Trophic Thunder

September 29, 2012 / by admin

Here is an excerpt from The Spine of the Continent that provides some background to the trophic cascade idea.  It picks up as a young forestry academic explains to me his reservations about the subject:

“I thought I’d take my usual contrarian stance and challenge these guys from the perspective of the plant ecologist,” explains John Lennon Campbell about the saucy argument he leveled at his elders in the matter of wolves. “Jim Estes was amused by my deliberate belligerence—he makes a very good case that not only does it make sense that higher organisms shape the ecosystem from the top down, but you always find it going on.” Michael Soulé, whose stalwart championing of the wolf comes with a resolutely emotional charge, was not as tolerant of Campbell’s parrying. But this is just the attitude that provokes Campbell to take exception. “I get worried that such an important thing as trophic cascades are being
used by conservation biologists to save wolves. I might say there’s a brain worm that controls elk that’s more influential on a particular population than wolves are—why wouldn’t we get excited by that? We’re scientifically enshrining top predators, almost exclusively charismatic ones.” Campbell says, “I was brought up by socialists who said labor is the source of everything; the primary producers. I say labor is the source of all wealth to invite the obvious; there is no economy without capital, just like there’s no
ecosystem without bottom-up effects. Plant production may start it all.”

“Trophic” is from the Greek trophikas, meaning “nourishment”; “cascade” is from the Latin cascare, “to fall”; and the term “trophic cascade” was coined by Robert Paine to describe how carnivores, at the top of the food chain, impact the herbivores, at the next level down, which impacts the plants and phytoplankton at the bottom of the food chain. That the critters with big, sharp teeth eat those with leaves in their mouths is an observation not in need of a scientific imprimatur, but in the 1960s a
trio with the names of Hairston, Smith, and Slobodkin (sounds like a Beckett scenario) proposed in a short paper that the reason terrestrial earth is covered in plants is that predators keep the herbivores from eating it all.  The paper stirred up enough debate that the authors became collectively known as HSS, and their idea “the green-world hypothesis.”

What people found so much to argue about was its directional change in thinking, since it was customary to view the dynamics of the food chain from the bottom up, probably since photosynthesis happens first in earth history, making phytoplankton and plants possible to begin with; and the existence of these also predates the herbivores and predators that have taken such advantage of their bounty. What Robert Paine did was test the hypothesis by removing sea stars from a defined area off the coast of Makah Bay on Washington’s Olympic Peninsula. (Formerly known colloquially as “starfish,” these multiarmed beings are not fish at all but animals: echinoderms. Despite their decorative demeanor, these are formidable predators and when it comes time to eat, the sea star throws its stomach over its prey and begins digesting.)  Paine then watched what happened there and in a control plot where he did not remove them. Left to their own devices, sea stars dined on mussels; in the area where they were removed, the mussels flourished. And then the mussels kept on flourishing, until they had decimated all the vegetation and crowded out all the other species on the water-washed rocks. Paine showed a deeper level of functioning going on in the linear sequence of predator-herbivore-plant, in which the predator’s role is intimately implicated in the persistence of the plants. This regulating and dynamic relationship among members of a food group brought the green-world hypothesis to dynamic life, with those at the top exerting demonstrable effects on those at the bottom.

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