Biology Now, 2e

368 ■ CHAPTER 20 Communities of Organisms

ECOLOGY

wolves back, young, woody plants are doing

better and growing taller,” says Ripple. “Plant

communities are beginning to recover.” Differ-

ent species are growing and spreading at differ-

ent rates of recovery, he adds, “but there’s enough

new growth that we suggest it is in support of our

basic hypothesis, that the presence or absence of

the top predator—the wolf in this case—makes a

difference in these plant populations.”

Since Ripple and Beschta’s discovery, the

scientific community has been debating two

potential reasons why the plants are flourishing

with the return of the wolves. The most straight-

forward possibility is that the elk population has

decreased: wolves kill elk, so there are fewer elk

to consume the plants. Yet some of the tree popu-

lations seemed to recover faster than the drop in

the elk populations would suggest. So a second

possibility is that the presence of wolves led to a

change in elk behavior called a fear effect. Often,

the presence of a predator in an area can affect

the behavior of its prey. In this case, it is possi-

ble that elk stopped grazing in areas where they

could easily be seen by wolves, such as along the

banks of the Lamar River.

“These two mechanisms, the population

density and fear behavior, are difficult to tease

apart, and we’re working on that,” says Ripple.

“Many today believe it [the change in plant

populations in Yellowstone] is due to a combina-

tion of the two.” Beschta agrees: “In my opinion,

they’ve both been going on.”

Safety in Numbers and Colors

Elk may avoid lingering at streams as a way

to evade their predators, but other prey have

far more elaborate strategies to avoid being

consumed. The poison dart frog, for example,

is among the most toxic animals on Earth, and

it evolved bright colors as warning coloration

to alert potential predators to the dangerous

chemicals in its tissues (Figure 20.11, top).

Such warning coloration can be highly effective.

Young blue jays, for example, quickly learn not to

eat brightly colored monarch butterflies, which

contain chemicals that cause nausea and, at

high doses, death.

Warning coloration

The bright colors of the poison

dart frog warn potential predators

of the deadly chemicals

contained in its tissues.

Camouflage

With its long legs outstretched,

this lichen-mimic katydid, a

relative of the cricket, sits

motionless on lichen-covered

branches to escape detection by

predators.

Mimicry

The viceroy butterfly (left) mimics

the color and pattern of the

monarch butterfly (right), which

contains toxic compounds.

Figure 20.11

Adaptive coloration responses to predation

Prey species have adapted many elaborate strategies to avoid being eaten

by predators, including warning coloration, mimicry, and camouflage.

Q1: How do predators know that brightly colored prey are usually

toxic?

Q2: Do you think mimicry works if the toxic species is in low

abundance? Why or why not?

Q3: Why is camouflage considered an adaptive response to predation?