When did we realize that the indiscriminate killing of animals had a negative effect on the environment? I know that for the majority of American history, the prevailing thought process was that certain animals like wolves, bears, and cougars are bad and dangerous and they should be killed off. We brought many animals to the brink of extinction in the 19th and early 20th century. After this it was like we suddenly came to our senses, started creating national parks, and reintroducing animals. Who brought on this change if there was a specific person?
There was growing interest in preserving wild spaces throughout the late 19th c., popularized by writers and outdoor enthusiasts like William Harrison Murray and George W. Sears. John Muir and some others founded the Sierra Club in 1892. In 1894 Congress banned hunting in Yellowstone Park with the National Park Protective Act . There was a famous 1896 court case, Geer vs Connecticut, which established that each state owned its wildlife and could regulate what was done with it, and manage it. Teddy Roosevelt was the first conservationist President: between 1901-1909 he created the US Forest Service and 150 National Forests, five National Parks, and the Antiquities Act under which he created 18 U.S. National Monuments and dozens of bird reserves.
So, more responsible management of the wilder areas was in the spirit of the times. But the biggest step specifically towards regulating hunting, the biggest change in the practice, was with the Migratory Bird Act of 1909, which ended market hunting of migratory waterfowl ( since they crossed state boundaries, they were under Federal jurisdiction). That Federal law was pretty soon followed by state laws banning market hunting in general. It was market hunting which had had the worst effect on game populations, and it was after it was banned that wildlife populations generally began to recover.
Old Time Market Hunter Reminisces: Says Protection Bringing Game Back