Biology Now, 2e

Of Wolves and Trees ■ 367

Figure 20.10

Predators can also be prey

While the cheetah is what we traditionally think of as a predator, it can also be prey to parasites,

and eventually to decomposers and detritivores.

Q1: What kind of predator is the cheetah in the figure?

Q2: What kind of predator is the louse in the figure?

Q3: What kind of predator are the elk that graze on aspen tree saplings?

This cheetah is a predator whose

many prey species include the

African hare.

The parasite

Hippobosca

longipennis, the

louse fly, lives off

carnivores like

the cheetah by

sucking their

blood, but does

not kill its host.

The animals eaten by predators are called

prey. All of the animal residents of Yellowstone

(and most plants) are eaten by other species,

except for grizzly bears, mountain lions, eagles,

and gray wolves. These four animals are all at

the top of the food chain. They are top predators,

although they are themselves prey to parasites

(Figure 20.10).

Back in the Park

Now that wolves, a top predator in the commu-

nity, were back in Yellowstone, how would their

prey, the elk, react? And how would that reac-

tion affect the elk’s food, the trees?

Using the plant measurements taken in the

park and comparing those measurements to

historical data, Beschta and Ripple found that

between 1998 and 2010, as the wolf population

in the park grew, the elk population decreased

and therefore fewer aspens were eaten. In 1998,

essentially 100 percent of the young aspen plants

were being preyed upon, but by 2010, only 18–24

percent were being eaten. In addition, average

aspen heights increased for all areas that the

scientists observed. Cottonwoods experienced

the same revival. In the 1970s, cottonwoods had

entirely stopped adding new young saplings, but

by 2012, some 4,660 young cottonwoods had

grown to over 2 meters high.

Together, the aspen and cottonwood data sets

convinced Ripple and Beschta that the reintro-

duction of wolves was responsible for a cascade

of species interactions leading to the restoration

of aspen and cottonwood populations. “With