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A paradox of biodiversity is that almost all species that have ever existed are now extinct, but there are more species in existence today than ever before. The explanation for this paradox is that the lifetime of a species is relatively short (1 million years) relative to the age of life on Earth (> 3 billion or 3 thousand million years) and that evolution is dynastic (i.e., one species gives rise to several species). Fossil records reveal characteristic longevities for a clade, which is composed of a species and all of its descendants "taken from the time the ancestral species first splits off from other species to the moment the last organism belonging to that species and all of its descendants disappears." Consequently, natural processes of speciation and extinction can be modeled with probability statistics.
Based on comparisons with those natural rates of evolution, there is an accelerated rate of extinction associated with human activities. E.O. Wilson has generalized that "in the small minority of groups of plants and animals that are well known, extinction is proceeding at a rapid rate, far above prehuman levels. In many cases the level is calamitous: the entire group is threatened." He has additionally noted that:
"From prehistory to the present time, the mindless horsemen of the environmental apocalypse have been overkill, habitat destruction, introduction of animals such as rats and goats, and diseases carried by these exotic animals. In prehistory the paramount agents were overkill and exotic animals. In recent centuries, and to an accelerating degree during our generation, habitat destruction is foremost among the lethal forces, followed by the invasion of exotic animals. Each agent strengthens the others in a tightening net of destruction. In the United States, Canada, and Mexico, 1,033 species of fish are known to have lived entirely in fresh water within recent historical times. Of these, 27 or 3 percent have become extinct within the past hundred years, and another 265 or 26 percent are liable to extinction….The changes that forced them into decline are:
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Destruction of physical habitat |
73% of species |
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Displacement by introduced species |
68% of species |
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Alteration of habitat by chemical pollutants |
38% of species |
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Hybridization with other species |
38% of species and subspecies |
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Overharvesting |
15% of species |
(These figures add up to more than 100 percent because more than one agent impinges on many of the fish populations.) When habitat destruction is defined as both the physical reduction in suitable places to live and the closing of habitats by chemical pollution, then it is found to be an important factor in over 90 percent of the cases. Through a combination of all of these factors, the rate of extinction has risen steadily during the past forty years.
Wilson, E.O. 1992. The Diversity of Life. Harvard University Press, Cambridge, MA.
Birds: It is estimated that there has been a 20% (2,000) reduction in bird species worldwide within the past two thousand years, primarily associated with human inhabitation of islands. The International Council for Bird Preservation has recently estimated that 11% (1,029) of the remaining bird species (9,040) are endangered. The population density of migratory songbirds in the mid-Atlantic United States has dropped by 50% within the past 60 years, and many of those species have also become extinct.
Freshwater Fish: Approximately 20% of the world's recent fresh water fish species are extinct or threatened. The problem appears to be especially critical in some tropical countries, where recent searches have found small numbers of species that were known to exist in those areas (e.g., 122 of 266 species in a Malaysian peninsula, and 3 of 18 species in Lake Lanoa, Philippines). More than half (>150) of the cichlid fishes in Lake Victoria are now projected to become extinct due to the introduction of the Nile perch as a sport fish in 1959.
Freshwater Mollusks: At least 12 freshwater mussel species in the United States are extinct, and 20% of the remaining species are endangered. For example, the construction of the Wilson Dam on the Tennessee River resulted in the extinction of 44 of the 68 mussel species in Muscle (sic) Shoals. These organisms are especially vulnerable to extinction from the damming or diversion of rivers and the introduction of alien species, because many "are specialized for life in narrow habitats and unable to move quickly from one place to another".
Plants: The Center for Plant Conservation has reported that between 213 and 228 plant species, out of a total of 20,000 in the United States, have bcome extinct; and that another 680 species and subspecies are in danger of becoming extinct by the year 2000. About three fourths of those species occur in: California, Florida, Hawaii, Puerto Rico, and Texas.
Hot Spots: The following "hot spots" are considered to be areas where the entire habitat, rather than one or a few organisms, are at risk. They represent areas "with many species found nowhere else and in greatest danger of extinction from human activity". The list, which is "far from complete", includes 2 habitats in California.
The two hot spots in California are threatened by urbanization, agriculture, mining, offroad vehicular traffic, and the invasion of foreign species, including eucalyptus.
San Bruno Mountain is a small refuge with several endangered plants, insects, and vertebrates. In contrast, the California floristic province is a large area extending from Baja California to southern Oregon, which "contains one fourth of all of the plant species found in the United States and Canada, combined" and half of those species (2,140) "are found nowhere else in the world".
There are numerous other "hot spot" candidates. These include the remnant rain forests of Central America Mexico, Hawaii, , Liberia, Queensland, and the West Indies; the Great Lakes of East Africa; Lake Baikal; "virtually every river drainage system in the world near heavily populated regions"; the Baltic and Aral seas; and numerous "isolated tracts of species-rich tropical deciduous forests, grasslands, and deserts". Note that many of these are freshwater habitats.
Coral Reefs
There are numerous compounding threats to the health of coral reefs. Natural threats include changes in weather patterns and climate (e.g., hurricanes, El Nio events, global warming and ice ages. While reefs typically recovered from natural disasters, concurrent anthropogenic impacts appear to be limiting or precluding some of those recoveries. These include the physical destruction of reefs from ships, dredging, mining of coral, and collecting; smothering of reefs by oil spills, increased sedimentation, and macroalgae blooms; and overfishing.
Declines in coral reefs have also been attributed to "coral bleaching". It is due to the loss in photosynthetic pigment of zooanthellae (single-celled algae that live symbitically within the coral). The bleaching is caused by a variety of stresses including; temperature changes, chemical pollution, and lowered salinity from fresh water discharges. Each of those stresses may be a result of human activities.
References (from Wilson, 1992):
Diamond, J. 1989. Quaternary megafaunal extinctions: Variations on a theme by Paganini. Journal of Archaeological Science 16: 167-175. Analyses of extinctions of megafauna by prehistoric humans.
Glynn, P.W. 1991. Coral reef bleaching in the 1980s and possible connectionss with global warming. Trends in Ecology and Evolution 6: 175-179. Natural and anthropogenic impacts on coral reefs.
IUCN. 1983. The IUCN Invertebrate Red Data Book. International Union of Nature and Natural resources, Gland, Switzerland. Report on the decline of freshwater mollusks.
McMahan, L.R. 1988. CPC survey revealss 680 native U.S. plants may become extinct within 10 years. Plant Conservation (Center for Plant Conservation) 3: 1-2.
Miller, R.R. et al. 1989. Extinctions of North American fishes during the past century. Fisheries 14: 22-38. Status of endangered fish.
Mooney, H.A. and J/.A. Drake (eds.). 1986. Ecology of Biological Invasions of North America and Hawaii. Springer, New York, NY. Report on the origins and impacts of exotic species.
Myers, N. 1990. The biodiversity challenge: Expanded hot-spots analysis. Environmentalist 10: 243-256. Threatened habitats.
Roberts, L. 1991. Greenhouse role in reef stress unproven. Science 253: 258-259. Natural and anthropogenic impacts on coral reefs.
Welsh, H.H. Jr. 1990. Relictual amphibians and old-growth forests. Conservation Biology 4: 308-319. Report on rare frogs and toads.
Williams, J.E. and R.R. Miller. 1990. Conservation status of the North American fish fauna in fresh water. Journal of Fish Biology 37: 79-85. Status of endangered fish
Williams et al. 1989. Fishes of North America. Endangered, threatened, or of special concern: 1989. Fisheries 14:2-20. Status of endangered fish.
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