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Sixth Extinction
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M
ost species of plants and animals that have ever lived on Earth are
now extinct. Though most have become extinct slowly, some have
been almost completely wiped out in a sudden, massive,
catastrophic event. These mass extinctions have happened at least five times
in the history of the Earth. The first mass extinction occurred about 450 million
years ago, when sudden changes in sea level, caused by melting
glaciers, killed
most
marine species. The most recent mass extinction was 65 million years
ago, when most scientists believe an
asteroid struck the Earth in Mexico and
extinguished the dinosaurs.
Now scientists claim a “Sixth Extinction” is underway. This extinction is not
caused by one big event but by a series of human actions that disrupt the
environment. Edward O. Wilson, a Harvard University scientist, estimates that
human harm will cause half of all species of life to be extinct by 2100. Scientists
say that human activities are causing biological extinction at a rate hundreds of
times faster than ever before.
Humans cause damage to the Earth in three primary
ways. The first is
exploitation. Humans hunt animals
for food, clothing, and sometimes just for fun. They also
collect plants and insects
---
such as butterflies
---
for
recreation. The second way humans impact the Earth is
by putting alien plant or animal species in new
environments. In Africa, for example, some fisherman
put perch from Egypt’s Nile River into Lake Victoria. Nile
perch like to eat other fish. Since they came, more than
200 native species of fish have disappeared from the lake.
The third way humans cause damage is by destroying
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catastrophic --- violent and destructive
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glacier --- a large body of ice
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marine --- relating to the sea
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asteroid --- a large rock floating in space
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exploitation --- using something for your own purposes
