Trophic level refers to an organism’s position on the food chain. It’s from the greek word τροφή meaning food. One popular strain of ecological thought follows trophic levels down the chain to study their impact: as in wolves eat elk. Elk eat willow. More wolves mean fewer elk and more willow, right? End of story.
Maybe not.
This kind of thinking leaves awesome gaps so wide that you could sail a scholarship through. For example, certain flat-tailed animals, (I won’t give the answer away) trigger a feed back loop that changes the top-down interaction. For example, wolves eat elk, fewer elk mean more willow, more willow means you can have more beaver, and beaver in addition to eating willow, create conditions that inspire more willow to grow!
Food chain of command changed from both ends! Enter Kristin Marshall Ph.D. Ecology Colorado University.
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 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…
