Wolves and Watersheds

With the reintroduction of wolves to Yellowstone National Park, watersheds are experiencing a renaissance in what scientists call “trophic cascade.” A recent article and short film on National Geographic examines how wolves are helping and changing rivers.

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  • Tobin

    Actually, this has been the subject of much criticism lately. Basically, this was touted a number of years ago, and while there are parts and pieces that are supported, like everything else in an ecological sense, there are multiple variables in play. Simply allowing the elk and bison to migrate out of Yellowstone like they did historically, would have many of the same effects.

  • Steve

    I’m sorry to report that this video is junk science. I’m on the board of a foundation focused on conservation, and through this role I’ve been working on some projects in Wyoming with several of the best ungulate scientists in the country, two of whom have spent a great deal of time studying the interaction of elk and wolves. In two separate studies, they have found that wolves do not cause elk to change their behavior or habitat use enough to cause the ecosystem changes with which they are credited in this video. Elk do act a bit differently when wolves are around, but only when they come quite close (within 1 km by the most comprehensive study). In that same study, 1 km encounters only happened about once every 9 days, so the ability of wolves to alter the way elk structure the plants they forage on, let alone entire ecosystems, is a bit constrained. Notably, wolves do not cause elk to avoid riparian areas; no definitive elk movement study has shown that. There has been a little tree/shrub regeneration in some parts of the Park, but that is believed to be the result of a huge decline in the elk population, not a change in elk behavior. The famous northern Yellowstone elk herd has been reduced from (depending on how you count it) about 20,000 animals to about 3 or 4,000 animals, due to combination of increased hunting, the worst drought since we started measuring such things in the late 1880s, and increased predation. Interestingly, Montana has issued many cow elk licenses to human hunters over the past decade in an effort to suppress the herd (ranchers control MT politics, and they don’t like elk because they can transmit brucellosis to cattle; of course, brucellosis was initially introduced to wild elk and buffalo from cattle). Predation by wolves and grizzlies has also had an impact, but smaller than that of human hunting. A reduction of 15,000 elk saves a lot of forage.

    Grizzlies are now believed to be having a bigger impact on the elk population in parts of the Park than wolves. As a result of the collapse of cutthroat trout in Yellowstone Lake, which has occurred because of an illegal introduction of lake trout sometime in the 1980s, grizzlies which used to feed on spawning cutthroat — exactly like grizzlies feed on spawning salmon in Alaska (lake trout spawn in the lake, so provide no food for bears) — have switched to elk neonates; calves up to 2 week old. Most wolf predation is considered “compensatory”, because they often kill old or sick animals. They also kill healthy animals, including mature bulls, but on a stressed winter range, the death of a healthy animal may allow another to survive the winter. Bear predation of elk neonates, on the other hand, is considered “additive” predation, because most of the calves would grow up to be perfectly healthy adults.

    Scientists are always trying to come up with new discoveries, and some get carried away, or simply do lousy research. Unfortunately, this phony trophic cascade story has resonated with the public and some parts of the conservation community. The convenient notion that wolves might be able to trigger these positive cascading effects without killing elk (i.e. by just scaring them out of riparian areas, keeping them on the move), has been, I think, part of the allure of the story. Recent research has more or less debunked this idea, but that part of the scientific discovery has been slow to work its way into the media and the public discourse.

    • Michael Brewick

      Very much appreciated, Steve. It is news to me that cattle ranchers are anti-elk, how interesting. I know from personal experience that many people in the area are convinced wolves are killing off all the elk. Of course, these are the same people who are killing them off, largely.