Extinction is a natural process. As evolution hums along, species disappear and new species emerge in an ongoing dynamic called "background extinction." Geologic history has also been punctuated by five great "mass extinctions"—precipitous declines in the number of species spurred by dramatic events such as an asteroid impact or changing sea levels.
Today we are witnessing what some experts believe to be the "sixth wave of extinction," a species diminution that appears to be the handiwork of humankind. Experts estimate that the current extinction rate is somewhere between 100 and 1,000 times higher than the background rate (others say it is even higher, up to 10,000 times the usual background extinction rate).
There are currently 3,071 "critically endangered" species in the world, according to the World Conservation Union, also known as the International Union for the Conservation of Nature and Natural Resources (IUCN), a collaboration of 83 countries, 800 nongovernmental organizations and 10,000 scientists and experts devoted to preserving Earth's biodiversity. According to the IUCN, species assessed at the critically endangered level "face an extremely high risk of extinction in the wild unless the pressures on them are relieved." Here are just a few of these species: