C H A P T E R 1 5 Extinction and Extirpation Extinction is the most obscure and local of all biological processes. We usually do not see the last individual of a species as it dies or is captured by a predator. We hear that a certain animal or plant is imperiled, perhaps already gone. | I Text Linzey Vertebrate Biology 15. Extinction and Extirpation The McGraw-Hill Companies 2003 CHAPTER 15 Extinction and Extirpation INTRODUCTION Extinction is the most obscure and local of all biological processes. We usually do not see the last individual of a species as it dies or is captured by a predator. We hear that a certain animal or plant is imperiled perhaps already gone. We return to the last known locality to search and when no individuals are encountered there year after year we pronounce the species extinct. Populations decline whenever deaths and emigration exceed births and immigration. The elimination of a species or subspecies from a region although it continues to exist elsewhere is known as extirpation. The cougar Puma concolor is thought by many to be extirpated from most of the eastern United States but cougars remain in Florida and many western states as well as in Canada and Central America. Extinction is the total disappearance of a species and has been the fate of most species since the origin of life. Dinosaurs passenger pigeons heath hens dodos Fig. mastodons and saber-toothed tigers are among the many vertebrates that have become extinct. Disappearance of entire species or even entire families orders or classes has occurred at times of extreme environmental change or more recently because of human action. With or without human interference extinction always has occurred. The last dinosaurs disappeared 65 million years ago over 60 million years before humans evolved. Judging from the fossil record Peter Raven Director of the Missouri Botanical Gardens calculated the average life span of a species at about 4 million years Raven 1995 . If there are about 10 million species in the world Raven calculated the normal rate of extinction at about 4 species a year. Many scientists believe that humans now have increased the pace of extinction far beyond natural levels so that species are now becoming extinct at rates 1 000 to 10 000 times .
