Extinction and Endangered Species - Mass Extinction

In the billions of years since life began on Earth, species have formed, existed, and then become extinct. Scientists call the natural extinction of a few species per million years a background, or normal, rate. When the extinction rate doubles for many different groups of plants and animals at the same time, this is described as a mass extinction. Mass extinctions have occurred infrequently in Earth's history and, in general, have been attributed to major cataclysmic geological or astronomical events. Five mass extinctions have occurred in the last 600 million years. These episodes, known as the Big Five, occurred at the end of five geologic periods:

• Ordovician (505–440 million years ago)
• Devonian (410–360 million years ago)
• Permian (286–245 million years ago)
• Triassic (245–208 million years ago)
• Cretaceous (146–65 million years ago)

After each mass extinction the floral (plant) and faunal (animal) composition of the Earth changed drastically. The largest mass extinction on record occurred at the end of the Permian, when an estimated 90% to 95% of all species became extinct. The Cretaceous extinction is perhaps the most familiar—it was at the end of the Cretaceous that many species of dinosaurs became extinct. The Cretaceous extinction is hypothesized to have resulted from the collision of an asteroid with the Earth.

The Sixth Mass Extinction?

Scientists estimate that hundreds, or even thousands, of species are being lost around the world each year. This suggests that we are currently in the midst of another mass extinction. Unlike previous mass extinctions, however, the current extinction does not appear to be associated with a cataclysmic physical event. Rather, the heightened extinction rate has coincided with the success and spread of human beings. Researchers predict that as humans continue to alter natural ecosystems through destruction of natural habitats, pollution, introduction of nonnative species, and global climate change, the extinction rate may eventually approach several hundred species per day. This would be a rate millions of times higher than normal background levels. The United Nations, in Global Biodiversity Outlook 2 (March 2006), concluded that without immediate intervention, more species of flora and fauna may disappear than were lost in the mass extinction that wiped out the dinosaurs sixty-five million years ago.

In 1948 an international conference on conservation resulted in formation of the International Union for the Protection of Nature. In 1956 the name was changed to the International Union for the Conservation of Nature and Natural Resources (IUCN; now the World Conservation Union). The IUCN is based in Gland, Switzerland, and is the world's largest conservation organization. As of 2006 more than eighty nations and 800 nongovernmental organizations are members. The IUCN reports that in the last 500 years, at least 816 species are known to have become extinct as a result of human activity. The actual number is probably much higher.