navigation, or public health.
Preventing the Spread of Invasive Plants
Prevention efforts include control of construction materials entering the park, equipment inspections at park entrances, allowing only certified weed-free hay to be transported through the park, restrictions on the use of hay in the backcountry, and planting native species where ground disturbance has occurred.
To improve nonnative plant management throughout the region, park staff work with land managers from other government agencies, the Greater Yellowstone Coordinating Committee’s Weed Subcommittee, and the National Park Service’s Rocky Mountain Exotic Plant Management Team. The park uses Integrated Pest Management—chemical, biological, sociological, and mechanical methods—to control some of the nonnative plants. The park also cooperates with adjacent state and county Weed Control Boards to share knowledge and technology related to nonnative plant detection and control.
Number in Yellowstone
Canada Thistle (Cirsium arvense)
Dalmatian Toadflax (Linaria dalmatica)
Houndstongue (Cynoglossum officinale)
Leafy Spurge (Euphorbia esula)
Ox-eye Daisy (Leucanthemum vulgare)
Spotted Knapweed (Centaurea maculosa)
Resource managers target the most invasive species for control or removal.
Words to Know
Native species have occurred, now occur, or may occur in a given area as a result of natural processes. Native plants evolve within their own ecological niche in concert with other native plants.
Nonnative species (also known as exotic, foreign, non-indigenous, alien, or weeds) live outside their natural range and arrived there by human activity, either deliberate or accidental. Nonnative species typically have few biological controls to keep their populations in check.
Invasive species have the potential to thrive and spread aggressively outside their natural range, and can include native species. A naturally aggressive plant may be especially invasive when it is introduced into a new habitat. An invasive species that colonizes a new area often has an ecological advantage because the insects, diseases, and foraging animals that naturally keep its growth in check in its native range are not present in its new habitat. An invasive plant that sustains itself outside cultivation is considered naturalized and has not “become” native.
Sheley, Roger L. and Janet K. Petroff. 1999. Biology and Management of Noxious Rangeland Weeds. Corvallis: OSU Press.
Whitson, Tom L. et al. 2002. Weeds of the West. Western Society of Weed Science. Jackson, WY/Grand Teton Lithography.