How did the free bison population disappear?

The total world population of the bison population in the early 1900s did not exceed 1,600 individuals. The wars and revolutions of the 20th century put them on the brink of complete annihilation.
In Pilyavino bison were destroyed in 1917, in the Gatchina hunting – in 1918, in the Crimea – in 1919. In Belovezhskaya Pushcha, the last free female of the bison was shot dead on February 9, 1921, by former security officer Bartholomeus Shpakovic. It is assumed that this was an act of protest against the new government.
In the Caucasus, wild bison were victims of poaching and epizootics of anthrax and foot and mouth disease. The last three free Caucasian bisons were killed by shepherds-Imeretins in 1927.
By 1928, the bison was completely exterminated in nature. Therefore, it was necessary to start the reconstruction of the bison from those animals that could be preserved in zoos and private zoos in Europe.