The Ecology Book

223

The sixth extinction

Some ecologists have estimated

the current rate of extinction of

animals and plants at 100–1,000

times the natural background

rate, with most of the increase

due directly or indirectly to

human activities. They argue

that this is evidence the world

is already in the middle of the

Holocene extinction, named for

the present geological epoch.

Many species of animals and

plants have been lost since

the start of the Industrial

Revolution in the 18th century.

These losses have been driven by

habitat change, climate change,

overfishing, overhunting, ocean

acidification, air pollution, and

the introduction of animals that

disrupt food chains. American

ecologist E.O. Wilson, known

as “the father of biodiversity,”

believes that if the species die-off

continues at the present rate, half

of all higher life forms will be

extinct by 2100. Stuart Pimm,

a British–American biologist and

modern extinctions expert, is

more cautious, claiming that we

are on the cusp of such an event

and can still act to stop it.

Sudan, the last male northern

white rhinocerous, died in 2018 (two

females remain). Poaching has taken

the species to the edge of extinction.

plants and insects, and massive

organic reefs thrived in the oceans.

The continents of Euramerica and

Gondwana were converging into

what would become Pangaea—

the last of the supercontinents.

In this period, a succession of

extinctions—possibly as many

as seven—took place over a longer

timescale than any other mass

extinction event, possibly up

to 25 million years.

The extinctions may have had

many causes, including reduced

oxygen in the oceans, falling sea

levels, atmospheric changes, the

draining of water produced by the

spread of plants, and asteroid

impacts. Most organisms lived in

the oceans, and shallow seas were

worst affected, with many reef-

building organisms, brachiopods,

trilobites, and the last of the

graptolite species dying off. Around

75 percent of marine species died,

and it would be another 100 million

years before corals re-established

themselves on a large scale.

“The Great Dying”

The most dramatic mass extinction

took place at the end of the Permian

period, 252 million years ago. Also

known as “The Great Dying,” it

resulted in the loss of 96 percent

of marine species and 70 percent of

land-living vertebrates. Insects

suffered the only mass extinction in

their history, and the last of the

trilobites, which had been in

decline for millions of years,

disappeared from the fossil record.

Potential causes for the mass

extinction include asteroid impact

and oxygen depletion in the

oceans. The extinction also

coincided with one of the biggest

periods of volcanic activity in

THE LIVING EARTH

Earth’s history. The eruptions,

which lasted nearly 1 million years,

flooded more than 0.8 million sq

miles (2 million sq km) of ancient

Siberia with basalt lava. The

resulting buildup of greenhouse

gases would have transformed the

atmosphere of Earth, likely resulting

in severe global warming and

contributing to species extinction.

Phased losses

All life today is descended from

the small minority of species

that remained at the start of the

Triassic period. During the

period’s final 18 million years,

ending about 201 million years

ago, at least half of all animal

species known to be living at that

time were wiped out in two or three

extinction phases. Climate change

caused by more basalt eruptions

and an asteroid impact have been

cited as causes. In the seas, many

reptiles, cephalopods, mollusks,

and reef-building organisms died

out. On land, most of the reptilelike

archosaurs and many large

amphibians became extinct. The

loss of the archosaurs, in particular,

opened up ecological niches that

the dinosaurs would fill. ■

Modelled future extinction

rates are projected to

be 10,000 times Earth’s

historical geological

background rate.

Ron Wagler

American academic

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