What reduces biodiversity

Flocks of thousands of passenger pigeons used to fly across the skies of North America. Few people today have ever seen one of these birds. The passenger pigeon, shown in Figure 5, has been extinct for almost 100 years. An extinct species is a species that was once present on Earth but has died out.

Extinction is a normal part of '' "'"'-iJ* nature. The fossil record shows that many species have become extinct since life appeared on Earth. Extinctions can be caused by competition from other species or by changes in the environment. A mass extinction that occurred about 65 million years ago wiped out almost two-thirds of all species living on Earth, including the dinosaurs. This extinction, shown on the graph in Figure 6, occurred in the Mesozoic Era. It might have been caused by a huge meteorite that crashed into Earth's surface. Perhaps the impact filled the atmosphere with dust and ash that blocked sunlight from reaching Earth's surface. This event might have caused climate changes that many species could not survive. Mass extinctions eventually are followed by the appearance of new species that take advantage of the suddenly empty environment. After the dinosaurs disappeared, many new species of mammals appeared on Earth.

["T^^^J^ffiJ^ifii What are some causes of extinction?

LOSS Of Species Not everyone agrees about the reason for the extinction of the dinosaurs. One thing is clear—human actions had nothing to do with it. Dinosaurs were extinct millions of years before humans were on Earth. Today is different. The rate of extinctions appears to be rising. From 1980 to 2000, close to 40 species of plants and animals in the United States became extinct. It is estimated that hundreds, if not thousands, of tropical species became extinct during the same 20-year period. Human activities probably contributed to most of these extinctions. As the human population grows, many more species could be lost.

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