Bison Create Their Own Green Up

Thanks to the University of Wyoming for this article.   On a typical June day in Yellowstone, it's not unusual to see hundreds of bison grazing in the Lamar Valley. The herds appear to aimlessly move back and forth through meadows threaded by a winding river, just passive figures in an idyllic scene. But, as it turns out, that's far from the full picture. In fact, with every blade of grass that bison bite off and swallow, they are fundamentally manipulating the landscape to maintain the best forage for themselves. They even change the way spring green-up occurs in Yellowstone's vast grasslands. Without bison moving freely on the landscape, the springtime season of plant growth in Yellowstone would be shorter, the habitat would not be as green, and the grasses would not be as nutritious. Remarkably, that allows bison to migrate differently than other species. Bison migrate from Gardiner Basin, Yellowstone National Park. NPS photo by Neal Herber

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3 thoughts on “Bison Create Their Own Green Up

  1. Nature is far more complex than any one study can capture, and this one raises some questions as fascinating as its conclusion.

    Granted, in the short term, the effect is an increase in forage quality. But what about the long term? Depending how long it takes the bison to return to those grazing lawns and graze that high-quality re-growth, they are likely over-grazing their preferred plants.

    If you go to northern Yellowstone, you’ll find a landscape that is heavily grazed. Even though elk numbers decreased steeply from what they were in the mid-1990s, they were largely replaced (perhaps out-competed?) by a rapidly growing bison population, and in recent years (except the most recent survey) elk have been increasing again. Grazing exclosures still show an obvious difference between grazed and ungrazed areas.

    If that level of grazing and browsing happened not with bison in a National Park, but with cattle on a National Forest, people would be howling from the rooftops about “overgrazing” (even if they don’t understand what that actually is).

    Nevertheless, something about that large aggregation of grazing animals appears to be advantageous. For one, it’s the prey species’ natural anti-predator behavior. Bison aggregate even more than elk do; they’re also less likely to run from predators–and they’re a lot harder for predators like wolves to kill.

    Ecologists have long debated whether nature is organized in a bottom-up or top-down fashion. It’s both: for example, deer and elk “surf the green wave” (bottom-up), but bison help maintain it (top-down).

    One has to wonder, then: might the (top-down) predation pressure exerted by wolves and grizzly bears help create the aggregation into groups, that then helps create those grazing lawns of high-quality forage?

  2. Could this bison related article be posted to North American Bison Group and Bison Bison FB sites,please.I am a member of each group an feel your bison articles would be of great interest to members of both groups. Thanks,Jim Wedeking

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