Overview of the Biodiversity Crisis
• More than 99% of species known to science are now extinct. (p. 1228)
• It is estimated that as much as 20% of the world's biodiversity may be lost during the next 50 years. (p. 1228)
• Shortly after humans arrived, 74-86% of the earth's megafauna became extinct. (p. 1228)
• Estimates indicate that about 85 species of mammals and 113 species of birds have become extinct in the past 400 years. (p. 1229)
• The majority of historic extinctions have occurred on islands, although most species now threatened by extinction live on continents. (p. 1229)
Species Endemism and Hotspots
• An endemic species is found naturally in only one geographic area. (p. 1230)
• Isolated, geographic areas often have high percentages of endemic species. (p. 1230)
• Worldwide, concentrations of endemic species occur in several hotspots that must be the focus of conservation efforts. Unfortunately, these areas currently also contain 20% of the world's human population. (pp. 1230-1231)
• Population pressure as well as habitat loss due to commercial exploitation play an important role in biodiversity loss. (p. 1231)
What's So Bad About Losing Biodiversity?
• The value of biodiversity can be divided into direct economic value (food, clothing), indirect economic value (ecosystem maintenance), and ethical and aesthetic value (beauty). (pp. 1232-1233)
57.2 The extinction crisis is a result of many factors.
Factors Responsible for Extinction
• Historically, overexploitation was the major factor responsible for extinction, but habitat loss is the most significant problem for most groups today. (p. 1234)
• Other factors, such as ecosystem disruption, pollution, loss of genetic diversity, catastrophic disturbance, and habitat fragmentation, can also contribute to species extinction. (p. 1234)
• Many amphibian species around the world are in decline, and because no single cause can be determined, fears are rising as to the overall health of the global environment. (p. 1235)
Habitat Loss
• Natural habitats may be adversely affected by humans in four ways: destruction, pollution, human disruption, and fragmentation. (pp. 1236-1237)
• Fragmentation of summer breeding grounds and loss of overwintering habitats appear to be contributing to an overall decline in the numbers of migratory songbirds. (p. 1238)
Overexploitation
• Species that are harvested by people have historically been at grave risk of extinction. (p. 1239)
• The existence of a commercial market often leads to overexploitation of a species. (p. 1239)
• Overharvesting has brought most species of large whales to the brink of extinction. (p. 1239)
Detrimental Effects of Introduced Species
• Although colonization and range expansion is a natural process, humans are increasingly causing the introduction of species to new, unintended areas. (p. 1240)
• Some species introductions can lead to the transformation of an ecosystem, the displacement of native species, and the production of a monoculture. (pp. 1240-1241)
• The introduction of Nile perch to Lake Victoria, along with eutrophication, caused a large reduction in the number of endemic cichlid species in Lake Victoria. (p. 1241)
Disruption of Ecosystems
• Species often become vulnerable to extinction when their web of ecological interactions becomes seriously disrupted. (p. 1242)
• Activities that detrimentally affect one species may indirectly affect other species as well. (pp. 1242-1243)
The Perils of Small Population Size
• Small populations are ill-equipped to withstand a catastrophic event, and single, natural events can lead to the extinction of a population or a species. (p. 1244)
• Small populations are also prone to the loss of genetic variation due to genetic drift. (p. 1244)
• As a population gets smaller, it becomes more vulnerable to demographic catastrophes, in turn causing a decline in genetic variation and a subsequent decline in reproductive rates, which set in motion a cascading effect. (p. 1245)
57.3 Successful recovery efforts need to be multidimensional.
Approaches for Preserving Endangered Species
• Many approaches exist for preserving and fostering endangered species, and these include habitat restoration and captive propagation. (p. 1246)
• Habitat restoration can involve many actions, including pristine restoration, removal of introduced species, and cleanup and rehabilitation. (p. 1246)
• Captive propagation has produced many success stories, including the peregrine falcon, the California condor, and wolves in Yellowstone National Park. (p. 1247)
Conservation of Ecosystems
• Conservation efforts are increasingly being turned toward preserving large tracts of land over long periods of time in order to preserve intact ecosystems rather than individual or particular species. (p. 1248)