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Two more decades of data shrink Yellowstone's wolf cascade to a 'trophic trickle'
Fewer elk let trees regrow in only parts of Yellowstone, a phys.org essay argues, far short of the wolf-driven cascade in a video seen 46 million times. Reintroductions sold on that version promise more than Yellowstone delivered.
The Scientist · Science desk
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What happened
- Wolves were eradicated from Yellowstone in the 1920s, after which elk multiplied and overgrazed riverbanks, turning willow, aspen and beaver ponds into bare grassland.
- The idea that returning wolves reversed this came from early-2000s studies built on trophic cascade theory, in which changes at the top of a food chain run to the bottom.
- George Monbiot's 2013 TED Talk supplied the voice-over for the viral video that carried the story well beyond the scientific literature.
- With two more decades of data, scientists found the wolf-elk-willow chain too simple, since humans, bears and mountain lions also kill elk.
- Feeding takes priority in elk behavior, according to the essay, so hungry elk seek good grazing whether or not predators are near.
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Why it matters
- precedent Proposals elsewhere that cite Yellowstone to promise reforestation from wolves are borrowing a result the park's own record supports only in patches.
- decision Anyone judging a reintroduction has to account for hunting and other predators before crediting vegetation change to the new arrival, or the verdict will overstate it.
- constraint Plans that count on predators frightening elk off riverbanks lose their main pathway if hungry elk keep feeding there regardless.
The cascade account makes two causal claims. The first runs through numbers: wolves kill elk, and fewer elk leave more and taller willows [5]. The second runs through fear: wolves drove elk out of the valleys, trees regrew where the elk stopped browsing, and beavers and bears followed [15].
The numbers claim has an attribution problem. Wolves were released from 1995 [3] into a park where they were far from the only thing killing elk [7]. A willow stand that recovered after that date had several predators changing around it at once. Crediting the whole effect to wolves means pulling their share apart from everyone else's. The essay makes a smaller claim. It credits the elk decline to "this varied suite of carnivores" and says it allowed some degree of tree recovery in parts of the park, well short of "the landscape-wide reversal" that popular accounts portray [8].
The park's lead wolf biologist said in The New York Times that the reality was more of a "trophic trickle" than a cascade [9]. The fear claim fares worse. It needs elk to give up good grazing to avoid wolves, and the essay's account of elk behavior leaves little room for that [10].
The popular version also leaves out people. The viral video shows no human being, and its narration says the wolves "turned up" and that "the number of beavers started to increase" [11]. In fact the wolves were flown in from Canada, and beavers had been deliberately released into the park's surroundings [12]. So one of the benefits the story credits to wolves has a human cause sitting right beside them [15].
None of this stays inside one park. Yellowstone's program set a worldwide precedent for conservation [3], and the story is widely cited as evidence that wolves can restore ecosystems and reforest the land [13]. The essay's author wrote that ecosystems are "dynamic, ever-changing and often unpredictable" [14].
I think the essay's reading is the better supported one, on the evidence it lays out, with one condition. It does not report how many stands regrew, by how much, or what share of elk deaths wolves account for, so this account leaves the size of the trickle unmeasured. The author's verdict is qualitative: wolves did not, by and large, restore Yellowstone [16].
What to watch
- Published Yellowstone willow and aspen measurements that separate the wolf effect from hunting, bear and mountain lion predation.
- Whether new wolf reintroduction proposals keep citing Yellowstone's cascade as grounds for expected reforestation.
- A response from researchers behind the early-2000s cascade studies to the 'trophic trickle' reading.