S e c o n d e d I t I o n 1 Reading for the Real World 1

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
6
glacier --- a large body of ice
7
marine --- relating to the sea
8
asteroid --- a large rock floating in space
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exploitation --- using something for your own purposes
Track 17
 

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natural habitats. They clear trees from forests to build 
houses, farms, cities, and roads. One of the most 
terrible instances of habitat destructions is the cutting 
down of the 
tropical
rainforests. It is estimated that 
these rainforests, which cover less than 10 percent of 
the Earth, contain almost half of all the world’s plant 
and animal species.
Human activity has had a devastating effect on the natural environment. 
Each year, an international organization of scientists releases a special “red list” 
that updates the 
status
of more than 44,000 animal and plant species. In 2008, 
the organization reported that 25 percent of our planet’s 5,487 mammals are on 
the