The Scientist - USA (2020-11)

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web hunting spiders, even without any immediate change in grass-

hopper numbers. Then, in a series of experiments in which Schmitz

glued the spiders’ mouthparts shut so they could still instill fear

but not consume their prey, he demonstrated that their mere pres-

ence was enough to allow grasses to flourish, as grasshoppers would

forgo a meal to avoid becoming one. (See “Lessons About Fear from

Our Deep Past,” The Scientist, October 2020.) Biologists now call

such fear-driven effects behaviorally mediated trophic cascades,

distinguishing them from density-mediated ones that involve the

predators’ consumption of their prey.

These behavioral effects come at a long-term cost to prey that

can ultimately lead to declines in population densities. Avoiding

the spiders caused some of Schmitz’s grasshoppers to starve, and

their populations eventually dwindled. In a menagerie of animals,

the sheer fear of predation can affect prey species’ metabolism,

stress hormone levels, neurophysiology, and reproduction. In the

forests of the southern Gulf Islands off the coast of British Colum-

bia, Canada, for instance, Western University ecologist Liana

Zanette and her colleagues have shown that hearing the sounds

of predatory birds and raccoons throughout a breeding season can

cause song sparrows (Melospiza melodia) to reduce the number of

young they produce by 40 percent.^6 “The role of predators is way

greater than we ever previously imagined,” she says.

What remains unclear is how common such trophic cascades

are in nature, Estes notes. “What I think is probably one of the

biggest outstanding questions in this part of ecology [is] just how

repeatable, how generalizable, these phenomena are.”

The gray wolf: An American hero?

A major challenge in studying the ecological role of large terrestrial

carnivores is that the vast majority of them are in decline. Much

knowledge of their influence comes from studies of local extinc-

tions. One remarkably common effect is that smaller predators—

which are often held in check by apex predators—take over and

wreak havoc. As coyote populations (Canis latrans) declined in

Colum-

Liana

sounds

can

RETURN OF THE JAGUAR: Key to restoring the second-

largest wetland in the world is the reintroduction of its

ancient predator. To that end, a jaguar breeding program

is underway. The resulting cubs will be released along

with three adults into the wild.

11.2020 | THE SCIENTIST

MATÍAS REBAK; REWILDING ARGENTINA FOUNDATION