IS YELLOWSTONE OVERGRAZED?
By George Wuerthner
There is a lot of misunderstanding, and in some cases purposeful distortion of the condition of Yellowstone's rangelands.
The recent bison slaughter has again brought this issue to the forefront. Critics claimthat bison are leaving Yellowstone because there's nothing for them to eat. They assert Yellowstone is over-stocked with ungulates and overgrazed as a consequence. The image conjured up is of barren slopes with little forage. Most of those making such assertions have spent very little time in Yellowstone, and base their conclusions upon preconceived notions of what is "normal" as well as false cause and effect analysis.
Let me start by saying there's plenty for bison, elk, and other ungulates to eat in Yellowstone--it's just that you have to move five feet of snow out of the way to get at it.
Yellowstone is largely a high plateau, with little lower elevation terrain. Heavy snow has driven bison and other ungulates to lower elevations where foraging is easier, but most of the terrain without snow is outside of the park. That's a subtle, but significant difference. Wildlife moving beyond Yellowstone's borders signals the failure on our part to provide sufficient low elevation winter range to support native populations. What Yellowstone needs is not more intensive management, but greater efforts to expand winter range opportunities beyond park borders.
Numerous scientific studies conducted in the past five years do not support the notion that Yellowstone is "overgrazed." Although there have been a few poorly designed studies that critics claim support their views, such research must be weighed along with the rest of the evidence—all of which supports a different conclusion.
The overwhelming evidence and research to date can find no particular degradation of Yellowstone's rangelands. This is not to suggest that these studies don't document some effects to the vegetation as a result of ungulate herbivory. They do. To find changes in the landscape but no serious degradation are not mutually exclusive.
One change that most researchers agree upon is that elk may have influenced the number and extent of mature aspen and willow on the winter range. This alone, however, is not necessarily an indicator of "overgrazing". Shrubs are accessible to ungulates feeding in deep snow, but overgrazing involves more than heavy cropping of shrubs. The vast majority of the scientific evidence compiled by a wide variety of researchers--many of them from outside of the park service has produced no evidence that overgrazing as traditionally defined is occurring in Yellowstone.
These findings include:
A truly overgrazed rangeland would exhibit downward trend in most, if not all of these indicators of rangeland health. The fact that such evidence cannot be measured should at least cause one to pause long enough to rethink the Yellowstone "overgrazing" issue.
The explanation for these findings is that native species use the landscape differently than domestic animals.
Native species browse shrubs and grasses heavily in the winter when plants are dormant. Ungulates then migrate with green up of plants, seldom grazing or browsing a plant more than once in a single season. In summer when grazing effects are most damaging to growing plants, ungulates are widely dispersed. Similarly, trampling of soils, and streambanks are less damaging in winter when snow and frozen soils limits impacts. Finally, natural periodic population reduction due to drought, fires, heavy snow, and predators means that native ungulate numbers are occasionally reduced—sometimes significantly, as in this winter.
There is no denying that elk and other wildlife have influenced the park's rangelands, but this is no different than finding that fires have changed the age and structure of it's forests, or that wolves and other predators affect the age and structure of prey populations. But fires and predators don't destroy ecosystems, indeed, they are fundamental to a healthy ecosystem. In other words, the mere finding of change in some aspects of the northern range, is not, by itself, necessarily something to be alarmed about.
To suggest there's some "right" number of animals that should exist in a place like Yellowstone demonstrates a misunderstanding how natural systems operate. Rather what we should be looking at is how well natural evolutionary and ecological processes operate. In Yellowstone, these natural forces are still largely intact and most of the factors that compromise them are a consequence of human manipulation and activities that exist beyond the park borders.
Let's keep a little perspective on the issue. Yellowstone has one of the healthiest native cutthroat trout fisheries in the West, its waters are among the purest, it's geological and scenic wonders some of the most intact, it is home to one of the largest bighorn sheep populations in the West, one of the largest elk herds, one of the large free-roaming wild bison herds, one of the last places where large predators like grizzlies and wolves still persist, where the influence of predators, wildfires and other ecological processes still operate. If Yellowstone is an example of "mismanagement" as critics suggest, then perhaps we need more mismanagement of this kind in the rest of the West.
George Wuerthner is a noted nature photographer, environmentalist, and expert on public lands issues, especially grazing . . . . Ralph Maughan