BILLINGS, Mont. - More than 2,400 bison from Yellowstone National Park's wild herds have been removed from the population this winter through slaughter, winter die-off and hunting, but park officials say they are not concerned about the animal's ability to rebound or its genetic diversity.

"Our job is to maintain a wild bison population, and we won't take any action that endangers the population as a whole," said Al Nash, park spokesman.

More than 1,700 bison were sent to slaughter or killed by tribal and Montana hunters this winter. Another 700 bison were estimated to have died in harsh weather conditions. Until the snow melts and the grass greens, another 255 cow and calf bison are being held at the park's Stephens Creek capture facility north of Gardiner, Mont., awaiting release.

With the remaining bison population at 2,300, state and federal agencies involved in the Interagency Bison Management Plan have stopped sending bison to slaughter.

"We've shifted our bison management plan and become more conservative," Nash said. "With the exception of some adult bulls, because of the challenge they present in captivity, we have halted shipping to slaughter. Consistent with the Interagency Bison Management Plan, 2,300 prompts us to look at additional nonlethal management activities."

Buffalo Field Campaign, a bison advocacy group, has noted that this winter's slaughter is the largest since the late 1800s and claims that the loss of bison threatens the animal's genetic diversity. The group is calling for the Park Service to maintain Yellowstone's two distinct herds - the northern and central - at 2,000 animals each.

The group bases its numbers on simulations by John Gross and Guiming Wang performed for the Park Service. The simulations found that to maintain 95 percent diversity in bison herds over the course of 200 years, a herd size of 2,000 would be optimum. Most bison slaughtered this year were from the northern herd.

A scientific review of the park's bison ecology in 2005 warned against the possibility of culling most of the northern herd if it exits the park. The review also noted that the northern herd "exhibits biological traits (genotypes, fetal growth, tooth wear patterns) distinctive from central range bison."

One bison advocate, Bob Jackson, claims there's a third distinct herd of park bison threatened by increasing intrusion from backcountry visitors.

"Yellowstone's mountain bison of Pelican Valley need to be recognized for what they are - a unique herd that is worth saving," Jackson said in a statement. Jackson, a former backcountry ranger in Yellowstone, raises bison in Iowa.

It's estimated that bison numbered 30 million to 60 million in North America before Euro-American settlement. Due to human slaughter, that number was reduced to a few hundred by the mid-1880s. One of the few remaining cores of wild bison founded the herds in Yellowstone National Park - a group of 23 bison that were bred in the early 1900s.

In 1967, the population dipped to 418 animals. But by the mid-1970s the herd had climbed past 1,000 and by 1980 hit 2,000. From there, it was only another eight years until the herd reached 3,000 animals, and by 2004 it had broken the 4,000 mark. The population is the largest free-roaming, genetically pure wild bison herd in the United States.

It is estimated that the herd has the capability to reproduce at the rate of 8 to 12 percent a year.

Despite the steady climb in the number of bison, Nash said the park is not overgrazed.

"Available forage has been impacted by the ongoing drought, but the park is not overgrazed," he said. "Bison are not leaving the park in winter because it's overgrazed."