A new study shows that restoring top predators is a big challenge but essential for maintaining healthy ecosystems.
(Photo: Matthew J. Lee/The Boston Globe via Getty Images)
June 20, 2014
Richard Conniff is the author of The Species Seekers: Heroes, Fools, and the Mad Pursuit of Life on Earth and other books.
have been suppressing predators since our terrified ancestors first
banded together around campfires. Oddly, though, we only began to notice
the catastrophic aftereffects in the 1960s. That’s when biologists
first demonstrated that taking out a top predator has a knock-on effect
for almost every plant and animal below it on the trophic ladder, or
It’s called a “trophic cascade,” and when settlers eradicated wolves
from the Lower 48, they set off a cascade on “a continental scale,”
according to a new study published in the Journal of Animal Ecology. Where
the wolf's howl once could be heard from the Arctic to the Gulf of
Mexico and from Cape Cod to the Olympic Peninsula, the night went
silent. And coyotes, once confined to the Great Plains,
were suddenly free to increase their populations almost astronomically,
extending their range from coast to coast and north into Alaska.
Wolves out, coyotes in. Almost a wash, right?
On the contrary, coyotes are “mesopredators,” meaning midsize, and
they favor smaller prey than do wolves. So the proliferation of coyotes
caused a corresponding decline in a host of other species, among them
sandhill cranes, snowshoe hares, long-billed curlews, and yellow-bellied
marmots. The replacement of wolves with coyotes is also a major reason black-footed ferrets, pygmy rabbits, San Joaquin kit fox, whooping cranes, and least terns are now endangered species.
The new study set out to examine the wolf-coyote dynamic on a much
larger scale than previous studies. Oregon State University wildlife
ecologists Thomas M. Newsome and William J. Ripple focused on
fur-trapping records over the past few decades from wildlife management
agencies in the Canadian provinces of Saskatchewan and Manitoba, an area
of almost 600,000 square miles.
They were particularly interested in how the presence of wolves affected two competing mesopredators, coyotes and red foxes.
It turned out that the foxes outnumbered coyotes by about four to one
when wolves were present, in the northern forests. On the other hand,
where wolves had been driven out by humans, in the southern third of the
study area, the coyotes outnumbered the foxes by about three to one.
But the really interesting area was a 125-mile-long transition zone.
There the wolves were still around but sporadically and at lower
densities—too low to suppress the coyotes.
That matters, according to Newsome, because it means that undoing
this particular trophic cascade may be more challenging than we imagine.
It’s conventional wisdom among environmentalists that the restoration of wolves
to Yellowstone National Park essentially “fixed” a broken environment,
moving elk away from streams, freeing overgrazed aspen groves to regrow,
and otherwise allowing the mix of species to recover to a more natural
balance. But a controversial opinion piece in The New York Times this
spring argued that this is mostly a myth. No environmental fix is that
easy, especially not when it involves wolves. The new study reinforces
“If our interest is in the broader ecological effects of restoring
wolves,” said Newsome in an interview, “this study suggests that they
need to occur over large continuous areas at ecologically effective
densities before they suppress coyotes.” Even around Yellowstone, much
less in New England or the Southeast, human activities have altered the
landscape irreparably and broken up suitable wolf habitat into small,
often widely distributed patches. Newsome doesn’t think that’s cause to
give up on wolf restoration. But it suggests that it will continue to be
complicated, and that wildlife managers may need to target restorations
carefully to achieve particular effects—for instance, to save a
particular species—and think much bigger.
Newsome, a Fulbright Scholar visiting from Australia, also hopes to
apply the large-scale analysis to the top predator back home. There the
elimination of dingoes from huge areas has caused foxes and house cats
to proliferate. That's a major reason Australia has seen 29 mammal
species—about 10 percent of its endemic mammals—go extinct over the past
two centuries. He says there’s increasing interest in dingo recovery,
not just as a way to protect other native species but also to control
overabundant native herbivores, such as kangaroos, and nonnative pigs,
goats, and rabbits.
Restoring any top predator means factoring in the negative effects on
ranching and other human enterprises, said Newsome. It also means
developing strategies for predators and livestock to coexist—for
instance, by developing guardian dog programs for ranch animals. The new
study makes clear that it also requires thinking about big landscapes.
It’s not just about national parks anymore.
It’s about entire continents.