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Trophic Cascade: Elk aren’t the only animals eating willows.
By Mike Tercek | June 13, 2007
Many ecologists in Yellowstone focus on elk, and these researchers often say that elk are the only animals that have any important effect on stream-side plants like willows. It’s true that we do have a lot of elk (more than 9000 on the northern range). But there is more to the story.
Before I explain why I think that bison are more important than most people realize, let me give a brief synopsis of the “trophic cascade” in Yellowstone.
For reasons that are not yet completely clear, willows on Yellowstone’s “Northern Range” have been getting taller for the first time in about 70-80 years. (For a google earth map of the “Northern Range” go here). Starting in the 1920s, when wolves were extirpated from the area, willows started getting shorter. In some areas they completely disappeared, while in others they shrank from 6 or more feet tall to less than waist high. Then in the mid-1990s, when the wolves came back, the willows started getting taller again. In some places they are now more than 4 meters (12 ft) tall.
One explanation for this change in willow height involves a “trophic cascade,” which basically means that the wolves scare elk out of certain areas and keep them from mowing down the willows. A “trophic level” is a level in the food chain or food pyramid. Examples of trophic levels are plants, herbivores (grazers), and predators. A “trophic cascade” refers to the fact that changes in one trophic level (the introduction of wolves, which are predators) has effects that trickle (or cascade) down into lower trophic levels. In this case, the effects went from the wolves, to the elk, and then to the plants.
This probably isn’t the whole story, and it might not even be the most important factor controlling willow height. But right now, I just want to point out that some researchers might want to re-think their assumption that elk are the only herbivores that have “large-scale effects” on the willows.
There are definitely fewer bison than elk in Yellowstone, but each bison does a lot more damage to a willow than an average elk. I have seen one angry bison rip out a 10ft tall bush. In a just a few minutes, a single bison can remove 10 years of willow growth. Also, I’ve noticed that even a relatively calm bison will rip out large branches and drop them on the ground. Bison waste more than they eat, while elk are relatively neat eaters.
Bison also tend to camp out in willow patches for much longer than elk. I spend all winter collecting samples on Blacktail Plateau and Crystal Creek, and I have seen a small group of bison in these places for almost the entire season. There might only be 20 - 30 bison near Crystal Creek in the winter, but they are there for 3 months. The elk, in contrast, only seem to be in the willow patches for a few weeks in February.
There are other animals that eat willow too. Insects seem to be affecting willows rather severely in some areas. Obviously they aren’t making the bushes shorter, but they do remove almost all of the leaves in some stands, and this will limit how much the plants can grow. I took these pictures last July near the confluence of Soda Butte Creek and the Lamar River:
Topics: Essays, General, Mike Tercek, Nature Notes |
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