The hundreds of bison that roam the far northern reaches of Arizona are descendants of the massive animals brought to the region in the early 1900s as part of a crossbreeding operation.

But a report issued Thursday by the National Park Service says that's just a snapshot of bison history in the region.

The report maintains the animals' designation as native wildlife but says bison weren't a dominant part of the landscape. Instead, they appeared for thousands of years in small, dispersed herds on the edge of their historic range.

The classification as "native" means the animals can't be removed entirely from Grand Canyon.

The report comes as officials are preparing to quickly reduce the population from as many as 600 animals to between 80 and 200 to prevent damage to park resources.