As of today, taxonomists have already described nearly 2 million species, although in fact their number varies, according to various estimates, from 5 to 100 million. But 90 to 99 percent of species ever existing on the planet have already become extinct. The overwhelming majority vanished as a result of the so-called normal or background extinction due to the limited period of biological species existence, which fluctuates from 1 million years with mammals through 11 million years with some marine invertebrates. Besides the background extinction, the fauna experienced five mass extinctions, as a result of which 50 to 95 percent of then existing species disappeared within a limited historical period. The first mass extinction occurred 440 million years ago, at the end of Ordovic, as a result of temperature fall and the ocean level lowering. The second wave took place during the late Devonian, again due to temperature fall and sea reliction. During the third wave of extinction, at the end of Permian, approximately 250 million years ago, 95 percent of marine species and nearly 70 percent of terrestrial ones disappeared. The catastrophe was probably caused by active reconstruction of the earth's crust and change of climate during formation of the supercontinent Pangaea. The forth extinction happened in the late Trias, and the fifth one - the most renowned extiction - hit 65 million years ago. Researchers are inclined to believe that the Earth came into collision with a large bolide at that time. As a result, sea shoals suffered from tsunami and acid rains, the seabed was covered by enormous amount of organic matter, and only 12 percent of the then existing species survived on land.
At present, according to numerous specialists' opinion, the sixth - pleistocene - wave of extinction is coming, which has been in many respects provoked by men. Given the current average extinction rate of 40 species a day, it would take only 16 thousand years for the extinction of 96 percent of the contemporary biota - exactly as much as died out during the period of disastrous Permian extinction. The major reason for the oncoming calamity is destruction of plants' and animal's ecotope. Scientists have estimated that the species life span for contemporary mammals and birds has decreased up to 10 thousand years, i.e. it became 100 to 1000 times shorter than that of fossil forms. If the habitat continues to be destroyed at the same pace, the life span of these species will soon make only 200-400 years. There are no such estimates for the invertebrates, but they are undoubtedly affected both by the global environment and climate change, and by disappearance of local biotopes.
Death reigns on land and at sea. Thus, about 1 percent of tropical rainforests disappears annually. Up to 70 plant and animal species become extinct every day, which makes about 3 species per hour. At present, one tenth of coral reefs - zones of the highest biological diversity at shallow water - perishes; about 30 more percent will be ruined in the next decades. Corals die out mainly due to global environment and climate change, reef fish catching, water contamination and warming, hurricanes, destruction of symbiotic organisms. Any events taking place in shallow water also affects sea depths. Perhaps only "autonomous" superorganisms of deep-water gas-hydrotherm have not been touched upon by anthropogenic impact and they will evidently be able to escape consequences of the planet's global environment and climate changes even in case of nuclear holocaust.
Nevertheless, the sixth mass extinction is oncoming. That will be the first extinction which did not happed due to natural reasons but as a result of activity of one biological species, whose quantity increases annually by 100 million individuals.