So it's reached the point where we have to ask: When did Yellowstone cease to be a national park and turn into a 3,400-square-mile zoo?

Pretend for a moment that you are a Yellowstone bison, part of the largest remaining wild herd in the world. It's been a long, hard winter in the high plateau, and you're getting hungry. You can't eat the snow, so you migrate to lower ground in search of food. And then you get shot as soon as you leave the west side of the park in Montana.

That is more-or-less the government's bison management plan these days. Since last fall, officials have killed 1,598 of the animals as they sought food at lower elevations. That's out of a total Yellowstone bison population of 4,700 ... well, make that about 3,000 now.

That's what you get for $16 million, which is what state and federal officials have spent on the management plan since 2002. The plan is aimed at preventing the transmission of the the disease brucellosis from bison to livestock. The disease can cause pregnant cows to abort their calves, though there is not a single documented case of bison-to-cattle transmissions.

This "kill every bison" strategy has caused some to question the long-term viability of the herds. Among them is the Government Accountability Office, which released a report last month criticizing state and federal agencies for not taking promised steps to stem the slaughter. This includes finding an effective brucellosis vaccine.

The Interagency Bison Management Plan, implemented earlier this decade, includes the Forest Service, National Park Service, Animal and Plant Health Inspection Service, Montana Fish Wildlife and Parks and Montana Department of Livestock.

Yesterday, three environmental groups filed an emergency petition with the Interior Department and asked Montana Gov. Brian Schweitzer to implement a moratorium on the slaughter. The groups say that because cattle no longer graze in the area of concern, there is little chance the bison could transmit brucellosis. There is also growing concern as bison calving season nears, since the area just outside the park is where female animals often go to give birth.

In the words of Natural Resources Defense Council spokesman Louisa Wilcox, "we ought to give them a break, not a bullet."