Most biologists accept the estimate of American evolutionary biologist Edward O. Wilson that the Earth is losing approximately 27,000 species per year. This estimate is based primarily on the rate of disappearance of ecosystems, especially tropical forests and grasslands, and our knowledge of the species that live in such systems. We can measure the rate of loss of tropical rain forests, for example, by analyzing satellite photographs of continents from different periods that show rates and amounts of habitat destruction—and from these measurements calculate the approximate number of species being lost each year.
This extraordinary rate of extinction has occurred only five times before in the history of complex life on Earth. Mass extinctions of the geological past were caused by catastrophic physical disasters, such as climate changes or meteorite impacts, which destroyed
