By 1902, poachers had illegally killed and reduced Yellowstone bison herd size to about two dozen animals. The U.S. Army, which administered Yellowstone, prior to the creation of the National Park Service, took over and protected the last remaining bison from further poaching. Bison from private herds were incorporated into Yellowstone, forming a northern herd. For decades, however, as Yellowstone bisons increased in number, herds were reduced, believing the animals had caused overgrazing. The practice of reducing the Yellowstone bison population ceased in 1968. However, by the 2000s, the reduction of Yellowstone bison herds resumed, "due to increasing numbers and litigation about migration into Montana." As of August 2019, Yellowstone had 4,829 bison from two herds, 3,667 northern bison, and 1,162 central bison.
