Biodiversity - Page 2 - By Mara Hurwitt


Geography And Climate Of A Home To Many
Overall, the greatest degree of species diversity is found in the warm tropical habitats nearest the equator. Diversity is lower in the Earth's temperate zones, and lowest in the polar regions, a phenomena known as the latitudinal gradient of biodiversity. The species richness of land ecosystems also decreases at higher elevations.

High amounts of annual rainfall also correlate to richer diversity in land habitats, with the highest concentration of different species found in the tropical rainforests. The most diverse marine ecosystems are located on the ocean's continental shelves, although deep-sea habitats also demonstrate significant species richness.

Rainforests And Coral Reefs
Science has yet to explain the remarkable degree of species diversity found in tropical rainforests and coral reefs. The answers to this ecological mystery involve both the origins of diversity through evolution and the maintenance of species diversity.

Tropical rainforests cover approximately 9 million square kilometers - a decrease of about 45% since man's arrival. Although they account for only about 6% of the world's land surface, these highly diverse ecosystems may contain over half of all living species. Some researchers place that figure as high as 90%, based on estimates of the number of microorganisms and insects believed to populate the rainforests. Likewise, approximately one quarter of marine species are thought to reside on the ocean's coral reefs.

Sadly, man is cutting and burning the world's rainforests at a rate of approximately 100,000 square kilometers annually. As Edward Wilson explains, this amounts to the destruction of an area of rainforest equal to the size of a football field every second. He conservatively estimates that between 0.2% and 0.3% of rainforest species - that is 4,000 to 6,000 individual species - are lost each year from deforestation alone. This is 10,000 times the rate of natural extinction which occurred before human intervention. The problem is so severe that, in 1990, the independent Science Advisory Board of the U.S. Environmental Protection Agency named habitat loss and species extinction as two of the world's most critical environmental problems.
Threats To Biodiversity
According to Stanford Professor Paul R. Erlich, the destruction of biodiversity is comparable to pulling the rivets out of an airplane. The missing rivet(s) will lead to stresses, which will affect performance, which will create other stresses, and so on, in a disastrous snowball effect.

The threats to biodiversity are myriad, but most are the direct or indirect result of human activity.

Population growth and migration, overexploitation, and the lack of sustainable resource consumption policies continue to stress sensitive ecosystems. Some of the highest rates of growth are occurring in the Earth's species-rich tropical regions. For example, 75 million people already inhabit the rainforests of the Upper Amazonia and Guyana Shield in South America, the Congo Basin in Africa, and the New Guinea/Melanesian Islands between Asia and Australia, and their populations are continuing to expand at nearly 250% of the world average.

The destruction of natural habitats to accommodate population growth and migration is the single largest factor in the loss of species and even entire ecosystems. In the continental United States alone, 98% of virgin forests and 54% of natural wetlands have been destroyed. However, lesser human intrusions, such as the removal of a single indigenous population or the introduction of a non-native species can also severely disrupt the functioning of natural ecosystems, due to the complex nature of their interrelationships and processes.

Human activity also produces pollution and contamination which can affect all levels of biodiversity. Toxic substances released into our air and water not only impact regional ecosystems, but can extend their debilitating effects beyond state and national borders, as in the case of acid rain. Continued discharge of other substances into the atmosphere, though not necessarily toxic, leads to the ozone depletion in the stratosphere and increased penetration of ultraviolet radiation to the land and ocean.

Scientists have identified the "greenhouse effect&quoy; where increased levels of carbon dioxide, methane, and other gases capture heat within the Earth's atmosphere. This climatic global warming is of particular concern in the cold temperate and polar regions, where possible climatic shifts could leave behind entire ranges of plant and animal species. Because climatic changes would be most severe near the poles, entire arctic ecosystems could be threatened with extinction.

Carbon released from the burning of fossil fuels and natural habitats is a key cause of global warming. According to a 1992 report by the U.S. Office of Technology Assessment, the burning of tropical forests outside the United States accounted for approximately 25% of all carbon released into the atmosphere during the preceding decade.

Global climate changes can alter the environment and disrupt ecosystems, resulting in loss of species and populations that are unable to either migrate or adapt to new ecological conditions. The ultimate effects of these climate changes are unknown but risk potentially serious consequences for humans as well as other species.

In his paper, Biological Diversity: The Oldest Human Heritage, Professor Edward Wilson writes,
Certainly over millions of years species adapted to alternative climatic warming and cooling, the expansion or shrinkage of continental shelves and the invasion of new competitors and parasites. Those that could not change became extinct, but at such a relatively slow rate that other better-adapted species evolved to replace them. In the midst of endless turnover, the balance of life was sustained. But now the velocity of change is too great for life to handle, and the equilibrium has been shattered. It has reached precipitous levels within a single human life span, merely a tick in geological time. Humanity is creating a radical new environment too quickly to allow the species to adjust. Species need thousands or millions of years to assemble complex genetic adaptations. Most of life is consequently at risk. We are at risk.
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