SOIL EROSION AND ITS EFFECTS
The loss of trees, which anchor the soil with their roots, causes widespread erosion throughout the tropics. Only
a minority of areas have good soils, which after clearing are quickly washed away by the heavy rains. Thus crop
yields decline and the people must spend income to import foreign fertilizers or clear additional forest. Costa
Rica loses about 860 million tons of valuable topsoil every year, while the Great Red Island, Madagascar, loses
so much soil to erosion (400 tons/ha) that its rivers run blood-red, staining the surrounding Indian Ocean. Astronauts
have remarked that it looks like Madagascar is bleeding to death, an apt description of a country with grave environmental
degradation and an agriculture-reliant economy that depends on its soils. The rate of increase for soil
loss after forest clearing is astonishing; a study in Ivory Coast (Cote d'Ivoire) found that forested slope areas lost 0.03 tons
of soil per year per hectare; cultivated slopes annually lost 90 tons per hectare, while bare slopes lost 138 tons
Plane view of deforestation-induced erosion in Madagascar
After heavy tropical rains fall on cleared forest lands, the run-off carries soil into local creeks and rivers.
The rivers carry the eroded soils downstream, causing significant problems. Hydroelectric projects
infrastructure lose productivity from siltation, while industrial installations suspend operations due to lack
of water. Siltation also raises river beds, increasing the severity of floods, and creates shoals and sandbars
that make river navigation far more troublesome. The increased sediment load of rivers smothers fish eggs, causing
lower hatch rates. As the suspended particles reach the ocean, the water becomes cloudy, causing regional declines in coral
reefs, and affecting coastal fisheries. The loss of coral reefs worldwide, often labeled the rainforests of the sea
, is especially
distressing to scientists because of their tremendous diversity and the important services they provide. Coastal
fisheries are affected not just by the loss of coral reefs and their communities, but by the damage inflicted on
mangrove forests by heavy siltation.
Besides damaging the fisheries industry, deforestation-induced erosion can undermine roads and highways
that cross through the forest.
Erosion is extremely costly for developing countries. Besides the damage to infrastructure, fisheries, and
property, erosion of precious topsoils costs tens of billions of dollars worldwide each year. For example, in the
late 1980s the Indonesian island of Java was losing 770 million metric tons of topsoil every year at an estimated
cost of 1.5 million tons of rice, enough to fulfill the needs of 11.5-15 million people.
Environmental deterioration can leave people as "environmental refugees
"—people who are displaced due to environmental degradation, including deforestation, sea-level rise, expanding deserts, and catastrophic weather events. Red Cross research shows more people are now displaced by environmental disasters than by war.
Erosion in Madagascar. Click image for more information. (Photo by R. Butler)
- Why do rainforests help prevent erosion?
- Why is erosion a problem?
- What are environmental refugees?
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Continued / Next: Species Loss, Extinction and Disease
J. Omang ("In the Tropics, still rolling back the rain forest primeval," Smithsonian (March 1987) reported the rate of erosion in Costa Rica.
Photograhper Frans Lanting made the comment that from space it looks as if Madagascar is bleeding to death from rampant erosion in A World Out of Time-Madagascar, New York: Aperture Foundation, Inc., 1990.
UNESCO/UNEP/FAO, in Tropical Forest Ecosystems, 1978 provides the erosion rates for different vegetation types in an Ivory Coast study.
A discussion on the worst coral bleaching on record in 1998 can be found in Wilkinson et al., (Wilkinson, C., O. Linden, H. Cesar, G. Hodgson, J. Rubens, and A. E. Stong, "Ecological and socioeconomic impacts of 1998 coral bleaching in the Indian Ocean: an ENSO impact and a warning of future change?" Ambio, 1999) the U.S. Department of State's "Coral Bleaching, Coral Mortality, and Global Climate Change," Bureau of Oceans and International Environmental and Scientific Affairs U.S. Department of State, March 5, 1999; and Wilkinson, C. and Hodgson, G. ("Coral reefs and the 1997-1998 mass bleaching and mortality," Nature and Resources Vol. 5, No. 2, Apr-June 1999).
Magrath and Areans (Magrath, W. and P. Arens., The costs of soil erosion on Java: a natural resource accounting approach, The World Bank, Washington, D.C., 1993) estimate the annual cost of erosion for Java in terms of rice production.