At times, people would mention bringing wolves back to Yellowstone to help control the elk population. Yellowstone's managers were not eager to bring back wolves, especially after so successfully extirpating them from the park. Elk control continued into the 1960s. In the late 1960s, local hunters began to complain to their congressmen that there were too few elk, and the congressmen threatened to stop funding Yellowstone. Killing elk was given up as control method which allowed elk populations to again rise. As elk populations rose, the quality of the range decreased affecting many other animals. Without wolves, coyote populations increased dramatically which adversely impacted the pronghorn antelope population. However, it was the overly large elk populations that caused the most profound changes to the ecosystem of Yellowstone with the absence of wolves. The campaign to restore the gray wolf in Yellowstone had its roots in a number of seminal studies related to the predator-prey ecology of the park. In 1940 Adolph Murie published "Ecology of the Coyote in the Yellowstone National Park". That study and his 1940–41 work "The Wolves of Mount McKinley" was instrumental in building a scientific foundation for wolf conservation. In 1944, noted wildlife biologist Aldo Leopold, once an avid predator control advocate, made the following comments in his review of "The Wolves of North America", Young and Goldman, 1944: By the 1960s, cultural and scientific understanding of ecosystems was changing attitudes toward the wolf and other large predators. In part, this included the emergence of Robert Paine's concept of the keystone species. In the early 1960s, Douglas Pimlott, a noted Canadian wildlife biologist was calling for the restorations of wolves in the northern rockies. In 1970 American wolf expert, David Mech published "The Wolf: The Ecology and Behavior of an Endangered Species" (1970, 1981), an enlightening study of the wolf and its impact on its environment. In 1978, when wildlife biologist John Weaver published his seminal study "Wolves of Yellowstone", he concluded the report with the following recommendation: The gray wolf was one of the first species to be listed as endangered (1967) under the Endangered Species Preservation Act of 1966. However, until the passage of the Endangered Species Act of 1973, there was no legal basis or process for re-introducing the gray wolf to Yellowstone National Park and the Greater Yellowstone Ecosystem. The Endangered Species Act obligated the U.S. Fish and Wildlife Service to develop restoration plans for each species designated as "Endangered".