Biology Now, 2e

Of Wolves and Trees ■ 363

populations and altered elk movements and

feeding patterns. In other words, with wolves

gone, elk were free to find and eat young aspen

shoots whenever they wanted, with no fear

of wolves.

In 2001, Beschta returned to the Lamar

Valley and collected data on another species of

tree, the cottonwood, which can live more than

200 years. He documented the same trend as

the aspens showed: in the 1920s, cottonwoods

suddenly stopped generating new, young trees.

In fact, since the 1970s, not a single new cotton-

wood had been established. “This was dramatic,”

recalls Beschta. “It’s a big deal when you can’t

have a single cottonwood in this large valley

make it to a mature tree.”

It is possible for a consumer to eat a species

to extinction, so if elk populations had contin-

ued to grow unchecked, they could have reduced

the aspen and cottonwood populations to zero,

disrupting the ecological community perma-

nently. But before that could happen, some -

thing dramatic occurred in Yellowstone: humans

brought the wolves back.

A Second Ripple Effect

In 1973, wolves became the first animals to be

protected under the Endangered Species Act. It

was the dawn of the modern conservation move-

ment, and the idea of returning wolves to Yellow-

stone grew in popularity. It took some time, but

eventually, lawmakers agreed to the plan, and

between 1995 and 1997, 41 wild wolves were

captured in Canada and released in Yellowstone.

It didn’t take long for the wolf populations to

recover: by 2007, an estimated 170 wolves lived

in and around Yellowstone; today that estimate

is 528. As of January 2016, at least 98 wolves

in 10 packs were living within the park limits

(Figure 20.7).

The loss, and then return, of the wolves had

significant impacts on species interactions in

the park. There are four central ways in which

species in a community interact: mutualism,

commensalism, predation, and competition.

Figure 20.6

Producers are the energy base of a food chain

All communities and ecosystems have producers, but they vary in abundance, depending on the circumstances.

Q1: Where do producers acquire the energy they need to perform their function in the food chain?

Q2: Why are producers necessary for life on Earth?

Q3: Given the lower abundance of producers in desert regions, compared to tropical rainforests, what would you predict

about the abundance of consumers in the two environments?

In tropical rainforests, the

abundant producers (plants)

store chemical energy harvested

from sunlight, which in turn is

readily available to consumers.

In deserts, the

sparseness of plant

life means that relatively

little chemical energy is

available for consumers.