July 04, 2004

Yellowstone Wolves

[Ed: This article was first published for Vox Populi Nebraska in May 2004. It seems fitting to republish this here on Independence Day as free running wolves in Yellowstone symbolizes freedom to me.]

Eighty years ago, Yellowstone National Park was rendered wolf-free. During the early 1900s, the war on wolves was in full bore. People during those days did not see wolves as anything other than dangerous competitors to humans and besides which, everyone knew that wolves would attack humans and eat young children if they had a chance. Wasn't that the lesson of the werewolves and Little Red Riding Hood?

2wolves.jpgPhoto courtesy of Yellowstone National Park

Yet, once the wolves were gone, the Yellowstone ecosystem started to change for the worse. One obvious change in the park ecology was the growing population of the elk herds. By 1956, human hunts were organized to reduce the size of the herds. Yet, despite the organized hunts, the ecosystem in the greater Yellowstone region continued to show further degradation. And then in 1968, all hunting in the park was stopped and the elk herds no longer had any limit except the food supply of the area. After decades without any wolf predation the elk population numbered close to 20,000 animals. It was a recipe for disaster. Now it is known that removing the wolves was a big mistake because wolves were a keystone species for the ecosystem.

What does it mean to be a keystone species? Biologically, a keystone species is one that that other principal species in a region base their behavior around thus affecting species lower in the food chain, i.e., the large carnivores affect the herbivores and they, in turn, affect the vegetation of a region. As a keystone species, wolves played an essential part in the ecology of the Yellowstone region and when they were removed from the scene, the rich biodiversity of this region started to diminish and die.

Biologists studying the interaction between species have been working to understand how complex food chain systems work. Recently they have been focused on a theory called "Trophic Cascade" which describes a framework that says a key predator not only influences the ecosystem by limiting the size of the population of the prey they feed on, but also by affecting the behavior of the prey. The theory says that a keystone species will in effect cultivate an ecosystem by making their prey more cautious and protecting areas from overuse by the prey. And at each level in a food chain, that same behavior modification is seen, with plants attempting to select areas to grow where herbivores cannot reach them or the plants change to find new defenses from their predators in order to survive.

What happened in Yellowstone when wolves no longer helped shape the ecosystem, and why did biologists recommend bringing back wolves?

Although Yellowstone's ecosystem was largely protected from human development, during the years without the wolves, the biodiversity of the region has suffered. How badly it was suffering was studied by forestry biologists who noticed that the numerous aspen groves that graced the western mountains were disappearing throughout the west. They studied the groves in Yellowstone and what they found were 85% of the aspens alive today dated from the period 1871-1920, but only 5% of the aspens found dated after that time (the others were from earlier). So they started to ask some questions: Perhaps the lack of new aspen groves related to the elk that fed on the aspen suckers? But hadn't elk always browsed on aspen suckers? Why was the effect of their browsing greater after 1920? And what happened in 1920 that caused this change? Answer: the wolves had been eliminated. What the biologists found was that when the wolves roamed the west, elk avoided the aspen groves because they knew it was possible for wolves to be hiding in the thickets waiting to pounce. So what the scientists realized was a healthy system included not just the aspens and the elk, but also required the wolves to keep it in balance. And it wasn't just limiting the number of elk; it was changing the behavior of the elk that was the key.

Although humans thought they were capable of managing the park by hunting, they were unable to mimic the complex web created by nature. When humans hunted the elk, they took the mature males with the largest antlers thus weakening the herds. When wolves hunted, they took the weak and lame, making the remaining elk stronger and more able. And furthermore, wolves changed the behavior of the elk, thus providing the opening for the aspens to thrive. No wonder the biologists recommended that if we were truly to save and protect the Yellowstone ecosystem for the future, wolves had to be reintroduced into the system.

So started one of the more dramatic environmental restoration stories. In 1995, wolves trapped in the Canadian forests were taken to the Yellowstone region and allowed to roam free. And the wolves have thrived in their ancient home and with them the biological diversity of Yellowstone is reviving. New stands of aspens and willows are growing tall. Beavers have returned to the areas where wolves roam, as they now can find aspens that are optimal for building dams. And as the thickets recover, they attract more songbirds and other smaller creatures each contributing to the web of life in the region. The recovery of the Yellowstone ecosystem by the reintroduction of wolves shows that we humans can reverse some of our worse mistakes and leave some areas on earth that show how beautifully complex natural systems can be for posterity to enjoy.

For more information, see this article published by the National Wildlife Federation and Wolves in Nature which studied the effect of the return of the wolves on the Yellowstone aspen groves. Another good article about the link between the aspen, the elk and the wolves can be found here.
For a scientific discourse on trophic cascade, read the paper, Trophic Cascades: the primacy of trait-mediated indirect interactions.

Posted by Mary at July 4, 2004 05:24 PM | Environment | Technorati links |


Thank You for this Mary

The ecosystem is so fine tuned that anything can upset it. Man as part of the ecosystem cannot avoid having an impact and the system is far to complex to predict what it will be. An example of course is global warming. We are constantly talking about CO2 emmisions but the fact that large areas of forest and grassland are being replaced with asphalt and roofs is also contributing. These numbers are off the top of my head (I will try to find the link) but are in the ballpark. The average temperature at the Portland weather station, in the middle of a metro area, has increased nearly 5 degrees in the last 30 years. At the Corvallis weather station, less than 100 miles away, but in an undevolped area, the average temperature has only risen between 1 and 2 degrees.

Posted by: Ron In Portland at July 4, 2004 05:50 PM

Ron, one place that the local climate has changed enormously is Phoenix, AZ from the number of people that have moved there. The extensive paving causes the nights to be 5-10 degrees warmer than it was and the irrigation for the lawns and all the swimming pools have increased the humidity of the area. It makes the area even less liveable than it was before the buildup.

We humans are a huge impact on the world. It's worth remembering that we can try to undo some of the damage and by being more aware of the ecosystems in which we live.

Portland has been doing some great work in reviving the streams and I know that the Tualatin River is now much healthier than it has been for years. And I believe that Johnson Creek now has spawning salmon again. (BTW: I like the fact you can see bald eagles in the Johnson Creek corridor.)

Posted by: Mary at July 5, 2004 06:49 PM


You may find this interesting:

Beavers rebound, strengthen habitat

BTW, I live near the Taulitan River a little west of Tigard and frequently see Bald Eagles fly over my home going between the many wild life refuges along the river.

Posted by: Ron In Portland at July 5, 2004 09:43 PM