The Ecology Book

143

Steller’s sea cow was a giant

sirenian discovered by the naturalist

Georg Steller in 1741. Its extinction is

the cause of debate: was it hunted to

death, or did its food source disappear?

invertebrates, which therefore ate

more vegetation. The leaf area

of the plants in the study was

consequently reduced by half.

Not all the “players” in trophic

cascades are obvious or visible.

Some are tiny and live underground.

For example, yellow bush lupines—

plants that live on the Californian

coast—are consumed by the

caterpillars of ghost moths, which

eat the lupines’ roots. In turn,

nematodes—wormlike

invertebrates—parasitize the

caterpillars. If these nematodes

are in the soil, they will limit the

population of caterpillars, and fewer

of the lupines’ roots will be affected.

Extinction events

In extreme cases, a trophic cascade

can lead to species extinction—as

in the case of Steller’s sea cows,

marine mammals that once lived in

the Bering Strait but were declared

extinct in 1768. It has recently been

argued that this extinction was

caused by a calamitous trophic

cascade, triggered by the hunting

to virtual extinction of sea otters for

the fur trade. The over-exploitation of

sea otters allowed the population

of sea urchins, their usual prey, to

rise above a critical threshold. Sea

urchins eat kelp, so the growth of

their population led to a collapse in

the extent of kelp forests—the sea

cows’ food source. Although the

sea cows themselves were not

being hunted, they soon became

extinct. Understanding how such

interventions, and the introduction of

nonindigenous species, can damage

trophic cascades is vital in shaping

conservation measures today. ■

ECOSYSTEMS

Early humans

and megafauna

In the last 60,000 years, which

includes the end of the last ice

age, about 51 genera of large

mammals became extinct in

North America. Most were

herbivores, including ground

sloths, mastodons, and large

armadillos, but many were

carnivores, such as American

lions and cheetahs, scimitar

cats, and short-faced bears.

Many of the extinctions

occurred between 11,500 and

10,000 years ago, shortly after

the arrival and spread of the

Clovis people, who were

hunters. One of the most

convincing theories about

these developments is the

“second-order predation

hypothesis,” which suggests

that the humans triggered a

trophic cascade. The people

killed the large carnivores,

which competed with them

for prey. As a result, predator

numbers were reduced

and prey populations rose

disproportionately, resulting

in overgrazing. The vegetation

could no longer support the

herbivores, with the result

that many herbivores starved.

Cave paintings in Altamira, Spain,

show the importance of bison to

early humans. The wild population

became extinct in 1927, but captive

herds have since been reintroduced.

Herbivores are

usually expected to be

well fed and carnivores

are usually expected

to be hungry.

Lawrence Slobodkin

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