I recently asked beaver-friend Bruce Thompson of Wyoming if I could share his thoughts on trophic cascades. He graciously agreed so you’d understand the concept better. The awesome graphics are from Earth Justice and perfect for the occasion. Enjoy!
While the term “trophic cascade” is new, the ecological concept is not. It is a process set in motion by the addition or removal of a top predator, which triggers reciprocal changes in the relative populations of predators and prey throughout a food chain. This “cascade” often results in impressive changes in ecosystem structure and nutrient cycling. As a simple example, an increase (or decrease) in carnivores causes a decrease (or increase) in herbivores (their prey) and an increase (or decrease) in plants (the “prey” of herbivores).
One case study from eastern US is how 19th century removal of wolves has been associated with an increase in white-tailed deer and a decline in plants eaten by the deer. Encyclopedia Britannica: “American zoologist Robert Paine coined the term trophic cascade in 1980 to describe reciprocal changes in food webs caused by experimental manipulations of top predators. In the 1980s others used the term to describe changes in aquatic ecosystems arising from factors such as sudden increases in predatory fish populations from stocking or dramatic declines in predatory fishes caused by overfishing.”
So, the phenomena is neither new nor in question, but as with anything as complex as an ecosystem (and involving human opinion) the precise mechanisms and predictable outcomes are.
To me, and most ecologists, it’s absurd to think that the removal of the wolf from so complex a system as Yellowstone (in the 1930′s, I believe) — or its reintroduction after more than a half-century’s absence — would not reverberate through the trophic system. Consider similar results from the removal of YOU from your household ecosystem. ; – )
Anyway, in its simplest use, the word “trophic” referees to anything having to do with eating. In ecology, the “trophic level” of an organism is the position it occupies in a food chain. So, “trophic cascade” refers to a sort of “domino effect” or cascading response within a system, triggered by a change in one or more of the major players within a food chain of that system.
Impacts associated with the trophic cascade in Yellowstone include:
- Scavengers like ravens, bald eagles, and grizzly bears, are benefitting from the carcasses left by wolf kills;
- Impacts from elk browsing on willow throughout the park has changed measurably since wolf introduction;
- In northern YNP, the number of a half-dozen songbird species that use willow for shelter and nesting was found to be greater in areas of willow recovery as opposed to those where willows remained suppressed, such as from ungulate browsing;
- The number of beaver colonies in the park has increased from one in 1996 to twelve in 2009. This is largely attributed to increased willow availability, which the beavers there are largely dependent upon for food and dam building.
- The work of beavers, acknowledged as a keystone species by most scientists, in turn reverberates through the system by positive changes in the water table, flood control, small mammal populations, nesting waterfowl, fish nesting habitat, soil development, etc, etc.
People — especially the media — will argue about the specifics of all this till the cows come home, but there is no question in my mind that multilevel shifts in food sources, food availability and use, and dependent wildlife populations have all shifted in innumerable ways since wolf introduction, and that the wolves are directly or indirectly responsible for most if not all of those changes.
That’s my story, short version.
Thanks for the great explanation Bruce! It all makes so much sense. But if you ask me the wolves are stealing wayyyyy too much credit. I mean all they do is make way for the real heroes. Right?