The full extent and impact of exotic plants in Yellowstone is not known. Many are found in disturbed areas such as developments, road corridors, and thermal basins; they also are spreading into the backcountry. Several exotics, such as the common dandelion, have spread throughout the park. Exotic plants can displace native plant species and change the nature of the vegetation communities, thus changing the available forage for grazers such as elk and bison. They also undoubtedly impact native invertebrate communities. Controlling more than 195 species, some of which have been well-established for decades, is unrealistic.
Due to the large number of exotic species, the park prioritizes them into several categories, thereby focusing control action on species posing the most serious threat or are most likely to be controlled. The park has made some progress in monitoring, mapping, and control of high priority target species. Using Integrated Pest Management chemical, biological, sociological, and mechanical methods the park can control some of the exotic plants. The park also cooperates with adjacent state and county Weed Control Boards to share knowledge and technology related to exotic plant detection and control. The Biggest Threats Dalmatian toadflax is prominent in northern portions of the park. Intense biological and chemical control efforts during the late 1960s and early 1970s were largely unsuccessful.
Dalmatian toadflax has since spread throughout the Mammoth area and is migrating into the park interior. Spotted knapweed is an aggressive species that, once established, can result in a virtual monoculture. It threatens to displace native grasses on the ungulate winter and summer ranges, which will have a major impact on wildlife. Aggressive control efforts target this species every year to prevent a catastrophic change in park vegetation. Canada thistle grows throughout the park and adjacent national forests. Its airborne seed enables this plant to spread widely throughout the park, invading wetlands and causing radical changes in the vegetation. Ox-eye daisy infestations were originally discovered in the Mammoth and Madison areas. Control efforts have substantially curtailed infestations in these two areas, but monitoring and evaluation continue. This exotic is unpalatable to wildlife; it becomes dominant in meadows. Houndstongue has become increasingly widespread. It appears to have been originally introduced in the park by contaminated hay used by both the National Park Service and concessioners in their horse operations. The seeds act like Velcro, easily attaching to the coats of animals, and thus has spread along animal corridors. Leafy spurge poses another potential threat. Although it has only been found in small infestations so far, it is extremely hard to control because of deep roots and dense vegetation. It becomes a monoculture, forcing out native forbs and grasses.