A bison herd from Elk Island National Park in Alberta returned to its ancestral home in Montana this week.

Seventy-one bison were released Wednesday on a prairie preserve in northeastern Montana.

The release marked a historic homecoming for the bison as their ancestors, part of the Pablo-Allard herd, were sold to the Canadian government in 1906.

The herd was among the last free-roaming herds of bison on the continent at the turn of the 20th century.

While tens of millions of bison once roamed the Great Plains, only an estimated 500,000 bison remain in North America, said the American Prairie Reserve, the group behind the release.

Of these, less than four percent live in conservation herds.

"We knew from the beginning that returning bison to the land would be an important step in restoring the reserve's full biodiversity," said APR president Sean Gerrity.

"When the Canadian government purchased the herd, it helped the species survive near extinction."

"Now we are bringing them back to help restore a complete grassland ecosystem."

The bison join a herd of 140 bison already roaming the reserve, descendants of a herd of 16 animals re-introduced in 2005.

The bison restoration project is part of APR's initiative to create the largest wildlife reserve in the continental U.S., culminating in three-million acres of private and public land and connecting one of the last large sections of untilled temperate grasslands on the continent.

The homecoming is important scientifically and genetically, said Bryce Christensen, APR manager.

The Elk Island bison herd is highly regarded in conservation circles because it is the only source herd for bison proven to be free of cattle gene introgression, a rare distinction that relates to widespread attempts to breed bison with cattle, said Christensen.

Cattle genes can negatively affect bison, involving effects such as altered metabolic rates, reduced fertility or changes in behavior.