The number of bison in Yellowstone National Park is again at a historic high as another winter of control efforts and controversy approaches.

The high numbers - biologists figure the population is around 4,200 - will likely help buffer the effect of sending some bison to slaughter because of a government plan to protect neighboring cattle from brucellosis.

At the start of last winter, park officials estimated about 4,000 bison in Yellowstone. Nearly 300 were sent to slaughter last spring and nearly 600 calves have been born since.

“We've certainly seen over the last few years the population can sustain some level of mortality - that includes natural mortality and management mortality - and still not cause the population to decline,” said Rick Wallen, a Yellowstone wildlife biologist.

But that doesn't mean park officials are eager to have the bison killed.

Instead, they'd like to see the state of Montana get more comfortable with allowing more bison to leave Yellowstone in the winter to occupy designated areas.

But for now, a state and federal plan signed in 2000 is providing a roadmap for what happens next.

Although government agents will continue to haze and, if necessary, slaughter bison that leave Yellowstone this year, more emphasis is being placed on vaccinations.

The Montana Department of Livestock is proposing vaccinating calves and yearlings that leave Yellowstone. The program, which could be implemented this winter, would vaccinate only young bison that are captured during ongoing efforts to keep bison from leaving the park.

“There's no intention to go out and capture more just for the sake of vaccinating,” said Tom Linfield, state veterinarian with the Department of Livestock.

The proposal, which has drawn opposition from animal rights groups and support from livestock officials, would use a vaccine known as RB51.

The vaccine's record of success is spotty.

“We all recognize there may be some shortcomings as far as efficacy,” Linfield said, but he added that the interagency plan for bison calls for a vaccination program once a safe vaccine has been identified.

Aside from acknowledging that the vaccine provides mixed results, state officials also said the bison vaccine won't succeed in eliminating brucellosis until a similar program is in place for elk, which also carry the disease.

Brucellosis is a contagious disease that can cause abortions and other problems in bison and cattle. Ranchers and Montana state officials have worried that bison that wander out of Yellowstone could spread the disease to nearby cattle. Such a development could jeopardize the state's brucellosis-free status and lead to restrictions on exports.

The 2000 state and federal plan gives authority for bison to be hazed, captured and, in some cases, sent to slaughter if they leave the park. Some bison are allowed to leave the north end of the park near Eagle Creek. On the western boundary, 100 bison are allowed to occupy a designated area as long as they have been captured and test negative for brucellosis.

The Buffalo Field Campaign, a pro-bison citizen group, and others have emphasized that there has never been a documented case of brucellosis being transmitted between a wild bison and domestic cattle. A better way, according to the group and some local property owners, is to manage the cattle to ensure they are kept away from bison.

Controversy tends to heat up in more severe winters when snow covers forage and bison move west and north in search of food at lower elevations.

A harsh winter in 1996-97 led to the deaths of more than 1,100 bison. The population dropped from 4,100 in 1994 to about 2,300 in 1997.

Several milder winters more recently gave the bison population a chance to rebound. Today, Wallen said, the bison appear healthy and robust despite apparently increasing predation by wolves, grizzly bears and other carnivores.

“In six years, the population (has) doubled,” Wallen said. “That's another indication that the population is doing really well.”

Although one research project has indicated that Yellowstone's “carrying capacity” may be around 5,000 bison, Wallen said wildlife management is more than just crunching numbers.
Complex factors such as weather, forage availability, population dynamics and other issues come into play when determining how many bison should be on the landscape.

Even though the bison population has grown in recent years, Wallen said, Yellowstone isn't full of bison yet.

“I would estimate that we're probably not at any maximum biological carrying capacity yet,” Wallen said.

It's possible that the park could one day approach a “carrying capacity,” but predators, severe winters, management and a proposed hunt will likely keep the population in check.

“I've got this hunch that over the next 50 years, the population will bounce between 2,300 and where we are now,” Wallen said.

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