In his discussion "Bison in Decline," on this website, Professor Dan Flores gives us an idea of the complexity of the questions and issues relating to the near-extinction of the American bison during the 19th century. At the turn of the 20th century, however, a simpler explanation was sufficient to rouse steadily-increasing numbers of Americans into action in behalf of the species.
Theodore Roosevelt (1858-1919), the 25th president of the United States (1901-1909), was one of the most eloquent and influential spokesmen for the early conservation movement. His Hunting the Grisly [sic] and Other Sketches, a collection of his experiences as a nimrod, opened with a piece on "The Bison or American Buffalo" (1893), in which he summarized the recent history of the species in the following two paragraphs.1
When we became a nation, in 1776, the buffaloes, the first animals to vanish when the wilderness is settled, roved to the crests of the mountains which mark the western boundaries of Pennsylvania, Virginia, and the Carolinas. They were plentiful in what are now the States of Ohio, Kentucky, and Tennessee. But by the beginning of the present century [the 1800s] they had been driven beyond the Mississippi; and for the next eighty years they formed one of the most distinctive and characteristic features of existence on the great plains. Their numbers were countless—incredible. In the vast herds of hundreds of thousands of individuals, they roamed from the Saskatchewan to the Rio Grande and westward to the Rocky Mountains. They furnished all the means of livelihood to the tribes of Horse [Plains] Indians, and to the curious population of French Metis, or Half-breeds, on the Red River, as well as to those dauntless and archetypical wanderers, the white hunters and trappers. Their numbers slowly diminished, but the decrease was very gradual until after the Civil War. They were not destroyed by the settlers, but by the railways and the skin hunters.
After the ending of the Civil War, the work of constructing trans-continental railway lines was pushed forward with the utmost vigor. These supplied cheap and indispensable, but hitherto wholly lacking, means of transportation to the hunters; and at the same time the demand for buffalo robes and hides became very great, while the enormous numbers of the beasts, and the comparative ease with which they were slaughtered, attracted throngs of adventurers. The result was such a slaughter of big game as the world had never before seen; never before were so many large animals of one species destroyed in so short a time. Several million buffaloes were slain. In fifteen years from the time the destruction fairly began the great herds were exterminated. In all probability there are not now , all told, five hundred head of wild buffaloes on the American continent; and no herd of a hundred individuals has been in existence since 1884.
1. (New York: The Review of Reviews Company, 1904), p. 3.