Trophic Cascades – Worth A Dam

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;

Bison numbers in the northern range have increased in proportion to the decline of elk numbers;

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.

Bruce

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?

With predators’ numbers falling in the wild, the rest of the food chain is being forced to adapt.

Do you think lions and tigers and bears are scary? As it turns out, their absence is even scarier. Predator ecology is gaining in popularity, as scientists discover just how crucial top predators — such as lions, tigers and, well, you know the rest — are to the success of an ecosystem.

A prime example of an essential predator is the gray wolf, Canis Lupus. Once a thriving and abundant species, gray wolf populations have been nearly eliminated from 48 U.S. states. The absence of this apex predator has a dramatic top-down effect on the entire ecosystem. With their natural predators removed from the picture, elk populations flourish.

Do you give the pastor credit when the choir sings a truly ‘Amazing Grace’? Sure he plays a role, but he wasn’t singing those notes or practicing in the loft, was he? Now don’t get me wrong. I am a big fan of wolves. I love their song and their fur and their social interaction. I loved ‘never cry wolf’ more than any movie I saw in college, and to this day I remember scenes and vivid images from it. But wolves shouldn’t get all the credit for restoring creeks. Wolves don’t build dams or raise the water table or augment the invertebrate community so that we get more fish and more mammals and more birds that eat fish. Beavers do that, and it drives me crazy when they get only an ‘incidental mention’.

In turn, cottonwoods and other trees become elk chow. Fewer trees result in fewer songbirds and fewer resources available for beavers to build dams with. Remove beaver dams from an ecosystem and small ponds also disappear. Eliminating ponds reduces habitats for succulent plants, a critical food source for grizzly bears.

Someday Earth Guardians or Wildlife watch will make a film about how good beavers actually are for the creek and wildlife and it will blow the mind of every single well-meaning person who has sent me the wolf-glorifying film. Wolves make it possible for beavers to weave their magic, and that’s very important. But beavers are the one who do all the work and make the difference. Beavers are the ones who deserve the credit!

Hrmph.

Wolf and Beaver – you can see the beaver’s tail slapping the surface (click to enlarge) Roads End Naturalist

Today’s donation is a whimsical painting by Suzanne Hunter of Phoenix Arizona called ‘Party in the Den’. Her Etsy shop ‘Red Raven Design‘ features “art with a sense of fun!” She painted the donation specifically for our festival, and her website says, “Yes, these hip beavers are having a cocktail party in the den! You never know what kind of crazy antics these revelers will get into. I did this painting specifically to benefit the beaver restoration project in Martinez, California. I’m all for animal and nature conservation and who doesn’t love the beautiful beavers!”

I love to imagine the beavers in their lodge toasting our success! The painting is colorful fun but honestly my favorite part is what looks like Martini glasses the beavers are holding. How appropriate is that considering our beavers hail from the home town of the Martini? Thank you Suzanne for your wonderful support! How did you know?