Habitat loss and fragmentation

Relatively undisturbed ecosystems have shrunk dramatically in area over past decades as human population and resource consumption have grown. Ninety-eight percent of the tropical dry forest along Central America's Pacific coast has disappeared. Thailand lost 22 percent of its mangroves between 1981 and 1985, and virtually none of the remainder is undisturbed. In freshwater ecosystems, dams have destroyed large sections of river and stream habitat. In marine ecosystems, coastal development has wiped out reef and near-shore communities. In tropical forests, a major cause of forest loss is the expansion of marginal agriculture, though in specific regions commercial timber harvest may pose an even greater problem.

Introduced species

Introduced species are responsible for many recorded species extinctions, especially on islands. In these isolated ecosystems, a new predator, competitor, or pathogen can rapidly imperil species that did not co-evolve with the newcomer. In Hawaii, some 86 introduced plant species seriously threaten native biodiversity; one introduced tree species has now displaced more than 30,000 acres of native trees.

Over-exploitation of plant and animal species

Numerous forest, fisheries, and wildlife resources have been over-exploited, sometimes to the point of extinction. Historically, both the great auk and the passenger pigeon succumbed to such pressure, and the Lebanon cedar that once blanketed 500,000 hectares now is found in only a few scattered remnants of forest. Over-exploitation of the Peruvian anchovy between 1958 and 1970 dramatically reduced the population size and the catch. Today, the Sumatran and Javan rhinos have been hunted to the verge of extinction, along with numerous other vertebrates. Many extinctions attend the human harvest of food, but the search for precious commodities -- notably, ivory -- and for pets, curiousities, and collector's items has also impinged on some populations and obliterated others.

Pollution of soil, water, and atmosphere

Pollutants strain ecosystems and may reduce or eliminate populations of sensitive species. Contamination may reverberate along the food chain: barn owl populations in the United Kingdom have fallen by 10 percent since new rodenticides were introduced, and illegal pesticide used to control crayfish along the boundaries of Spain's Cota Donana National Park in 1985 killed 30,000 birds. Some 43 species have been lost in Poland's Ojcow National Park, due in part to severe air pollution. Soil microbes have also suffered from pollution as industry sheds heavy metals and irrigated agriculture brings on salinization. Acid rain has made thousands of Scandinavian and North American lakes and pools virtually lifeless, and, in combination with other kinds of air pollution, has damaged forests throughout Europe. Marine pollution, particularly from non-point sources, has defiled the Mediterranean and many estuaries and coastal seas throughout the world.

Global climate change

In coming decades, a massive "side-effect" of air pollution -- global warming -- could play havoc with the world's living organisms. Human-caused increases in "greenhouse gases" in the atmosphere are likely to commit the planet to a global temperature rise of some 1 to 3 degrees Celsius (2 to 5 degrees Fahrenheit) during the next century, with an associated rise in sea level of 1 to 2 meters. Each 1 degree Celsius rise in temperature will displace the limits of tolerance of land species some 125 kilometers towards the poles, or 150 meters vertically on the mountains. Many species will not be able to redistribute themselves fast enough to keep up with the projected changes, and considerable alterations in ecosystem structure and function are likely. In the United States rising seas in the next century may cover the entire habitat of at least 80 species already at risk of extinction. Many of the world's islands would be completely submerged by the more extreme projections of sea level rise -- wiping out their fauna and flora. And protected areas themselves will be placed under stress as environmental conditions deteriorate within and suitable habitat for their species cannot be found in the disturbed land surrounding them.

Industrial agriculture and forestry

Until this century, farmers and pastoralists bred and maintained a tremendous diversity of crop and livestock varieties around the world. But on-farm diversity is shrinking fast thanks to modern plant-breeding programs and the resulting productivity gains achieved by planting comparatively fewer varieties of crops that respond better to water, fertilizers, and pesticides. Similar trends are transforming diverse forest ecosystems into high-yielding monocultural tree plantations -- some of which now resemble a field of maize as much as a natural forest -- and even fewer tree genes than crop genes have been preserved off-site as an insurance policy against disease and pests.

Sources:

• Reid, W.V., and M.C. Trexler. 1991. Drowning the National Heritage: Climate change and U.S. coastal biodiversity. World Resources Institute, Washington, D.C., USA.

• IPCC [Intergovernmental Panel on Climate Change]. 1990. Climate Change: The IPCC Scientific Assessment. Houghton, J.T., G.J.Jenkins, and J.J. Ephraums (eds.). Cambridge University Press, Cambridge, U.K.

• Thorsell, J. 1990. The IUCN register of threatened protected areas of the world. Paper presented to the 34th Working Session of the IUCN Commission on National Parks and Protected Areas, Perth, Australia, November 26-27, 1990.

• Reid, W.V., and K.R.Miller. 1989. Keeping Options Alive: The scientific basis for conserving biodiversity. World Resources Institute, Washington, D.C., USA.

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