THE GREAT EXTINCTIONS

The "big five" are not Earth's only extinctions but they have been the most devastating.
Fossils of marine animals indicate that each event wiped out at least 17 percent of families.
(A family such as canids can have thousands of species.) Scientists are still puzzling over
what caused the big five-climate change,perhaps caused by cosmic impacts,is a leading
suspect-but experts agree that human kind has ignited the sixth.Mass extinction can last for
millennia,and it takes millions of years for new species to make up the loss.The animals
illustrated here represent what has already vanished and what could be next.

Global threat to Coral Reefs

Coral reefs thrive in the clear, shallow coastal waters of tropical seas. The reefs of southeast Asia-the world's richest-and those in the caribbean have suffered the greatest degradation.

when climatic extremes such as El Nino increase water temperature,symbiotic algae flee the corals' tissue. Without the algae,corals lose their color and a source of energy. Bleached,they may eventually die.

If sediment from storms and coastal development blocks sunlight,corals weaken and become vulnerable to infections such as deadly black band disease.If sewage or agricultural runoff promotes an increase in plankton and a consequent population explosion of crown-of-thorns sea stars.Death comes by predation. Adult sea stars feed mainly on corals.

the vanishing praire dog

seen both as vermin and as victims,black-tailed prairie dogs scan for predators in the south Dakota Badlands.Habitat destruction,shooting and poisoning-the method federal agents in Arizona used to exterminate this pyramid of dogs in the early 1900s-have eliminated the rodents from about 98 percent of their range.