Though images of bison aren't the first thing that pop into mind when you think of Grand Canyon, the animals that lumber like walking boulders have become a significant attraction for visitors to the North Rim.

The bison are part of a herd that was introduced to the Kaibab Plateau in the early 1900s, courtesy of ranchers who wanted to toughen up their cattle herds by mixing in some bison genes. Back then, bison were stuck in a genetic bottleneck with, at one point, fewer than 110 animals in existence. Things are different now: Bison are in no danger of extinction, and the animals are flourishing with a current population of half a million.

The Arizona State Game and Fish Department manages the bison at Grand Canyon for both hunting and wildlife viewing. Game managers estimate the herd of 300 as growing by 10 percent every year, a rate that takes into account deaths by hunting, encounters with vehicles and natural causes.  At this rate, the herd should double in size every seven years.

At first glance, the thrill of seeing bison at the North Rim fits right in with the mission of the National Park Service. National parks were created with a mandate to "conserve the scenery and the natural and historic objects and the wild life therein..." But there's a catch: All these bison were introduced; they were never among the wildlife that came with the park.

Meanwhile, Grand Canyon National Park harbors unique plant communities that don't cope well with animals that can weigh 2,200 pounds.

When park botanist Nancy Brian compiled a list of 63 "special status" plants found in Arizona in 2000, 24 of them could be found on the Kaibab Plateau. Recent work indicates that another seven species should be added to this list, and that all seven are endemic to the Kaibab Plateau or Grand Canyon, meaning that they grow nowhere else in the world. Unfortunately, the places many of these Kaibab plants need to survive -- the most sensitive springs, meadows and ponds -- are also the places most likely to be harmed by bison that eat, trample and wallow in them.  After bison linger in an area for a while, it tends to resemble well-used ranch land.

Bison impacts also extend deep into the Grand Canyon itself.  Springs 1,000 feet below the rim create unusual habitat for rare plants, and those areas have become damaged by bison trampling and browsing.  As they move around, the bison create multiple trails at many remote overlooks on the west side of the North Rim.  The condition of these steep slopes is not unlike the condition of the Tonto Platform before the Park Service removed wandering burros from the park.

Here's my solution: Just as park managers rounded up burros and moved them out of the park a few decades ago, managers now need to remove bison from the North Rim. It will not be an easy job, to say the least. In 2005, the Park Service hired a group of cowboys to round up the offending bison. Ten days in the saddle yielded no animals, a failure reminiscent of the drive to remove deer back in the 1920s.  A sturdy fence could block movement of bison into the park, or onto sensitive areas, but would likely be prohibitively expensive to build and maintain. A fence might also harm other wildlife and destroy wilderness values. Another solution would be to push most of the bison out of the region and cull any animals still found on the Kaibab Plateau. This, of course, might be politically unpalatable.

Ron Sieg, who manages the bison herd for the Arizona Game and Fish Commission, says there's just no easy solution to the bison problem. So he proposes finding the option that's "least bad." Park advocates will want to see Grand Canyon protected at all costs, hunters will not want to lose their chance to hunt bison "in the wild," and animal-rights advocates will insist that bison must not be harmed.

Perhaps the greatest challenge for park managers is to take any action, and to commit to it, even if it means removing one of the country's important symbols from one of its iconic parks. But the Game and Fish Department needs to stop a bad situation from getting much, much worse.

Bison may be great to look at on the North Rim, but they don't belong there and it's time they went.

Glenn Rink is a contributor to Writers on the Range, a service of High Country News, hcn.org. He is a botanical consultant in Flagstaff, Arizona.