Trophic cascades in action

This is a classic example of a popular theory in community and food web ecology– trophic cascades. The story goes like this: top predators (in this case, wolves) prey upon herbivores (elk) and control their population size. Herbivores feed on plants, and when herbivores are controlled by predation, plants do better. If top predators are removed, herbivore populations increase, more plants are consumed, and overall plants do worse.

Back to our story. In 1995, something really miraculous happened in Yellowstone. There was enough interest and political will to allow Park biologists to reintroduce a few wolves, and then a few more the following year (you can find more backstory in this book). Wolves quickly became established on the northern range, and their population grew. They preyed upon the large elk herd, and elk numbers declined (other factors contributed, like people hunting elk outside the park boundaries).

Declining elk numbers should mean that plants should do better, right? That’s what the ecological theory predicts. But it turns out the story is a bit more complicated.

Beaver face: Ann Cameron Siegal

Beaver dams have key feedbacks to willow stands. They raise water levels behind the dam, giving willow roots easier access to water, and increase flooding, a disturbance required for willow reproduction.

Who was it that said “beavers change things; that’s what they do“. Oh, right that was ME. I’m delighted that your missing link turned out to be castor in nature! Kristin when you’re done researching Yellowstone, maybe you’d appreciate a trip to Martinez? We would be happy to help you get to the bottom of this…

Yellowstone Wolves

Need Help From Beavers

To find out whether wolves could rescue the willows, Marshall and her colleagues charted willow growth at four sites in the northern range. At each site, the researchers fenced some plots to provide total protection from browsing elk and other animals. They also built dams—hauling in logs by helicopter—in streams near some plots to mimic the effect of beavers. Some plots were dammed and fenced; control plots were neither dammed nor fenced, making wild wolves their only possible protection from elk.

After 10 years, the fenced willows that weren’t close to dams, though they’d suffered no browsing at all, on average were far shorter than 2 meters, the team reports online today in Proceedings of the Royal Society B: Biological Sciences. That’s the threshold height that makes willows tall enough to reproduce despite voracious elk. Unfenced willows along dammed streams didn’t make the threshold either. Only a combination of dams and fences provided the right conditions for the willows to grow to a self-sustaining height.

Another fine study shows [basically] that man cannot reinvent the wheel without beavers in the mix. Who would have thought? So wolves are important to the recovery of yellowstone but can’t do their magic unless beavers do theirs. Hmm. And pretend dams are interesting opportunities to carry logs by helicopter but beavers make the ones that really matter. Are you shocked?

If beavers need willows, willows also need beavers. Beaver dams help create mud flats where new willows can sprout; they also raise the water table, supplying more water to willow roots. When wolves vanished, the willows of the northern range faced a double whammy: too many elk, too few beaver. The result was a scarcity of the thick, lush willow patches needed for a healthy riparian zone.

Wait a minute. Maybe its not just wolves that need beavers to plant the trees that feed the elk that they harvest. Maybe its all of us! Didja think of that?

All agree, however, that beavers might help the willows and riparian zones make a comeback. But until willows are vigorous, beavers could starve. It’s hard to see a way out of this “chicken-and-egg” problem, Marshall says. Perhaps if Yellowstone got a very wet year, encouraging willow growth, combined with a year that saw a low level of elk browsing, beavers could establish a foothold in the small streams of the northern range, as they have in other parts of the park. “It’s feasible that it could happen on its own,” she says. “It’s just not likely in the next few years.”

I’ll tell you what. Instead of using those tax and grant dollars to carry logs and measure trees, why don’t you sponsor a pizza party for every boyscout in the state and involve them in planting stakes of willow as far along the waterline as you and the wolves can see. Then when the sprouts burst along the riverside beavers will settle in and have enough to eat and your problem will be solved.