Yellowstone officials say more than half of the park’s 4,700 bison have died this winter.

The new estimate comes after recent aerial surveys that wildlife managers used to revise estimates of winter-killed bison from roughly 400 animals to roughly 700 animals.

The winter-kill estimate is in addition to roughly 1,700 animals killed by humans this winter, most of which wildlife managers captured and sent to slaughter for attempting to leave the park near the North and West entrances.

The news comes after Park Service officials announced a deal Thursday that could eventually allow 100 animals to migrate out of the park’s North Entrance, through land owned by Church Universal and Triumphant, and onto a portion of the Gallatin National Forest.

The 2,300 Yellowstone bison that remain still have to contend with several weeks of snowy conditions in most areas of the park, in addition to grizzly bears and wolves, which often prey on bison in the early spring because their weakened condition leaves them more vulnerable to attack.

With a population of 2,300 animals, the Park Service has passed a threshold in the 2000 Interagency Bison Management Plan that allows wildlife managers to use more non-lethal management techniques such as hazing. If the population sinks below 2,100, officials are required to use more non-lethal techniques. The population objective for the Yellowstone bison herd is 3,000 animals.

According to Yellowstone spokesman Al Nash, park officials won’t send anymore bison to slaughter for leaving the North Entrance of the park this year, with the exception of bulls, which he said are too large and too aggressive to hold at the park’s Stephens Creek Bison Capture Facility. Earlier this month, officials said they would start holding pregnant bison at the facility in order to try and maintain a viable population of animals.

Nash said there are currently about 240 animals being held at the facility until snow melts in the park. Officials say the facility can handle a maximum of about 400 bison for a short period of time.

“In the Gardiner and Mammoth areas, you are starting to see some green plants,” said Nash, who said some clear patches of ground are also starting to appear in Lamar Valley. “There’s still a lot of snow. We are days away [from when plants will emerge] at this point. It’s not imminent, but it’s coming soon.”

Officials with the Buffalo Field Campaign call this winter’s management actions the worst slaughter of bison since the 1800’s.

Darrell Geist, habitat coordinator for the Buffalo Field Campaign, said the slaughter of more than half of Yellowstone’s bison bodes poorly for the long-term genetic health of the herd. “Every bison removed from the population can no longer contribute genetic characteristics to the herd,” he said.

Further, he said bison have an important ecological niche in the park that benefits other animals species, especially carnivores such as grizzly bears, wolves and eagles that rely on bison winter-kill for an extra boost of nutrition in the spring. Bison also help keep open spaces free of trees for other ungulates, he said.

Yellowstone National Park and Montana Department of Livestock officials say they try to keep bison from leaving the park to prevent the spread of brucellosis to cattle. Brucellosis is a bacteria that causes some ungulates to abort their fetuses. Researchers aren’t sure if the disease readily passes from bison to cattle outside of laboratory conditions.