Now in it's fifth year, the annual bison roundup at Rocky Mountain Arsenal refuge is something of an institution.

"I think pretty much everyone who gets the opportunity to work a bison roundup relishes that opportunity," said refuge manager David Lucas.

This year, the bison herd is 89 strong, including 20 calves. It's a thriving population, but it's also too many bison for the 2,500 acres of space in the arsenal. So on Tuesday, all the bison were herded up -- but 15 of the youngest won't return.

Listen To Bison Snort And Bang

It may sound cruel, but research has shown that removing young animals from the herd is the best way to conserve diversity, says Lee Jones, a U.S. Fish and Wildlife Service biologist. The genetic testing that determines how many relatives each bison has is more important though.

"If [the bison] has got a lot of relatives in the herd, it's highly related to the herd," said Jones. "So in order to keep inbreeding down, which is when too many animals are too closely related, we keep the animals that are the least related to the herd as a whole."

Refuge Manager David Lucas says the herd's getting another 700 acres this year, and by the end of next year, they'll have up to 6,800. Once that happens, the arsenal will be able to support 110 bison. 

"Bison kind of got left out when we conserved all the other big game species," said Jones. "We have elk in the mountains, we have deer in the mountains, we have bald eagles that we brought back. "

Someday, Jones hopes that they'll be enough land to restore bison on a larger scale.

"Until we have that opportunity, our job is to hang on to the genetics of what that species is and to conserve that species in the best way we can."