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In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: Shoshone National Forest is an integral part of the Greater Yellowstone Ecosystem, which has 1,700 documented species of plants. Since the elevation of the land in the forest ranges from 4,600 to 13,804 ft (1,402 to 4,207 m), which is more than 9,000 ft (2,700 m), the forest has a wide variety of ecosystems. Lower elevations often have sagebrush and grass-dominated vegetation types, while forested areas are dominated by various combinations of tree and shrub species. These include lodgepole pine, which along with Rocky Mountain juniper, and quaking aspen are found at elevations up to 9,000 ft (2,700 m). At higher elevations subalpine fir, Engelmann spruce, whitebark pine and limber pine, are common, each occurring up to timberline. The region above timberline makes up 25 percent of the total acreage of the forest and of that 13 percent is listed as just either barren, rock or ice. The types of plant species is highly dependent on the amount of water available, and trees are more commonly found on higher slopes due to the longer lasting snowfall which keeps the soil moister for a longer time into the summer months. Along lower elevation riparian corridors, cottonwoods and willows are typically dominant. Numerous plant species are endemic to the region including some that are rare. 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 1,000,000 acres (400,000 ha) 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.
Output:
What bugs had killed between 25 and 100 percent of the trees in the impacted areas?