Biologists are concerned that the world is facing an extinction crisis, in which a large fraction of the Earth's species will disappear by 2100. The passenger pigeon, which numbered in the billions and was once the most abundant bird in North America gradually went extinct under our watch, with the last individual dying in the Cincinnati zoo in 1919. The Northern Right whale, once one of the North Atlantic's most important marine species, now numbers only 200-300 individuals, and the death of just one female is newsworthy. Although scientists agree that an unprecedented number of species are at risk of imminent extinction, and that the rate of biodiversity loss rivals the great extinction events of geological history, the science of extinction faces several big unknowns. Most notably, although scientists have named and described about 1.75 million species, they have still barely scratched the surface of Earth's total biological diversity. In fact, about 15 000 new species are described each year, and these discoveries are not limited to obscure or small-bodied groups - for example, a new species of large mammal is discovered approximately every three years. Clearly, it is hard to know how many species are going extinct when we do not even know how many species there are. It is not surprising then that estimates of extinction rates vary by an order of magnitude, ranging from lower than 10 species lost per day to over 100 species lost each day.

Even though quantifying global extinction rates is highly uncertain, we have a good understanding of what factors are likely to make a species endangered. Moreover, as nations have become alerted to the extinction crisis, international institutions and governments around the world have started to identify endangered species (species at risk of going extinct in the near future) and many countries have laws that afford special protection to these at-risk species. Our scientific understanding of species at-risk or endangered species comes from four main arenas: (1) our theoretical understanding of the population biology of small populations, (2) statistical studies relating the number of resident species to habitat availability and geography, (3) monitoring and tracking of endangered species in the United States, which was the first country to pass a law protecting endangered species and has since accumulated over 30 years of data on its endangered species, and (4) global lists and analyses of endangered species compiled by the International Union for the Conservation of Nature and Natural Resources (IUCN). If we can develop a predictive science of endangered species, then there is hope of avoiding the demise of the thousands of at-risk species currently on the brink of extinction throughout the world.

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