Why Black Bears Are Nature's Landscape Architects
In a remote mountain meadow in central Colorado, treehopper nymphs and western thatching ants have come to a mutual understanding, and it makes for a case study in just how nuanced the natural world can be, and how small changes—changes we might consider trivial—can send huge effects cascading through a habitat.
The nymphs are the immature stage of treehopper insects, and they’re not wonderful to look at, with spines, and hairs, and legs sticking out everywhere from their tiny black-and-gray bodies (think Jeff Goldblum in The Fly). While you’re having a John Denver moment about the beautiful shrubbery on that mountain meadow, the nymphs are clustered along the stems and branches, busily sucking out the sugar-rich phloem. But the ants think they’re adorable, and they keep the nymphs safe from marauding beetles and spiders. In return, the ants get to eat the sweet honeydew the nymphs excrete.
This lovely bit of mutualism is part of a larger food web extending, improbably, to the black bears living nearby. Black bears, it turns out, like to eat ants. Lots of ants. In Colorado’s Rocky Mountain National Park ants make up nearly a third of the diet for black bears, by volume. And, yep, grizzlies do it too. A study of Ursus arctos in Scandinavia found that 16 percent of summertime food for one of the scarier predators on earth consists of ants. The bears invade large ant mounds and use their sticky tongues to lap up the tiny, honeydew-packed morsels within, like sprinkles on a dirt ice cream cone. Yum.
By doing so, the bears cause a trophic cascade, according to a paper published last month in the journal Ecology Letters. That is, the effect of their feeding cascades from one organism to another down the food chain. Fewer ants mean fewer babysitters for the hungry nymphs. The nymphs then become an all-you-can-eat buffet for beetles, spiders, and other small predators. The shrubs in turn become bigger and more productive, because the bears have spared them from the nuisance of being sucked dry by treehopper nymphs. “I’ve seen lady beetles just go through a colony of 20, eating one after another,” said Josh Grinath, who discovered the cascade while researching his Ph.D. dissertation at Florida State University.
Trophic cascades aren’t exactly news. Aldo Leopold described one—about what happens when we remove wolves from a habitat—in his classic 1949 book A Sand County Almanac. The University of Washington’s Robert Paine established a scientific basis for trophic cascades with his 1960s study, showing that removing starfish from a coastal habitat caused a half-dozen other species to go kaflooey. But Grinath’s study takes the concept a step further, combining the effects of a top predator with an incidence of mutualism.
He didn’t run into the exact same story in every season of his study. (The truth is, nature doesn’t give the hindquarters of a Rattus rattus for a simple story.) In some years, the bears raided only about a quarter of the ant mounds in the study site, and in other years they got 86 percent of them, probably because drought made berries and other favorite foods scarce. The treehopper nymph population rose and fell accordingly. One year, the hungry bears even rooted around some of Grinath’s equipment and left a bite mark in a two-gallon bucket.
What’s the take-home? The study shows that “we need to not disturb these predators,” said Grinath. We may consider them scary or a nuisance, but ultimately, “they are maintaining ecosystems in a way that humans want them to be maintained,” even if such benefits are not immediately obvious. More to the point, they are maintaining them the way nature wants them to be maintained.
The paper “contributes to a larger corpus of work that’s beginning to show that top predators in ecosystems have some pretty sweeping effects that we haven’t, up until now, really appreciated,” said Os Schmitz, a professor at the Yale School of Forestry and Environmental Studies, who wasn’t involved in the study. “We really need to step back and appreciate animals like bears in a different light.”
One trophic cascade in particular has become an object of public attention—and seems to get oversimplified by almost everybody. Until recently, Yellowstone National Park was overrun with elk, because the wolves that preyed on elk had been extirpated from the park. And overgrazing by the elk decimated the willow saplings that used to grow thick along the edges of streams. After wildlife managers reintroduced wolves in the 1990s, it became a popular just-so story among conservationists that the wolves had brought the elk population under control and that the willows in turn are coming back as a result, meaning the streams are healthier, and the species in the streams, and…you get the idea.
“But hang on, there,” biologist Arthur Middleton said, in a New York Times piece last year. Elk populations may be down (although that has as much to do with the loss of native cutthroat trout from the park as it does with wolves), but the willows and aspens are still struggling. That’s mainly because beaver populations declined as streamside trees and shrubs disappeared. Lack of beavers meant lack of dams, which meant faster-moving rivers, which cut more deeply into the earth, which lowered the water table to the point that the landscape is no longer suitable habitat for willow trees. Ecosystems change, often permanently. No number of wolves by themselves will bring those willows back.
Pretending we can restore an entire ecosystem by putting one ingredient back into the mix is a fool’s game. We need to pay attention to entire ecosystems and to real science, not just-so stories, and we need to have the patience and persistence to rebuild accordingly. Better still, we need to learn not to do the damage in the first place, because our misdeeds may never be undone.