The world's ecosystems yield a flow of essential services that sustain and fulfill human life, from seafood and timber production to soil renewal and personal inspiration. Although many societies have developed the technological capacity to engineer replacements for some services, such as water purification and flood control, no society can fully replace the range and scale of benefits that ecosystems supply. Thus, ecosystems are capital assets, worthy of at least the level of attention and investment given to other forms of capital. Yet, relative to physical, financial, human, and social capital, ecosystem capital is poorly understood, scarcely monitored, and, in many cases, undergoing rapid degradation and depletion.
Recognition of ecosystem services dates back at least to Plato. This recognition of human dependence on ecosystems, in the past and today, is often triggered by their disruption and loss. Direct enjoyment of services, such as the extraction of timber, fish, and freshwater, can reduce the quantity and quality produced. The provision of ecosystem services can also be affected indirectly and inadvertently. Deforestation, for instance, has exposed the critical role of forests in the hydrological cycle -mitigating flooding and reducing erosion. Release of toxic substances has uncovered the nature and value of physical and chemical processes, governed in part by microorganisms that disperse and break down hazardous materials. Thinning of the stratospheric ozone layer has sharpened awareness of the value of its service in screening out harmful ultraviolet radiation.
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