Columns

Big predators have a powerful mystique, a theme explored by David Quammen in his book Monsters of God. If the police had shot a coyote on Shattuck Avenue, would anyone have set up a shrine in its memory? 

As it happens, at about the time the mountain lion made its ill-fated venture downtown I was reading The Wolf’s Tooth, an interesting study of keystone predators and trophic cascades by wildlife biologist Cristina Eisenberg. The book’s title and epigraph come from Robinson Jeffers’ poem “The Bloody Sire”: “What but the wolf’s tooth whittled so fine/ The fleet limbs of the antelope?.” If the antelope Jeffers had in mind was the American pronghorn, it was a different tooth entirely; but I’ll get to that later. 

Keystone predators are species whose behavior controls the population of prey species and sends ripples (“cascades”) through the local food web to the benefit or detriment of non-prey organisms or plants. Remove those predators and a whole sequence of dominos fall. The phenomenon was first described for starfish and mollusks in Pacific Northwest tide pools and has been generalized to other marine and terrestrial ecosystems. 

Probably the best-publicized case of the consequences of reintroducing an absent keystone predator is the return of the wolf to Yellowstone National Park. Without wolves, the park’s elk grew complacent and tended to hang around the same creeks all day, browsing the aspen down to stumps. Reintroduced wolves created what has been called an “ecology of fear.” The elk became nervous, tending to move around more. That allowed the aspen to rebound. Riparian songbird diversity increased. Beavers returned to previously abandoned drainages to do their own ecosystem engineering. The wolves also killed or intimidated Yellowstone’s coyotes, reducing coyote predation on pronghorn calves. 

Eisenberg, a wolf person, has studied their ecological role in the northern Rockies and worked with landowners who are open to living with wolves. But she reviews the literature on other potential candidates for keystone species. She says black and brown/grizzly bears are too omnivorous to have a measurable impact on mammalian prey species. (The polar bear, an obligate carnivore, would be a different story.) Mountain lions, which prey primarily on mule or white-tailed deer, appear to control prey populations in some locations (including Zion National Park) but not in others. It’s not clear what makes the difference. 

Whatever the current North American predator-prey dynamics are, they used to be a great deal more complex. Whereas the mountain lion is now our only major feline predator, it had company in the Pleistocene. Lions—the same species as the modern African and (barely surviving) Asian lion—were widespread, and well-represented at sites like Rancho La Brea. So were sabertooth cats like Smilodon fatalis, our official state fossil: robust, long-fanged beasts with no living counterpart. The sabertooth’s less common relative, the scimitar cat (Homotherium serum), preyed on young mammoths and mastodons. Jaguars ranged as far north as Washington State and Nebraska. 

There was even a North American cheetah (Miracinonyx trumani), the predator that may have driven the pronghorn’s adaptations for speed. Pronghorns are faster than they need to be to escape wolves and other living species. Cheetahs would have given them a run for their lives. 

And that’s only the cats. The modern timber wolf coexisted with the dire wolf, which must have occupied a somewhat different ecological niche. The extinct short-faced bear may have been a more dedicated carnivore than its living cousins. A North American species of hyena could have been a predator or scavenger—or both. 

Pleistocene North America, in short, was much more like the contemporary African savanna in megapredator diversity. With so many contenders—lions, leopards, cheetahs, wild dogs, and spotted hyenas in the African case—it’s harder to assign keystone status. 

The cause of the demise of the great North American predators and their prey is still hotly debated, of course. The recent proposal that a meteorite strike caused the Pleistocene extinction seems to have been discredited, leaving human overkill and rapid climate change as the main contenders. Imagine, though, what a world we’d have had if things had gone otherwise: a pride of American lions strolling by Peet’s, or a lone Smilodon.