Fearsome predators afraid of us? Although we
likely retain a primal
fear of predators from the days when our
forebears lived among giant ice age carnivores,
today we overcompensate for that fear with a
penchant for killing that is unknown in the wild. At
a time when humans have become the dominant
influence on the planet—leading many scientists to
dub this epoch the Anthropocene,
or Age of Humans—it’s perhaps not surprising that we
distinguish ourselves as killers too.
We kill adult animals, the reproductive future of
a species, at up to 14 times the rate seen in wild
predators, Chris Darimont and his colleagues
reported in a 2015 Science paper.
We kill large carnivores at 9 times the rate they
kill each other (mostly through intra-species
battles). The wide-ranging ecological and
evolutionary consequences of our extreme predatory
behavior, the scientists argued, “uniquely define
humans as a global ‘super predator.’” In the
Anthropocene, Darimont told me, “humans have turned
carnivores into prey.”
Only three people have died in mountain lion
attacks in California since 1986, according
to the California Department of Fish and Wildlife.
Pumas, on the other hand, have a long history of
dying at the hands of humans. Bounty hunters had
largely eradicated the felids east of the Rockies by
1900, and hunted them for decades in California
after they became confined to the West. Today,
they’re typically killed by government officials
after picking off someone’s pet or livestock. “The
highest cause of mortality for pumas in our area is
getting shot for eating goats,” Smith says. It’s no
wonder the big cats bolt at the sound of a human
voice.
“Understanding fear in the things that should be
fearless is one of the coolest and newest [research]
areas,” says Joel Brown, an evolutionary ecologist
at the University of Illinois who was not involved
in the puma research. Brown has long studied the
larger ecological implications of being afraid, a
phenomenon he calls “the
ecology of fear.”
Scientists used to think mostly about predators’
ecological effects in terms of the direct impacts of
killing, Brown says. “We now know that fear
responses are often more important than the direct
killing effect,” he says. The mere presence of a
predator—signaled by a scent, sudden movement or an
approaching shadow—triggers a range of responses in
prey species as they try to avoid becoming food.
“The mere risk of predation dictates where they
forage, when they forage, how much they’re willing
to forage and how vigilant [they are],” says Brown.
...
Theoretical models from the 1970s assumed that
risk of predation influenced how animals foraged.
This assumption was tested a decade later in pikas,
small mountain-dwelling rodents that nest among
boulders and also happen to be the inspiration for
the Pokemon Pikachu. Nancy
Huntly, now an ecologist at Utah State University,
created experimental enclosures for the skittish
herbivores by carrying boulders out to meadows, far
from their dens. Pikas took advantage of these new
refugia and promptly moved down the meadow.
In a
now classic experiment from
1997, Oswald Schmitz, an ecologist at Yale
University, showed that fear can ripple through
trophic levels in the food web. Schmitz glued
together the mouthparts of grasshopper-eating
spiders, to see how grasshoppers would respond to
predators that couldn’t kill them. The grasshoppers
didn’t distinguish between the intact and
incapacitated spiders, he found. They changed their
feeding behavior when either spider was present,
which in turn affected the biomass of the grasses
they ate.
Fear can ripple not just through a food web but
through future generations. In 2011, Liana Zanette,
an expert on predator-induced fear who helped Smith
design her puma study, showed that simply
hearing the sounds of predators lowers breeding
success in songbirds. Zanette used the same type of
setup on songbirds in Vancouver’s Gulf Islands. Her
team removed real predation risk by protecting nests
with electric fences to zap hungry raccoons and
fishing nets to thwart raptors. Then they
manipulated the birds’ perception of risk by
alternating recordings of raccoons, hawks and other
predators—which typically eat half the songbirds’
offspring every year—with those of nonthreatening
animals like hummingbirds and loons.
“The fear effect was extremely costly for these
animals,” says Zanette, who is at Western University
in Ontario. Females ate less, and so laid fewer
eggs. They spent most of their time looking for
predators instead of foraging for their nestlings.
As a result, these songbird parents produced 40
percent fewer offspring over the breeding season
compared to animals that heard nonthreatening
sounds.
Last year,
Zanette’s team used this experimental setup in the
same ecosystem to test
the idea that fear of large carnivores can
ripple through the food web. They focused on
raccoons, opportunistic omnivores which their
songbird experiments revealed were particularly fond
of songbird eggs. It turns out they also love
intertidal crabs and fish. With top predators long
gone on the Gulf Islands, the fearless coons are
free to chow down 24 hours a day, Zanette says.
So she and her student Justin Suraci tried to put
the fear of predators back into the gluttonous
bandits. They set up speakers and cameras along the
shoreline, then played recordings of either dogs
(which occasionally kill raccoons) or seals and sea
lions (which don’t). “When raccoons heard the sounds
of barking dogs, they fed 66 percent less than when
they heard the sounds of barking seals,” Zanette
says. “And there was a massive increase in the
intertidal fishes and crabs, all the stuff raccoons
loved to eat.”
If fear produces such dramatic effects through a
mesopredator like a raccoon, what might it produce
through a top predator like a puma? “We would expect
these fear effects to be a common pattern across
every single species in the animal world, because
being killed by a predator immediately in an attack
is such an extremely powerful evolutionary force,”
says Zanette. Perhaps stating the obvious, she adds:
“If you die instantly in a predator attack, your
fitness falls to zero.”
If people are frightening a top predator to such
an extent that it’s eating less of its cache, she
says, that’s clearly going to affect the predator
population. But altering the behavior of a large
carnivore and how it moves through the landscape
will also affect the fear responses of animals in
the middle of the food chain and how much they can
eat, she says: “And that’s going to cause a trophic
cascade.”
On the positive side, the fact that a top
predator fears us enough to avoid us when we’re out
and about means they can coexist with us, says
Smith. But it’s a balance. If they become too
fearful to traipse through human landscapes, their
habitat and hunting grounds will become even more
fragmented, drastically reducing their chances of
long-term survival.
Smith tries to understand what it’s like to live
with people from the puma’s point of view. “Imagine
a zombie apocalypse where there are these dangerous
things that they can’t comprehend, and they have to
hide and slink around like in a zombie movie to find
food and navigate the landscape,” she says. “We have
all these weird sounds and technology, and kill them
all the time, but probably in ways they can’t
predict or perceive. They’re kind of living in this
postapocalyptic world, trying to escape us.”
Read more:
http://www.smithsonianmag.com/science-nature/how-fear-humans-can-ripple-through-food-webs-reshape-landscapes-180963987/#5EDCk5HO4sZKmpbs.99
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