Biology Now, 2e

Of Wolves and Trees ■ 361

A Key Loss

In the early 1900s, ranchers and homesteaders

killed wolves all across the United States and

eliminated them in many eastern states. Then,

in 1915, the U.S. government began subsidiz-

ing wolf extermination programs all over the

1962 ’64 ’ 66 ’ 68 ’ 70 ’ 72 ’ 74

0

Year

Number of species^5

10

15

20

25

Sea star removed

Sea star not removed

How a predator maintains diversity

Loss of a keystone species reduces diversity

Sea stars completely

eliminated mussels in

submerged areas of this

marine community,

enabling other intertidal

species to thrive there.

A Pisaster sea star

feeding on a mussel.

Without this keystone

species, the mussels

crowd out other species

in their community.

When the sea star Pisaster

was removed from a

community experimentally,

the number of species

dropped from 18 to 1, a

mussel.

Figure 20.4

The star of the community

The sea star Pisaster ochraceus is an example of a keystone species. In a classic experiment conducted

along the Pacific coast of Washington State in 1963, sea stars were removed from one site while an

adjacent site was left undisturbed.

Q1: How many species were left in 1966 in the community where sea stars were not removed?

Q2: How many species were left in 1966 in the community where sea stars were removed?

Q3: How do your answers to questions 1 and 2 demonstrate the importance of a keystone

species for the maintenance of diversity in a community?

country, and a systematic slaughter began.

Under the national program, states paid boun-

ties of up to $150 for individual wolf pelts.

Wolve s, fe a red a nd hat ed by pr ivat e la ndow n-

ers for killing livestock, were trapped, shot,

and skinned. The extermination happened

quickly: the last known wolf den in Yellowstone