For hundreds of years game animals have been hunted throughout the world. Deer, elk, rabbit, squirrel, ducks, geese and wild turkeys are hunted in the United States. Unregulated hunting has seriously threatened some species of wildlife. Here are a few examples. Herds of buffalo that once numbered in the millions were nearly wiped out by hunters in the 19th century. They were killed for enjoyment and profit. By 1895 only 400 remained in the United States. In the west pronghorn and deer populations were nearly wiped out to feed the growing population. Some game birds such as the passenger pigeon were hunted to extinction. After all of these animal populations were decimated some people began to take notice. The general public caused some states to pass laws protecting some animals. The government also started to set aside land for national parks and wildlife refuges. In 1872 Ulysses S. Grant signed a bill that created Yellowstone National Park. This was the first ever national park. Several other national parks were created shortly after. Other laws were passed to set dates for hunting seasons and set bag limits. These laws also outlawed inhumane hunting methods. Once protected by laws and regulations game animals began to flourish. The spreading of human settlements also helped animals flourish. The settlers killed off predatory animals like wolves and grizzly bears. The animals that spread the most were elk and deer. These animals spread so rapidly that the area they were living in became over populated. This caused many of them to die of starvation. In 1933 Aldo Leopold ,an American biologist, developed a theory that one unit of habitat can only support a certain number of animals. Any extra animals needed to be harvested by hunters because they would probably die from starvation anyway. Once this theory was proven hunting laws were redesigned to allow hunter to harvest enough animals a year without harm