Mass extinctions have occurred at least five times over the last half-billion years, including the episode that ended the reign of the dinosaurs. During these five episodes, huge numbers of species were suddenly annihilated. In addition to mass extinctions, many species have died out slowly over the millennia, due mostly to the winnowing processes of natural selection and survival of the fittest.
Each time a mass extinction has occurred, the earth has recovered completely from it. In fact, there are more species alive today than ever before in the earth’s history – perhaps as many as 100 million.
Many scientists believe that the earth is currently experiencing yet another episode of mass extinctions. But the extinctions today are being caused by humans rather than by “natural” events. In addition, most extinctions today are occurring at rapidly accelerated rates. Even the so-called sudden mass extinctions of the past took place over many thousands of years. Extinctions today can be measured in terms of a human lifetime. For example, since the discovery of North America by the Europeans in the late 1400s, more than 60 species of North American vertebrates have become extinct, nearly all within the last 100 years. At least 10 percent of all the animals with backbones in the United States are now in danger of becoming extinct.
Some scientists believe that extinctions are taking place worldwide at the rate of at least one species every day, and that this rate may increase. If that is true, then within the next few decades, we may well lose more than a quarter of the earth’s total number of species.
Why be concerned about this? After all, the earth has recovered from earlier mass extinctions. But these recoveries took many millions of years. And with the kinds of losses we are currently experiencing, a full recovery in any span of time that is meaningful to humanity is highly unlikely.
Exercise 5. Read the text “Mass Extinction” once again and fill in the diagram below:
  Mass Extinction
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  how
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 Naturally
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| when | ||||
| today |
   how long
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  What are the consequences?
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Exercise 6. Read the 1st part of the following dialogue “Mass Extinction. Is It Really a Problem Today?” and translate it into Russian:
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| Disastrous Consequences | | | IV. The boiling point is the temperature at which a substance begins to change from the liquid into the gaseous state. |