It is estimated that over 99 percent of all species that ever existed have become extinct. What causes extinction? When a species is no longer adapted to a changed environment, it may perish. The exact causes of a species’ death vary from situation to situation. Rapid ecological change may render an environment hostile to a species. For example, temperatures may change and a species may not be able to adapt. Food resources may be affected by environmental changes, which will then cause problems for a species requiring these resources. Other species may become better adapted to an environment, resulting in competition and, ultimately, in the death of a species.

The fossil record reveals that extinction has occurred throughout the history of Earth. Recent analyses have also revealed that on some occasions many species become extinct at the same time- a mass extinction. One of the best- known examples of mass extinction occurred 65 million years ago with the demise of dinosaurs and many other forms of life. Perhaps the largest mass extinction was the one that occurred 225 million years ago, when appropriately 95 percent of all species died. Mass extinction can be caused by a relatively rapid change in the environment and can be worsened by the close interrelationship of many species. If, for example, something were to happen to destroy much of the plankton in the oceans, then the oxygen content of Earth would drop, affection even organisms not living in the ocean. Such a change would probably lead to a mass extinction. One interesting, and controversial, finding is that extinctions during the past 250 million years have tended to be more intense every 26 million years. This periodic extinction might be due to intersection of the Earth’s orbit with a cloud of comets, but this theory is purely speculative. Some researchers have also speculated that extinction may often be random. That is, certain species may be eliminated and others may survive for no particular reason. A species’ survival may have nothing to do with its ability to adapt. If so, some of evolutionary history may reflect a sequence of essentially random events.

Why is “plankton” mentioned in the second paragraph?

Câu hỏi liên quan

• The word “captures” in the fourth paragraph is closest in meaning to ______.

• The word "evidenced" in line 22 is closest in meaning to .............

• A housewife’s services in a family of five people are worth ............

ZUNIA12

• What does the word magni stand for?

• The word "norm" in paragraph 1 is closest in meaning to ..........

• We can see from the passage that in the countryside of Britain ______.

• In the passage, the author gives the explanation of the concept of mandatory volunteer programs by ______.

• According to the passage, what is implied about tea harvesting?

• Scientists concluded from the experiment with chimpanzees and chocolate chips that chimpanzees __________.

• According to the passage, what is TRUE about origin of tea drinking?

• During the procedure described in paragraph 3, when clay particles are placed into water they ______.

• In order to show that learning to write requires effort, the author gives the example of ................

• The word it in the first paragraph refers to ___.

• The word “disaster” in paragraph 4 is closest in meaning to _____.

• The word "fascinating" in the second paragraph could be replaced by ...............

• According to the passage, a regional university rector in Germany is elected by _____.

• The phrase "All in all "in the passage  is closest in meaning to .............

• hat caused thousands of deaths in 2003?

• What can be said about the mother and son in the passage?

• The word “advantage” in the last paragraph most closely means .................