Among them, the whitlow grass, fremont bladderpod, shoshonea, and the north fork Easter daisy provide vivid white and yellow flowers during the spring and summer. Exotic species of flora that are not native to the region include Canada thistle, Musk thistle, Spotted knapweed, Leafy spurge and Yellow toadflax. These non-native plant species are considered noxious, impacting native plant communities and the species that thrive on them. Native species such as the mountain pine beetle are having an enormous negative impact on some tree species. A survey of the forest performed in 2010 indicated that over of timberland had been impacted by insects such as the mountain pine beetle, spruce bark beetle and Douglas fir beetle, and that the insects had killed between 25 and 100 percent of the trees in the impacted areas. The forest service is addressing the situation by performing controlled burns, selling dead trees as firewood, timber harvesting and spraying the highest value areas. Fauna Since the migration of the endangered gray wolf into Shoshone National Forest after the successful wolf reintroduction program in the Yellowstone region commenced in the mid-1990s, all of the known 70 mammal species that existed prior to white settlement still exist in the forest. Altogether, at least 335 species of wildlife call Shoshone National Forest their home, including the largest population of Bighorn sheep and one of the few locations Grizzly bears can still be found in the contiguous U. S. At least 700 grizzly bears are believed to exist in the Greater Yellowstone Ecosystem, which includes Shoshone National Forest, with approximately 125 grizzlies in the forest. The grizzly is listed as a threatened species by the U.S. Fish and Wildlife Service, and the forest is one of their last strongholds. For what are considered to be "nuisance bears", non-lethal traps are set to capture them so that they can be relocated to remote areas, away from civilization. In the case of the grizzly, each captured bear is tranquilized and then ear tagged with an identifying number. Each number is registered, and if the bear continues to return to areas where they pose a risk of imminent threat to human safety, they are exterminated. The grizzly recovery efforts implemented by federal agencies have often resulted in major disagreements with local landowners and surrounding municipalities. This situation occurs less frequently with the smaller and less aggressive black bear. An active management program, in