that maintaining biological diversity is important to sustain ecosystem functioning, although the information on this matter is still very sparse and unclear. Fourth, all these achievements require that, in plans for providing and allocating the study area's water resources, a balance must be struck among environmental, short-term economic, and other objectives. To assess these balances and identify appropriate tradeoffs, a significant amount of new scientific information will be needed.
Ecosystem services are ecosystem processes and functions beneficial to humans, primarily in contributing to the sustainability of people's lives and their intensively managed ecosystems. When activities destroy or impair the ability of natural ecosystems to provide these goods and services, the goods and services must be replaced by artificial means. Examples of such replacements are wastewater treatment plants, water filtration and purification systems, erosion control programs, and so on. Wide experience has shown that the artificial replacements for natural ecosystem goods and services are usually very expensive and often inferior to the natural ones. Because natural ecosystems provide these goods and services at no immediate financial cost, they appear to be free and their value and importance are often underestimated or overlooked entirely. For example, the value of ground water properly includes its extractive values (e.g., municipal, industrial, and agricultural uses) as well as the natural, in-situ services it provides (e.g., providing habitat and supporting biota, preventing subsistence of land, buffering against periodic water shortage, and diluting or assimilating ground-water contaminants) (NRC, 1997). To take advantage of these crucial services, they must be understood and protected.
Ecosystem services can be classified into those related to air, soil, and water. One particular service, absorbing and detoxifying pollutants, can be related to air, soil, water, or some combination of the three. Some services are global in extent and of crucial survival value, namely, the maintenance of the gaseous composition of the atmosphere, and regulation of global air temperatures and global and local climatic patterns.
Although increasing quantities of cash crops are produced on soilless substrates in the study area (using growth chambers, or ''greenhouses"), soil is one universal substrate for terrestrial biological production. Soils are produced by weathering of rocks in the Earth's crust. Organisms directly affect this weathering and also mediate the effects of water and air on weathering. Thus, one important ecosystem service is production and maintenance of soil. Soil can be lost by wind and water erosion at a rate orders of magnitudes faster than it is generated. Normally, soil erosion