There are lots of reasons to be interested in the fate of the Yellowstone bison herd. Here's a very quick overview:
1. The Yellowstone bison herd was the only wild herd in the U.S. to survive westward expansion. There were 23 wild bison left in the country in 1901. They were protected inside Yellowstone, and today's herd of close to 5,000 are their descendants.
2. These bison are "genetically pure" -- their DNA shows no introgression with cattle. Most bison in the country today have some cattle genes.
3. Other wild bison herds were killed off and then re-introduced later. These animals have had a continuous connection to the greater Yellowstone area for millennia.
4. The Yellowstone bison herd is several times larger than any other conservation herd in the U.S. (such as the Henry Mountains herd in Utah or the Wind Cave herd in South Dakota).
5. The herd has recovered to the extent that they're wild migratory instincts are waking up.
6. Many of the animals in the Yellowstone bison herd have brucellosis – a bacterial disease which both elk and bison in the greater Yellowstone area originally contracted from cattle. This is one of the reasons their urge to migrate out of Yellowstone National Park has been highly controversial.