Top: This bull is so swollen with the abundance of new grass that it looks as though it is pregnant. A good supply of browse in early summer is vital to the chances of the bull surviving the rigors of rut and the possibility of a harsh winter.

From the NPS Northern Range Newsletter: "The natural regulation of wildlife may involve both an animal's internal physiology and its external environment. To varying degrees, wildlife populations exhibit "self regulation," which means their growth tends to slow down as the population becomes more dense and to increase if their numbers decline - a biofeedback response. For example, as more elk inhabit the same range, the cows carry less fat and produce fewer calves, and the calves that are born weigh less and are therefore less likely to survive. As the northern Yellowstone elk population has grown, researchers have documented a decline in pregnancy rates and increases in the winter mortality of calves, yearlings, and older bulls.

Environmental factors such as climate and predation also play a large role in controlling an animal population. During a predation study done from 1987 to 1990, about one third of the elk born on the Northern Range were lost within one month to predation by grizzly bears, black bears, coyotes, and golden eagles, and an average of 20% of the population died each winter, mostly from undernutrition in the very young and very old.

But while a policy of natural regulation may work for elk on the Northern Range, it's not appropriate in all wildlife management situations. National Park Service policy and federal legislation will continue to require intervention in certain circumstances - for example, to restore wolves and native fish, to suppress exotic plants and animals, to fight fire in specified situations, and to cull bison. Hunting on public lands adjacent to the park can also be used to complement natural regulation. The challenge is to pay careful attention to the consequences of ecosystem processes while resisting the temptation to step in to "fix a problem" that may be more complex or of a different type than first appears."

"An increase in elk numbers has also been blamed for erosion and heavy sedimentation in Northern Range rivers. To determine if there was any scientific basis for this, a team of researchers from government agencies and Trout Unlimited mapped erosive lands in the Yellowstone River drainage from the park to Livingston, Montana from 1985 to 1987. They found that most of the sediment that muddies park rivers comes from four steep and geologically unstable areas such as the Grand Canyon of the Yellowstone River and the higher elevations of the Lamar River watershed. In none of these places are ungulates significant soil movers.

Consequently, even if the Northern Range were rid of all ungulates, its rivers would still become muddy, especially during spring snowmelt and intense thunderstorms, and following fires when vegetation is reduced. These processes are part of the natural functioning of a wildland ecosystem.

Sediments in rivers throughout the park are within the normal range observed in other western streams, and the Lamar Valley is still considered a blue-ribbon trout fishery, comparing favorably with that of other sport fisheries that have no wildlife herds grazing nearby."

Below: Cow elk cross the snowmelt swollen waters of the Gardner River in northern Yellowstone.