Biology Now, 2e

362 ■ CHAPTER 20 Communities of Organisms

ECOLOGY

Grass, willow, berries

Coyote

Vole

Elk

Pronghorn

Snowshoe hare

Red fox

Gray wolf

The species to which

an arrow leads eats

the species from which

the arrow originates.

Red arrows follow a

single food chain

within this food web.

Figure 20.5

Food webs show how energy moves through a community

Food webs are composed of many food chains that show one species eating

another. M

Q1: What species eats the coyote?

Q2: What species does the coyote eat?

Q3: What do you think would happen to a community that lost its

coyotes?

was destroyed in 1926. At least 136 wolves,

maybe more, were killed during the eradication

campaign in Yellowstone. Once it was done, the

park was wolf-free.

Seven decades later, staring at the poster

of a wolf standing among the aspens, Ripple

wondered whether the loss of that keystone

species had contributed to the decline of aspen

trees. He immediately looked up the histori-

cal records to see whether the timing matched.

Lo and behold, the last wolf had been killed at

about the same time the aspens stopped regen-

erating, in the mid-1920s. Suddenly, it seemed

obvious why the aspens had declined: wolves kill

elk and elk eat aspens—three species in the same

food chain.

A food chain is a simple list of who eats

whom. In scientific terms, it is the direct path

by which nutrients are transferred through the

community. A food web, on the other hand, is

a more complex diagram of all the food chains

in a single ecosystem and how they interact and

overlap (Figure 20.5).

In the wolf-elk-aspen food chain, aspen are

the producers, the organisms at the bottom

of the food chain that use energy from the

sun to produce their own food through photo-

synthesis. In Yellowstone and on land all

over Earth, photosynthetic plants like trees,

grasses, and shrubs are the major producers

(Figure 20.6). In aquatic biomes, photosyn-

thetic plankton are the major producers, as

we’ll see in Chapter 21.

Further up the food chain are consumers,

organisms that obtain energy by eating all

or parts of other organisms or their remains.

Elk and wolves are both consumers: elk eat

aspens, and wolves eat elk. In the Yellowstone

food chain, elk are primary consumers: they

eat producers. Wolves are secondary consum-

ers because they eat primary consumers. This

sequence of organisms eating organisms can

continue: a bird that eats a spider that ate a

beetle that ate a plant is a tertiary consumer;

a killer whale that eats a leopard seal that ate

a sea bass that ate a krill that ate a phyto-

plankton is a quaternary consumer. We will

explore the f low of energy up the food chain in

Chapter 21.

The more they discussed it, the more Ripple,

Larsen, and Beschta believed that the loss

of wolves in Yellowstone had allowed the elk

population to f lourish and eat so many young

trees that the aspen population could not

regenerate. “We developed a hypothesis that

maybe the killing of wolves actually affected

the reproduction of aspen trees,” says Ripple.

Any change in species diversity will have a

ripple effect (no pun intended) throughout

the community, and the wolves of Yellow-

stone were no exception. Ripple and Larsen

published their hypothesis in 2000, suggesting

that the loss of wolves had led to increased elk