Ecosystem Services:Benefits Supplied to Human Societies by Natural Ecosystems
Published by the Ecological Society of America Number 2, Spring 1997 Ecosystem Services: Benefits Supplied to Human Societies by Natural Ecosystems Issues in Eco logy
Issues in Ecology Number 2 Spring 1997 Ecosystem Services: Benefits Supplied to Human Societies by Natural Ecosystems SUMMARY Human societies derive many sential goods from natural esy fuelwood.timber. ang pharm al produ These represent important an sof the econ without which human civilizations would cease to thrive.These include the purification of air and water,detoxification and decomposition of wastes,regulation of climate,regeneration of soil fertility.and production and maintenance of biodiversity,from which key ingredients of our agricultural,pharmaceutical,and industrial enterprises are derived.This array of services is generated by a complex interplay of natural cycles powered by solar energy and operating across a rocess of waste disposal,for ple,involves the life cycles of bacte ria as we ide cycles of major chemic eleme as carbon an are worth many ars annually.Yet because most of these benefits are not traded in economic markets,they carry no pric tags that could alert society to changes in their supply or deterioration of underlying ecological systems that generate them.Because threats to these systems are increasing.there is a critical need for identification and monitoring of ecosystem services both locally and globally,and for the incorporation of their value into decision-making processes Historically the nature and value of Earth's life tems have largely been their disruption oss highlighted their importance m o defores has h elatedly reve led e regulating the water cycle- in particular,in mitigating floods,droughts,the erosive forces of wind and rain.and siting of dams and irrigation canals.Today,escalating impacts of human activities on forests,wetlands,and other natural ecosystems imperil the delivery of such services.The primary threats are land use changes that cause losses in biodiversity as well as disruption of carbon,nitrogen,and other biogeochemical cycles:human-caused invasions of exotic species: releases of toxic substances:possible rapid climate change:and depletionof stratospheric oe. Based on available scientific evidence,we are certain that: Ecosystem services are essential to civilization. Ecosystem services operate on such a grand scale and in such intricate and little-explored ways that most could not be replaced by technology. Human activities are already impairing the flow of ecosystem services on a large scale. If current trends continue,hum alter virtually all of Earth' emaining natural ecosystems within a few decades In addition,based on current scientific evidence.we are confident that: Many of the human activities that modify or destroy natural ecosystems may cause deterioration of ecological services whose value.in the long term.dwarfs the short-term economic benefits society gains from those activities Considered very large mbers of species and populations arere quired to sustain ecosvstem services. The functioning of many ecosystems could be restored if appropriate actions were taken in time We believe that land use and development policies should strive to achieve a balance between sustaining vital ecosystem services and pursuing the worthy short-term goals of economic development
Issues in Ecology Number 2 Spring 1997 1 Ecosystem Services: Benefits Supplied to Human Societies by Natural Ecosystems SUMMARY Human societies derive many essential goods from natural ecosystems, including seafood, game animals, fodder, fuelwood, timber, and pharmaceutical products. These goods represent important and familiar parts of the economy. What has been less appreciated until recently is that natural ecosystems also perform fundamental life-support services without which human civilizations would cease to thrive. These include the purification of air and water, detoxification and decomposition of wastes, regulation of climate, regeneration of soil fertility, and production and maintenance of biodiversity, from which key ingredients of our agricultural, pharmaceutical, and industrial enterprises are derived. This array of services is generated by a complex interplay of natural cycles powered by solar energy and operating across a wide range of space and time scales. The process of waste disposal, for example, involves the life cycles of bacteria as well as the planet-wide cycles of major chemical elements such as carbon and nitrogen. Such processes are worth many trillions of dollars annually. Yet because most of these benefits are not traded in economic markets, they carry no price tags that could alert society to changes in their supply or deterioration of underlying ecological systems that generate them. Because threats to these systems are increasing, there is a critical need for identification and monitoring of ecosystem services both locally and globally, and for the incorporation of their value into decision-making processes. Historically, the nature and value of Earths life support systems have largely been ignored until their disruption or loss highlighted their importance. For example, deforestation has belatedly revealed the critical role forests serve in regulating the water cycle -- in particular, in mitigating floods, droughts, the erosive forces of wind and rain, and silting of dams and irrigation canals. Today, escalating impacts of human activities on forests, wetlands, and other natural ecosystems imperil the delivery of such services. The primary threats are land use changes that cause losses in biodiversity as well as disruption of carbon, nitrogen, and other biogeochemical cycles; human-caused invasions of exotic species; releases of toxic substances; possible rapid climate change; and depletion of stratospheric ozone. Based on available scientific evidence, we are certain that: • Ecosystem services are essential to civilization. • Ecosystem services operate on such a grand scale and in such intricate and little-explored ways that most could not be replaced by technology. • Human activities are already impairing the flow of ecosystem services on a large scale. • If current trends continue, humanity will dramatically alter virtually all of Earths remaining natural ecosystems within a few decades. In addition, based on current scientific evidence, we are confident that: • Many of the human activities that modify or destroy natural ecosystems may cause deterioration of ecological services whose value, in the long term, dwarfs the short-term economic benefits society gains from those activities. • Considered globally, very large numbers of species and populations are required to sustain ecosystem services. • The functioning of many ecosystems could be restored if appropriate actions were taken in time. We believe that land use and development policies should strive to achieve a balance between sustaining vital ecosystem services and pursuing the worthy short-term goals of economic development
Issues in Ecology Number 2 Spring 1997 Ecosystem Services:Benefits Supplied to Human Societies by Natural Ecosystems by Gretchen c daily susan alexander paul r fhrlich enco.Pamela A.Matson.Harold A.Moon INTRODUCTION purification of air and water mitigation of droughts and floods Many societies today have technological capa- generation and preservation of soils and renewal of bilities undreamed of in centuries past.Their citizens their fertility. have such a global command of resources that even foods detoxification and decomposition of wastes flown in fresh from all over the planet are taken for pollination of crops and natural vegetation. pled from the limita dispersal of seeds. nd soils.These de 。and moy sed so much attention upor men t of nutrients e vast majority of potential agricultura human-engineered and exotic sources of fulfillment that pests. they divert attention from the local biological underpin- maintenance of biodiversity. nings that remain essential to economic prosperity and protection of coastal shores from erosion by waves other aspects of our well-being. protection from the sun's harmful ultraviolet rays These biological underpinnings ar enc mpassed artial stabilization of climate. in the system s services. ather extremes and thei acts d proce through whic 0 ral ecosystems,and the species that are part of them help sustain and fulfill human life.These services main Although the distinction between "natural"and tain biodiversity and the production of ecosystem goods "human-dominated"ecosystems is becoming increasingly such as seafood,wild game.forage.timber,biomass fu- blurred,we emphasize the natural end of the spectrum els,natural fibers,and many phar naceuticals.industrial for three related reasons.First,the services flowing from oducts and thei The ha cosystems are grea ndervalued by P tant partsof For the most part. they are not tra ded in forr market: the human economy.In addition to the production of and so do not send price signals that wam of changes in goods,ecosystem services support life through(Holdren their supply or condition.Furthermore,few people are and Ehrlich 1974;Ehrlich and Ehrlich 1981): conscious of the role natural ecosystem services play in forest in Colorado,filtering and pu- rifying air and water
12 Issues in Ecology Number 2 Spring 1997 Ecosystem Services: Benefits Supplied to Human Societies by Natural Ecosystems by Gretchen C. Daily, Susan Alexander, Paul R. Ehrlich, Larry Goulder, Jane Lubchenco, Pamela A. Matson, Harold A. Mooney, Sandra Postel, Stephen H. Schneider, David Tilman, George M. Woodwell INTRODUCTION Many societies today have technological capabilities undreamed of in centuries past. Their citizens have such a global command of resources that even foods flown in fresh from all over the planet are taken for granted, and daily menus are decoupled from the limitations of regional growing seasons and soils. These developments have focused so much attention upon human-engineered and exotic sources of fulfillment that they divert attention from the local biological underpinnings that remain essential to economic prosperity and other aspects of our well-being. These biological underpinnings are encompassed in the phrase ecosystem services, which refers to a wide range of conditions and processes through which natural ecosystems, and the species that are part of them, help sustain and fulfill human life. These services maintain biodiversity and the production of ecosystem goods, such as seafood, wild game, forage, timber, biomass fuels, natural fibers, and many pharmaceuticals, industrial products, and their precursors. The harvest and trade of these goods represent important and familiar parts of the human economy. In addition to the production of goods, ecosystem services support life through (Holdren and Ehrlich 1974; Ehrlich and Ehrlich 1981): • purification of air and water. • mitigation of droughts and floods. • generation and preservation of soils and renewal of their fertility. • detoxification and decomposition of wastes. • pollination of crops and natural vegetation. • dispersal of seeds. • cycling and movement of nutrients. • control of the vast majority of potential agricultural pests. • maintenance of biodiversity. • protection of coastal shores from erosion by waves. • protection from the suns harmful ultraviolet rays. • partial stabilization of climate. • moderation of weather extremes and their impacts. • provision of aesthetic beauty and intellectual stimulation that lift the human spirit. Although the distinction between natural and human-dominated ecosystems is becoming increasingly blurred, we emphasize the natural end of the spectrum, for three related reasons. First, the services flowing from natural ecosystems are greatly undervalued by society. For the most part, they are not traded in formal markets and so do not send price signals that warn of changes in their supply or condition. Furthermore, few people are conscious of the role natural ecosystem services play in Figure 1-Aspen (Populus tremuloides) forest in Colorado, filtering and purifying air and water. Photo by J. Robert Stottlemeyer/Biological Photo Service
Issues in Ecology Number 2 Spring 1997 Figure 2-Woman carrying treetrunk for boat- making in a fishing village on Chiloe Island.Chile Natural forests remain an important source of wood for construction,fuel,and other uses generating those ecosystem goods that are traded in the directly for food,drink.spices,fiber,timber,pharmaceu marketplace.As a result,this lack of awareness helps ticals,and industrial products such as waxes,rubber,and drive the conversion of natural ecosystems to oils.Even if one were highly selective,the list could amount human-dominated systems (e.g.,wheatlands or oil palm to hundreds or even thousands of species.And that would fields).whose only be a start.since one would then need to consider inpart,in standard which those usedi Dacterla. ngi,and invertebrates that help ruptions of these systems-such as introductions of ex make soil fertile and break down wastes and organic otic species,extinctions of native species,and alteration matter:the insects,bats.and birds that pollinate flow of the gaseous composition of the atmosphere through ers:and the grasses,herbs.and trees that hold soil in fossil fuel burning are difficult or impossible to reverse a hird if av The clea of this is that no nd ntinue, kno tions species ven appr will dra tically alter Earth' remain ing natural ecosys- mately how many-are required to sustain human life tems within a few decades(Daily 1997a,b). Rather than selecting species directly.one might The lack of attention to the vital role of natural try another approach:Listing the ecosystem services ecosystem services is easy to understand.Humanity came needed by a lunar colony and then guessing at the types into being after most ecosystem services had been in and numbers of species required to perform each.Yet for hundreds of millions to billion deter re critical to the functionin are so funda I to life that mining which species of a parti osystem easy to take service for gran nted,and s large in s us lake oil fertility as an example hard to imagine that human activities could irreparably crucial to the chemical conversion and physical transter disrupt them. Perhaps a thought experiment that re of essential nutrients to higher plants. But the abun moves these services from the familiar backdrop of the dance of soil organisms is absolutely staggering.Under Earth is the best way to illustrate both the importance a square-yard of pasture in Denmark,for instance.the and complexity of ecosystem services,as well as how soil is inhabited by roughly 50.000 small earthworms are to em. their relatives.50.OO nd L ngs trying to n rou And t tally is only the begi Assume for the sake of argument that the moon had ning.The number of soil animals is tiny compared to the already miraculously acquired some of the basic condi number of soil microorganisms:a pinch of fertile soil may tions for supporting human life,such as an atmosphere contain over 30.000 protozoa,50.000 algae,400,000 a climate.and a physical soil structure similar to those fungi and billions of individual bacteri on Earth.The big question facing hum an colonists wo (Overgaard-Nielsen955:Rouatt and then be.which of Earth's millic on would Cha y 1993).Which must colonists bri ng to the to be rted to the moon to make sterile sur to assure lus and plant growth.soil renewa face habitable? waste disposal,and so on?Most of these soil-dwelling One could tackle that question systematicall species have never been subjected to even cursory in by first choosing from among all the species exploited spection:no human eye has ever blinked at them through
3 Issues in Ecology Number 2 Spring 1997 Figure 2-Woman carrying treetrunk for boatmaking in a fishing village on Chiloe Island, Chile. Natural forests remain an important source of wood for construction, fuel, and other uses. Photo by Taylor Ricketts generating those ecosystem goods that are traded in the marketplace. As a result, this lack of awareness helps drive the conversion of natural ecosystems to human-dominated systems (e.g., wheatlands or oil palm fields), whose economic value can be expressed, at least in part, in standard currency. The second reason to focus on natural ecosystems is that many human-initiated disruptions of these systems -- such as introductions of exotic species, extinctions of native species, and alteration of the gaseous composition of the atmosphere through fossil fuel burning -- are difficult or impossible to reverse on any time scale relevant to society. Third, if awareness is not increased and current trends continue, humanity will dramatically alter Earths remaining natural ecosystems within a few decades (Daily 1997a, b). The lack of attention to the vital role of natural ecosystem services is easy to understand. Humanity came into being after most ecosystem services had been in operation for hundreds of millions to billions of years. These services are so fundamental to life that they are easy to take for granted, and so large in scale that it is hard to imagine that human activities could irreparably disrupt them. Perhaps a thought experiment that removes these services from the familiar backdrop of the Earth is the best way to illustrate both the importance and complexity of ecosystem services, as well as how ill-equipped humans are to recreate them. Imagine, for example, human beings trying to colonize the moon. Assume for the sake of argument that the moon had already miraculously acquired some of the basic conditions for supporting human life, such as an atmosphere, a climate, and a physical soil structure similar to those on Earth. The big question facing human colonists would then be, which of Earths millions of species would need to be transported to the moon to make that sterile surface habitable? One could tackle that question systematically by first choosing from among all the species exploited directly for food, drink, spices, fiber, timber, pharmaceuticals, and industrial products such as waxes, rubber, and oils. Even if one were highly selective, the list could amount to hundreds or even thousands of species. And that would only be a start, since one would then need to consider which species are crucial to supporting those used directly: the bacteria, fungi, and invertebrates that help make soil fertile and break down wastes and organic matter; the insects, bats, and birds that pollinate flowers; and the grasses, herbs, and trees that hold soil in place, regulate the water cycle, and supply food for animals. The clear message of this exercise is that no one knows which combinations of species -- or even approximately how many -- are required to sustain human life. Rather than selecting species directly, one might try another approach: Listing the ecosystem services needed by a lunar colony and then guessing at the types and numbers of species required to perform each. Yet determining which species are critical to the functioning of a particular ecosystem service is no simple task. Let us take soil fertility as an example. Soil organisms are crucial to the chemical conversion and physical transfer of essential nutrients to higher plants. But the abundance of soil organisms is absolutely staggering. Under a square-yard of pasture in Denmark, for instance, the soil is inhabited by roughly 50,000 small earthworms and their relatives, 50,000 insects and mites, and nearly 12 million roundworms. And that tally is only the beginning. The number of soil animals is tiny compared to the number of soil microorganisms: a pinch of fertile soil may contain over 30,000 protozoa, 50,000 algae, 400,000 fungi, and billions of individual bacteria (Overgaard-Nielsen 1955; Rouatt and Katznelson 1961; Chanway 1993). Which must colonists bring to the moon to assure lush and continuing plant growth, soil renewal, waste disposal, and so on? Most of these soil-dwelling species have never been subjected to even cursory inspection: no human eye has ever blinked at them through
Issues in Ecology Number Spring 1997 hand hasever typed out aname erated by sport fishing activities.it raises the total to or description of the m,and most man inds hav citedin Postel and spent a mo treflecting on them Yet the sobering Carpenter).The future of these fisheries isinq fact is.as E..Wilson put it:they don't need us,but we tion,however,because fish harvests have approached or need them (Wilson 1987). exceeded sustainable levels virtually everywhere.Nine of the world's major marine fishing areas are in decline THE CHARACTER OF ECOSYSTEM SERVICES due to overfishing,pollution.and habitat destruction (UNFAO 1993:Kaufman and Dayton 1997). ottmt the moto Eth,e attentior he land, more closely at the service an impo rce of marketable goods the only planet we know that is habitable.Ecosystem mals used for labor (horses,mules,asses,camels,bul services and the systems that supply them are so inter- locks.etc.)and those whose parts or products are con connected that any classification of them is necessarily sumed (as meat,milk,wool,and leather).Grasslands rather arbitrary.here we briefly explore a suite of were also important as the original source habitat for overarching services that operate in ecosystems world most domestic animals such as poats.sheep.and wide. horses. as well as many crops such rley rye,oats,and other grass 1997八. Production of Ecosystem Goods a wide variety of terrestrial habitats,people hunt game Humanity obtains from natural ecosystems an animals such as watertowl.deer.moose.elk.fox.boar array of ecosystem goods-organisms and their parts and other wild pigs,rabbits,and even snakes and mon and products that grow in the wild and that are used keys.In many countries,game meat forms an important directly for human benefit.Many of these.such as fishe and s nly tra aded in Part ets and.many paces.is n econom mportant sport. markets. catch, example n used directly amounts to about 100 metric tons and is valued by humans as food,timber,fuelwood,fiber,pharmaceu at between $50 billion and $100 billion:it is the leading ticals and industrial products.Fruits,nuts,mushrooms mgiAeamh2e7 honey.other foods,and spices are extracted from many forest species.Wood and other plant materials are used mary source of protein(UNFAO 1993).The commercial in the construction of homes and other buildings.as well harvest of fre ter fish worldwide in 1990 totaled as for the re of furniture far nd was d at al (UNFA).Interestingly the vue of paper,cloth,thatch out cent of the world's energy consumption is supplied by the freshwater sport fishery in the U.S.alone greatly fuelwood and other plant material:in developing coun exceeds that of the global commercial harvest,with di- tries.such "biomass"supplies nearly 40 percent of en rect expenditures in 1991 totaling about $16 billion ergy consumption (Hall et al.1993).although the por When this is added to the value of the employment gen tion of this derived from natural rather than Figure 3-Alpaca grazing on the Chilean alti- plano Grassland ecos they re the of most domesticanima and many crops.such as wheat,barley,and oats
4 Issues in Ecology Number 2 Spring 1997 Figure 3-Alpaca grazing on the Chilean altiplano. Grassland ecosystems are an important source of animal products; they are also the original habitat of most domestic animals and many crops, such as wheat, barley, and oats. a microscope, no human hand has ever typed out a name or description of them, and most human minds have never spent a moment reflecting on them. Yet the sobering fact is, as E. O. Wilson put it: they dont need us, but we need them (Wilson 1987). THE CHARACTER OF ECOSYSTEM SERVICES Moving our attention from the moon back to Earth, let us look more closely at the services nature performs on the only planet we know that is habitable. Ecosystem services and the systems that supply them are so interconnected that any classification of them is necessarily rather arbitrary. Here we briefly explore a suite of overarching services that operate in ecosystems worldwide. Production of Ecosystem Goods Humanity obtains from natural ecosystems an array of ecosystem goods organisms and their parts and products that grow in the wild and that are used directly for human benefit. Many of these, such as fishes and animal products, are commonly traded in economic markets. The annual world fish catch, for example, amounts to about 100 million metric tons and is valued at between $50 billion and $100 billion; it is the leading source of animal protein, with over 20% of the population in Africa and Asia dependent on fish as their primary source of protein (UNFAO 1993). The commercial harvest of freshwater fish worldwide in 1990 totaled approximately 14 million tons and was valued at about $8.2 billion (UNFAO 1994). Interestingly, the value of the freshwater sport fishery in the U.S. alone greatly exceeds that of the global commercial harvest, with direct expenditures in 1991 totaling about $16 billion. When this is added to the value of the employment generated by sport fishing activities, it raises the total to $46 billion (Felder and Nickum 1992, cited in Postel and Carpenter 1997). The future of these fisheries is in question, however, because fish harvests have approached or exceeded sustainable levels virtually everywhere. Nine of the worlds major marine fishing areas are in decline due to overfishing, pollution, and habitat destruction. (UNFAO 1993; Kaufman and Dayton 1997). Turning our attention to the land, grasslands are an important source of marketable goods, including animals used for labor (horses, mules, asses, camels, bullocks, etc.) and those whose parts or products are consumed (as meat, milk, wool, and leather). Grasslands were also important as the original source habitat for most domestic animals such as cattle, goats, sheep, and horses, as well as many crops, such as wheat, barley, rye, oats, and other grasses (Sala and Paruelo 1997). In a wide variety of terrestrial habitats, people hunt game animals such as waterfowl, deer, moose, elk, fox, boar and other wild pigs, rabbits, and even snakes and monkeys. In many countries, game meat forms an important part of local diets and, in many places, hunting is an economically and culturally important sport. Natural ecosystems also produce vegetation used directly by humans as food, timber, fuelwood, fiber, pharmaceuticals and industrial products. Fruits, nuts, mushrooms, honey, other foods, and spices are extracted from many forest species. Wood and other plant materials are used in the construction of homes and other buildings, as well as for the manufacture of furniture, farming implements, paper, cloth, thatching, rope, and so on. About 15 percent of the worlds energy consumption is supplied by fuelwood and other plant material; in developing countries, such biomass supplies nearly 40 percent of energy consumption (Hall et al. 1993), although the portion of this derived from natural rather than Photo by Taylor Ricketts
Issues in Ecology Number 2 Spring 1997 human-dominated ecosystems is undocumented in addi. As described in the previous section biodiversity tion.natural products extracted from many hundreds of is a direct source of ecosystem goods.It also supplie ies ontribute diver inputs to industrv gums and the genetic and biochemical res that under ssential oils and rings.resins nd oleo curre ultral and pha utical resins,dyes,tannins,vegetable fats and w axes,insect may allow us to adapt these vital enterprises to globa cides,and multitudes of other compounds(Myers 1983: change.Our ability to increase crop productivity in the Leung and Foster 1996).The availability of most of face of new pests.diseases.and other stresses has de these natural products is in decline due to ongoing habi- pended heavily upon the transfer to our crops of gene tat conversion from wild cror relatives that confer resistance to thes challenge Such extractions fro nbiodiversity's Generation and Maintenance of Biodiversity library account for nnual t Biologicl diversity.or biodiversity for short.re fers to the variety of life forms at all levels of organiza 1992).Biotechnology now makes possible even greate tion,from the molecular to the land- use of this natural storehouse of ge. scape level.Biodiversity is generated netic diversity via the transfer to crops and maintained in natural ecosystems of genes from any kind of organisn where organisms counter a wide not simply crop relative and it variety of livi ing cond tions pro 6 play a majd role in fu events that shape their evolution in ture yield increase By the e turn unique ways.Out of convenience or the.farm-evelsae f th necessity.biodiversity is usually quan products of agricultural biotechnol tified in terms of numbers of species ogy,iust now entering the market and this perspective has greatly influ- ce,are expected to reach at least enced atio It is i rtant to ren however, 1991 cited (World Bank tal.1996 In addition to sustaining the produc to humanity are delivered througl tion of conventional crops,the populations of species residing in liv biodiversity in natural ecosystems may ing communities within specific physi include many potential new foods cal settings-in other words.through Human beings have utilized around comple syste 7 000 plant sn ies for food ov systems (Daily and) story and ar .70.000 beings to realize most of the nts are aesthetic,spiritual,and economic ben Wilson 1989).Only about 150 food efits of biodiversity,natural ecosys Figure 4-Harpoon-whaling in olants have ever been cultivated on a tems must therefore be accessible.the The o re a key sourc large scale,however.Currently.82 continued existence of coniferous tree of animal protein for the human popu plant species contribute 90 percen species somewhere in the world would lation. of national per-capita supplies of food not help the inhab itants of a tov ants ott-Allen undated by ng be ause of the clearing of a pin a much smalle er numbe forest upstream.Generally,the flow of ecosystem goods of these supply the bulk of the calories humans consume and services in a region is determined by the type,spa- Many other species,however,appear more nutritious or tial layout.extent.and proximity of the ecosystems sup better suited to the growing conditions that prevail in plying them.Because of this,the preservation of only important regions than the standard crops that dom one minimum viable population of each non-human spe nate world food supply today. Because of increasin and the world' eprtectedra wot iation ofirigate nds and the otential for d security it.Indeed,such a strategy.taken to extreme,would leac may come to depend on drought-and salt-tolerant vari to collapse of the biosphere,along with its life support eties that now play comparatively minor roles in agricul services. ture
As described in the previous section, biodiversity is a direct source of ecosystem goods. It also supplies the genetic and biochemical resources that underpin our current agricultural and pharmaceutical enterprises and may allow us to adapt these vital enterprises to global change. Our ability to increase crop productivity in the face of new pests, diseases, and other stresses has depended heavily upon the transfer to our crops of genes from wild crop relatives that confer resistance to these challenges. Such extractions from biodiversitys genetic library account for annual increases in crop productivity of about 1 percent, currently valued at $1 billion (NRC 1992). Biotechnology now makes possible even greater use of this natural storehouse of genetic diversity via the transfer to crops of genes from any kind of organism not simply crop relativesand it promises to play a major role in future yield increases. By the turn of the century, farm-level sales of the products of agricultural biotechnology, just now entering the marketplace, are expected to reach at least $10 billion per year (World Bank 1991, cited in Reid et al. 1996). In addition to sustaining the production of conventional crops, the biodiversity in natural ecosystems may include many potential new foods. Human beings have utilized around 7,000 plant species for food over the course of history and another 70,000 plants are known to have edible parts (Wilson 1989). Only about 150 food plants have ever been cultivated on a large scale, however. Currently, 82 plant species contribute 90 percent of national per-capita supplies of food plants (Prescott-Allen and Prescott-Allen 1990), although a much smaller number of these supply the bulk of the calories humans consume. Many other species, however, appear more nutritious or better suited to the growing conditions that prevail in important regions than the standard crops that dominate world food supply today. Because of increasing salinization of irrigated croplands and the potential for rapid climate change, for instance, future food security may come to depend on drought- and salt-tolerant varieties that now play comparatively minor roles in agriculture. 5 Issues in Ecology Number 2 Spring 1997 Photo by Taylor Ricketts Figure 4-Harpoon-whaling in Flores, Indonesia. The oceans are a key source of animal protein for the human population. human-dominated ecosystems is undocumented. In addition, natural products extracted from many hundreds of species contribute diverse inputs to industry: gums and exudates, essential oils and flavorings, resins and oleoresins, dyes, tannins, vegetable fats and waxes, insecticides, and multitudes of other compounds (Myers 1983; Leung and Foster 1996). The availability of most of these natural products is in decline due to ongoing habitat conversion. Generation and Maintenance of Biodiversity Biological diversity, or biodiversity for short, refers to the variety of life forms at all levels of organization, from the molecular to the landscape level. Biodiversity is generated and maintained in natural ecosystems, where organisms encounter a wide variety of living conditions and chance events that shape their evolution in unique ways. Out of convenience or necessity, biodiversity is usually quantified in terms of numbers of species, and this perspective has greatly influenced conservation goals. It is important to remember, however, that the benefits that biodiversity supplies to humanity are delivered through populations of species residing in living communities within specific physical settings in other words, through complex ecological systems, or ecosystems (Daily and Ehrlich 1995). For human beings to realize most of the aesthetic, spiritual, and economic benefits of biodiversity, natural ecosystems must therefore be accessible. The continued existence of coniferous tree species somewhere in the world would not help the inhabitants of a town inundated by flooding because of the clearing of a pine forest upstream. Generally, the flow of ecosystem goods and services in a region is determined by the type, spatial layout, extent, and proximity of the ecosystems supplying them. Because of this, the preservation of only one minimum viable population of each non-human species on Earth in zoos, botanical gardens, and the worlds legally protected areas would not sustain life as we know it. Indeed, such a strategy, taken to extreme, would lead to collapse of the biosphere, along with its life support services.
Issues in Ecology Number 2 Spring 1997 Tuming to medicinal res urces 20.000 years ago,for example,much of Europe an showed that top 150 th Am red by mi -thick ice the United States are on natura sources While the imate has been la vel stable since 74%on plants.I8%on fungi.5%on bacteria,and 3% the invention of agriculture around 10,000 years ago on one vertebrate (snake)species. Nine of the top ten periodic shifts in climate have affected human activities drugs in this list are based on natural plant products and settlement patterns.Even relatively recently,from (Grifo and Rosenthal,in press.as cited in Dobson 1995) 1550-1850.Europe was significantly cooler during a The commercial value of pharmaceuticals in the devel period known as the Little lceAge.Many of thesechar n per year( nate are thought to be aused by altera mately Earth's orbita the energy outpu t of the human population relies on traditional medical systems, sun,or even by events on the Earth itself sudden per and about 85%of traditional medicine involves the use turbations such as violent volcanic eruptions and aster of plant extracts(Farnsworth et al.1985). oid impacts or more gradual tectonic events such as the Saving only a single popula tion of each species could have an othe cost. Different pop gh all these the same species may pro change e diffe life year ent types or quantities of defensive (Schneider and Londer 1984). An chemicals that have potential use as life itself has played a role in this buff- pharmaceuticals or pesticides ering. (McCormick et al.1993):and they Climate.of course.plays a may exhibit different tolerances maior role in the evolution and distri onmental stresses such the pla et.Yet m salinity. scienti d agree hat life i se is a principal factor in the regulati therapeutic antibiotic took a full 15 of global climate.helping to offset the years after Alexander Fleming's fa- effects of episodic climate oscillations mous discovery of it in common bread by responding in ways that alter the mold.In part.this was because greenhouse gas concentrations in the ntistshad great ere for instance nat extracting. the sul systems may have helped to stabi stance in needed quantities One key climate and prevent overheating of the to obtaining such quantities was the Figure 5-Trapping and releasing butterflies Earth by removing more of the green discovery.after a worldwide search in a mixed-agriculture landscape in Costa house gas carbon dioxide from the of a ponulation of Fleming's mold that Rica.Monitorine the impact of humar atmosphere as the sun grew brighter produced more penicillin than the activities on biodiversity and ecosysten over millions of vears (Alexander et Similarly. services is needed worldwide:butterflie ,1997).ife also evert a de ary in thei may be useful indi cators for monitoring. or positive feedback to resist pests and diseas traits im reinforces climate change,particularly portant in agriculture.Many thousands of varieties of during transitions between interglacial periods and ice rice from different locations were screened to find one ages.One example:When climatic cooling leads to drops with resistance to grassy stunt virus,a disease that posed in sea level.continental shelves are exposed to wind and a serious threat to the world's rice crop(Myers 1983). rain.causing greater nutrient runoff to the oceans.thes Des pite these the vth of r anktor es of crops remain unpro form c ium carb nate shells tected and heavily threatened populations would remove more carbon dioxide e from the oceans and the atmosphere.a mechanism that should Climate and life further cool the planet.living things may also enhance Earth's climate has fluctuated tremendously since warming trends through such activities as speeding up humanity came into being.At the peak of the last ice microbial decomposition of dead organic matter.thus
6 Issues in Ecology Number 2 Spring 1997 Figure 5-Trapping and releasing butterflies in a mixed-agriculture landscape in Costa Rica. Monitoring the impact of human activities on biodiversity and ecosystem services is needed worldwide; butterflies may be useful indicators for monitoring. Photo by Paul R. Ehrlich Turning to medicinal resources, a recent survey showed that of the top 150 prescription drugs used in the United States, 118 are based on natural sources: 74% on plants, 18% on fungi, 5% on bacteria, and 3% on one vertebrate (snake) species. Nine of the top ten drugs in this list are based on natural plant products (Grifo and Rosenthal, in press, as cited in Dobson 1995). The commercial value of pharmaceuticals in the developed nations exceeds $40 billion per year (Principe 1989). Looking at the global picture, approximately 80% of the human population relies on traditional medical systems, and about 85% of traditional medicine involves the use of plant extracts (Farnsworth et al. 1985). Saving only a single population of each species could have another cost. Different populations of the same species may produce different types or quantities of defensive chemicals that have potential use as pharmaceuticals or pesticides (McCormick et al. 1993); and they may exhibit different tolerances to environmental stresses such as drought or soil salinity. For example, the development of penicillin as a therapeutic antibiotic took a full 15 years after Alexander Flemings famous discovery of it in common bread mold. In part, this was because scientists had great difficulty producing, extracting, and purifying the substance in needed quantities. One key to obtaining such quantities was the discovery, after a worldwide search, of a population of Flemings mold that produced more penicillin than the original (Dowling 1977). Similarly, plant populations vary in their ability to resist pests and disease, traits important in agriculture. Many thousands of varieties of rice from different locations were screened to find one with resistance to grassy stunt virus, a disease that posed a serious threat to the worlds rice crop (Myers 1983). Despite numerous examples like these, many of the localities that harbor wild relatives of crops remain unprotected and heavily threatened. Climate and Life Earths climate has fluctuated tremendously since humanity came into being. At the peak of the last ice age 20,000 years ago, for example, much of Europe and North America were covered by mile-thick ice sheets. While the global climate has been relatively stable since the invention of agriculture around 10,000 years ago, periodic shifts in climate have affected human activities and settlement patterns. Even relatively recently, from 1550-1850, Europe was significantly cooler during a period known as the Little Ice Age. Many of these changes in climate are thought to be caused by alterations in Earths orbital rotation or in the energy output of the sun, or even by events on the Earth itselfsudden perturbations such as violent volcanic eruptions and asteroid impacts or more gradual tectonic events such as the uplift of the Himalayas. Remarkably, climate has been buffered enough through all these changes to sustain life for at least 3.5 billion years (Schneider and Londer 1984). And life itself has played a role in this buffering. Climate, of course, plays a major role in the evolution and distribution of life over the planet. Yet most scientists would agree that life itself is a principal factor in the regulation of global climate, helping to offset the effects of episodic climate oscillations by responding in ways that alter the greenhouse gas concentrations in the atmosphere. For instance, natural ecosystems may have helped to stabilize climate and prevent overheating of the Earth by removing more of the greenhouse gas carbon dioxide from the atmosphere as the sun grew brighter over millions of years (Alexander et al. 1997). Life may also exert a destabilizing or positive feedback that reinforces climate change, particularly during transitions between interglacial periods and ice ages. One example: When climatic cooling leads to drops in sea level, continental shelves are exposed to wind and rain, causing greater nutrient runoff to the oceans. These nutrients may fertilize the growth of phytoplankton, many of which form calcium carbonate shells. Increasing their populations would remove more carbon dioxide from the oceans and the atmosphere, a mechanism that should further cool the planet. Living things may also enhance warming trends through such activities as speeding up microbial decomposition of dead organic matter, thus
Issues in Ecology Number 2 Spring 1997 soaked up by soils and gradually meted out to plant roots or into aquifers and surface streams.Thus,the soil itself slows the rush of water off the land in flash bare soil is soil from the full destructive force of raindropsand hod it in place.When landscapes are denuded.rain com pacts the surface and rapidly turns soil to mud (espe cially if it has been loosened by tillage):mud clogs sur face cavities in the soil,reduces infiltration of water.in cre ,and further enhances ed downslope Detache clog arried off b running water (Hille Erosion causes costs not only at the site where soil is lost but also in aquatic systems.natural and human-made,where the material accumulates.Local costs of erosion include losses of production potential diminished infiltration and water availability.and losses Figure 6-Bark of the Pacific vew tree (Taxus breuifolia) of nutrients. which is the source of the new anti-cancer drug,taxol Downstrear cos smay include or lower quality water supplies:si tion that impairs drain Willamette National Forest,Oregon age and maintenance of navigable river channels.har releasing carbon dioxide to the atmosphere (Schneider bors.and irrigation systems:increased frequency and and Boston 1991;Allegre and Schneider 1994). The severity of floods:and decreased potential for hydroelec relative influence of life's stabilizing and destabilizing feed- tric power as reservoirs fill with silt(Pimentel et al.1995). backs remains uncertain:what is clear is that climate Worldwide.the st of reservoir and natural e lost to siltati bility per year pacit system is an importan In addition to protecting soil from erosion,living ecosystems vegetation-with its deep roots and above-ground evapo rating surface-also serves as a giant pump,returning tems also exert direct physical influences that help to water from the ground into the atmosphere.Clearing of moderate regional and local weather.For instance,tran- plant cover disrupts this link in the water cycle and leads spiration(release of water vapor from the leaves)of plants e loss fron the regio nd the in surface temperature. In the Amazon. for example. 50%of the mean annual rainfall is recycled by the forest itself via evapotranspiration-that is.evaporation from wet leaves and soil combined with transpiration (Salati 1987).Amazon deforestation could so dramatically re- duce total pitation that the forest might be tself following complet dest ction (Shukla by forests,which provide shade and surface cooling and also act as insulators,blocking searing winds and trap- ping warmth by acting as a local greenhouse agent. Mitigation of Floods and Droughts An ount of water, about 119.000 Figure 7-Herbal pharmacist in Dai.Yunnan Province. surface-enough to cover the land to an average depth China.An estimated 80 percent of the world's popu- lation relies on natural medicinal products of I meter(Shiklomanov 1993).Much of this water is
7 Issues in Ecology Number 2 Spring 1997 Figure 6-Bark of the Pacific yew tree (Taxus brevifolia), which is the source of the new anti-cancer drug, taxol, Willamette National Forest, Oregon. Photo by Michael Graybill & Jan Hodder/ Biological Photo Service Figure 7-Herbal pharmacist in Dali, Yunnan Province, China. An estimated 80 percent of the worlds population relies on natural medicinal products. Photo by Catherine M. Pringle/Biological Photo Service releasing carbon dioxide to the atmosphere (Schneider and Boston 1991; Allegre and Schneider 1994). The relative influence of lifes stabilizing and destabilizing feedbacks remains uncertain; what is clear is that climate and natural ecosystems are tightly coupled, and the stability of that coupled system is an important ecosystemservice. Besides their impact on the atmosphere, ecosystems also exert direct physical influences that help to moderate regional and local weather. For instance, transpiration (release of water vapor from the leaves) of plants in the morning causes thunderstorms in the afternoon, limiting both moisture loss from the region and the rise in surface temperature. In the Amazon, for example, 50% of the mean annual rainfall is recycled by the forest itself via evapotranspirationthat is, evaporation from wet leaves and soil combined with transpiration (Salati 1987). Amazon deforestation could so dramatically reduce total precipitation that the forest might be unable to reestablish itself following complete destruction (Shukla et al. 1990). Temperature extremes are also moderated by forests, which provide shade and surface cooling and also act as insulators, blocking searing winds and trapping warmth by acting as a local greenhouse agent. Mitigation of Floods and Droughts An enormous amount of water, about 119,000 cubic kilometers, is rained annually onto the Earths land surfaceenough to cover the land to an average depth of 1 meter (Shiklomanov 1993). Much of this water is soaked up by soils and gradually meted out to plant roots or into aquifers and surface streams. Thus, the soil itself slows the rush of water off the land in flash floods. Yet bare soil is vulnerable. Plants and plant litter shield the soil from the full, destructive force of raindrops and hold it in place. When landscapes are denuded, rain compacts the surface and rapidly turns soil to mud (especially if it has been loosened by tillage); mud clogs surface cavities in the soil, reduces infiltration of water, increases runoff, and further enhances clogging. Detached soil particles are splashed downslope and carried off by running water (Hillel 1991). Erosion causes costs not only at the site where soil is lost but also in aquatic systems, natural and human-made, where the material accumulates. Local costs of erosion include losses of production potential, diminished infiltration and water availability, and losses of nutrients. Downstream costs may include disrupted or lower quality water supplies; siltation that impairs drainage and maintenance of navigable river channels, harbors, and irrigation systems; increased frequency and severity of floods; and decreased potential for hydroelectric power as reservoirs fill with silt (Pimentel et al. 1995). Worldwide, the replacement cost of reservoir capacity lost to siltation is estimated at $6 billion per year. In addition to protecting soil from erosion, living vegetationwith its deep roots and above-ground evaporating surfacealso serves as a giant pump, returning water from the ground into the atmosphere. Clearing of plant cover disrupts this link in the water cycle and leads
Issues in Ecology Number 2 Spring 1997 These subalpine forests mitigate flood,drought, and temperature extremes: they soak up rain and snowmelt and mete it out ing cooling afternoon thunderstorms. to potentially large increases in surface runoff,along with Services Supplied by Soil nutrient and soil loss.A classic example comes from the Soil represents an important component of a experimental clearing of a New Hampshire forest,where nation's assets.one that takes hundreds to hundreds of herbicide was applied to prevent regrowth for a 3-year The result was a 40 per thousands of years to build up and yet very few years to be lost Some civilizations have draw a in a verage stre am flow During one four from fertile sol conve the h as more e much larger scale,extensive deforestation in the Hima Today.soil degradation induced by human activities af- layan highlands appears to have exacerbated recent flood- flicts nearly 20 percent of the Earth's vegetated land ing in Bangladesh,although the relative roles of human surface (Oldeman et al.1990). and natural forces remain debatable(lves and Messerli In addition to moderating the water cycle.as described 1989) In addition som of the world,such a above pro vides five othe rinte ser vices (Daily re expenen g an increased f equency eds and prov s phys cal support as they sprout and mature into adult plants sive deforestation. The cost of packaging and storing seeds and of anchor Wetlands are particularly well-known for their role in flood control and can often reduce the need to con- hoenm struct flood control structures.Floodplain forests and for ple.slow the of flood dto help as sess the u of this se ice The cost uppor nd stan nds us sed in such oper plain rather than washed i nto downstream bays or oceans tions total about USs55.000 per hectare (for the Nutri n addition,isolated wetlands such as prairie potholes in ent Film Technique Systems:FAO 1990). the Midwest and cypress ponds in the Southeast,serve Second.soil retains and delivers nutrients to as detention areas during times of high rainfall.delaying plants.Tiny soil particles(less than 2 microns in diam saturation of upland soils and overland flows into eter).which are p cha ge e that is ing vegeta and proper spositively charged ents ral water regimes intact can reduce the severity and du as calcium and magnesium-near the surface.in prox ration of flooding along rivers(Ewel 1997).A relatively imity to plant roots,allowing them to be taken up gradu small area of retained wetland.,for example.could have ally.Otherwise.these nutrients would quickly be leached largely prevented the severe flooding along the Missis- away.soil also acts as a buffer in the application of sippi River in 1993. fertilizers,holding onto the fertilizer ions until they are 8
Services Supplied by Soil Soil represents an important component of a nations assets, one that takes hundreds to hundreds of thousands of years to build up and yet very few years to be lost. Some civilizations have drawn great strength from fertile soil; conversely, the loss of productivity through mismanagement is thought to have ushered many once flourishing societies to their ruin (Adams 1981). Today, soil degradation induced by human activities afflicts nearly 20 percent of the Earths vegetated land surface (Oldeman et al. 1990). In addition to moderating the water cycle, as described above, soil provides five other interrelated services (Daily et al. 1997). First, soil shelters seeds and provides physical support as they sprout and mature into adult plants. The cost of packaging and storing seeds and of anchoring plant roots would be enormous without soil. Human-engineered hydroponic systems can grow plants in the absence of soil, and their cost provides a lower bound to help assess the value of this service. The costs of physical support trays and stands used in such operations total about US$55,000 per hectare (for the Nutrient Film Technique Systems; FAO 1990). Second, soil retains and delivers nutrients to plants. Tiny soil particles (less than 2 microns in diameter), which are primarily bits of humus and clays, carry a surface electrical charge that is generally negative. This property holds positively charged nutrientscations such as calcium and magnesiumnear the surface, in proximity to plant roots, allowing them to be taken up gradually. Otherwise, these nutrients would quickly be leached away. Soil also acts as a buffer in the application of fertilizers, holding onto the fertilizer ions until they are 8 Issues in Ecology Number 2 Spring 1997 Figure 8-Early summer in the Colorado Rockies. These subalpine forests mitigate flood, drought, and temperature extremes; they soak up rain and snowmelt and mete it out gradually to streams and to the atmosphere, creating cooling afternoon thunderstorms. Photo by Gretchen C. Daily to potentially large increases in surface runoff, along with nutrient and soil loss. A classic example comes from the experimental clearing of a New Hampshire forest, where herbicide was applied to prevent regrowth for a 3-year period after the clearing. The result was a 40 percent increase in average stream flow. During one four-month period of the experiment, runoff was more than 5 times greater than before the clearing (Bormann 1968). On a much larger scale, extensive deforestation in the Himalayan highlands appears to have exacerbated recent flooding in Bangladesh, although the relative roles of human and natural forces remain debatable (Ives and Messerli 1989). In addition, some regions of the world, such as parts of Africa, are experiencing an increased frequency and severity of drought, possibly associated with extensive deforestation. Wetlands are particularly well-known for their role in flood control and can often reduce the need to construct flood control structures. Floodplain forests and high salt marshes, for example, slow the flow of floodwaters and allow sediments to be deposited within the floodplain rather than washed into downstream bays or oceans. In addition, isolated wetlands such as prairie potholes in the Midwest and cypress ponds in the Southeast, serve as detention areas during times of high rainfall, delaying saturation of upland soils and overland flows into rivers and thereby damping peak flows. Retaining the integrity of these wetlands by leaving vegetation, soils, and natural water regimes intact can reduce the severity and duration of flooding along rivers (Ewel 1997). A relatively small area of retained wetland, for example, could have largely prevented the severe flooding along the Mississippi River in 1993.
Issues in Ecology Number 2 Spring 1997 required by plants.Hydroponic systems supply water recycling of nutrients-the fourth service soils provide- and nutrients to plants without need of soil,but the mar are two aspects of the same process.The fertility of error is much smaller of -that is.their ability to su ply nutrients to plant is largely the u of the activities of diverse spec of bacteria.fungl.algae,crustacea.mites,termites.spring ent concentrations,pH.and salinity of the nutrient solu tails,millipedes,and worms,all of which,as groups,play tion in hydroponic systems,as well as the air and solu- important roles.Some bacteria are responsible for "fix tion temperature,humidity,light,pests,and plant dis. ing"nitrogen,a key element in proteins,by drawing it eases.Worldwide,the area under hydroponic culture is out of the atmosphere and converting it to forms usable only a few thousand hecta and is unlikely to plants and. atol hu be ngs and other the Certain type of fungi play e about roles in supplying nutrients to many kinds of trees. 1993) worms and ants act as “mechani Third.soil plays a central cal blenders.breaking up and mix role in the decomposition of dead ing plant and microbial material and organic matter and wastes,and this other matter (enny 1980).For example,as much as 10 metric harmles y potential humar onne erial may pass pathogens.People generate a tre through the bo mendous amount of waste,includ on a hectare of land each year.re ing household garbage,industrial sulting in nutrient rich“casts”thal waste,crop and forestry residues enhance soil stability,aeration,and and sewage from their own popula- drainage (Lee 1985) tions and their billions st cated a key fac h the Earth's tion of the amount of dead organi lement cycle of carbon matter and waste (mostly agricul- nitrogen,and sulfur.The amount tural residues)processed each year of carbon and nitrogen stored in is 130 billion metric tons,about 30 soils dwarfs that in vegetation,for percent of which is associated with example.Carbon in soils is nearly human activities (derived fron double(1.8 times)that in plan Vitousek et al.6).Fort ure 9-Bacteria (Bradurhizobium jap and nit abou there is a wide array of decompos d3.550 times greater (Schles ing organisms-ranging from vul 1991).Alterations in the carbor tures to tiny bacteria- -that extract gen into a form that can be utilized by plants. and nitrogen cycles may be costly energy from the large,complex organic molecules found over the long term.and in many cases.irreversible on a in many types of waste.Like assembly-line workers,di- time scale of interest to society.Increased fluxes of car verse microbial species process the particular compounds bon to the atmosphere, such as occur when land is cor whose che mical bonds the ong to ed to a tonMany ndustrl wastes incun or when wet othe are dra co buildup of ke y greenhous gase namely carbon dioxide and methane,in the atmosphere pesticides.oil.acids.and paper.are detoxified and de (Schlesinger 1991).Changes in nitrogen fluxes caused composed by oroanisms in natural ecosystems if the con by production and use of fertilizer,burning of wood and centration of waste does not exceed the system's capac other biomass fuels and clearing of tronical land lead to ity to transform it.Some modern wastes.ho increasing atmospheric concentrations of nitrous oxide such as stics and the her nhouse ga that is also produc ctof the pesticide shield.Thes The simple inorganic chemicals that result from and other changes in the nitrogen cycle also result in natural decomposition are eventually returned to plants acid rain and excess nutrient inputs to freshwater sys. as nutrients.Thus,the decomposition of wastes and the tems,estuaries,and coastal marine waters.This nutrient
9 Issues in Ecology Number 2 Spring 1997 Figure 9-Bacteria (Bradyrhizobium japonicum) in a soybean root nodule cell, magnified 3,550 times. These bacteria fix atmospheric nitrogen into a form that can be utilized by plants.Photo by L. Evans Roth/Biological Photo Service required by plants. Hydroponic systems supply water and nutrients to plants without need of soil, but the margin for error is much smallereven small excesses of nutrients applied hydroponically can be lethal to plants. Indeed, it is a complex undertaking to regulate the nutrient concentrations, pH, and salinity of the nutrient solution in hydroponic systems, as well as the air and solution temperature, humidity, light, pests, and plant diseases. Worldwide, the area under hydroponic culture is only a few thousand hectares and is unlikely to grow significantly in the foreseeable future; by contrast, global cropped area is about 1.4 billion hectares (USDA 1993). Third, soil plays a central role in the decomposition of dead organic matter and wastes, and this decomposition process also renders harmless many potential human pathogens. People generate a tremendous amount of waste, including household garbage, industrial waste, crop and forestry residues, and sewage from their own populations and their billions of domesticated animals. A rough approximation of the amount of dead organic matter and waste (mostly agricultural residues) processed each year is 130 billion metric tons, about 30 percent of which is associated with human activities (derived from Vitousek et al. 1986). Fortunately, there is a wide array of decomposing organismsranging from vultures to tiny bacteriathat extract energy from the large, complex organic molecules found in many types of waste. Like assembly-line workers, diverse microbial species process the particular compounds whose chemical bonds they can cleave and pass along to other species the end products of their specialized reactions. Many industrial wastes, including soaps, detergents, pesticides, oil, acids, and paper, are detoxified and decomposed by organisms in natural ecosystems if the concentration of waste does not exceed the systems capacity to transform it. Some modern wastes, however, are virtually indestructible, such as some plastics and the breakdown products of the pesticide DDT. The simple inorganic chemicals that result from natural decomposition are eventually returned to plants as nutrients. Thus, the decomposition of wastes and the recycling of nutrientsthe fourth service soils provide are two aspects of the same process. The fertility of soilsthat is, their ability to supply nutrients to plants is largely the result of the activities of diverse species of bacteria, fungi, algae, crustacea, mites, termites, springtails, millipedes, and worms, all of which, as groups, play important roles. Some bacteria are responsible for fixing nitrogen, a key element in proteins, by drawing it out of the atmosphere and converting it to forms usable by plants and, ultimately, human beings and other animals. Certain types of fungi play extremely important roles in supplying nutrients to many kinds of trees. Earthworms and ants act as mechanical blenders, breaking up and mixing plant and microbial material and other matter (Jenny 1980). For example, as much as 10 metric tonnes of material may pass through the bodies of earthworms on a hectare of land each year, resulting in nutrient rich casts that enhance soil stability, aeration, and drainage (Lee 1985). Finally, soils are a key factor in regulating the Earths major element cyclesthose of carbon, nitrogen, and sulfur. The amount of carbon and nitrogen stored in soils dwarfs that in vegetation, for example. Carbon in soils is nearly double (1.8 times) that in plant matter, and nitrogen in soils is about 18 times greater (Schlesinger 1991). Alterations in the carbon and nitrogen cycles may be costly over the long term, and in many cases, irreversible on a time scale of interest to society. Increased fluxes of carbon to the atmosphere, such as occur when land is converted to agriculture or when wetlands are drained, contribute to the buildup of key greenhouse gases, namely carbon dioxide and methane, in the atmosphere (Schlesinger 1991). Changes in nitrogen fluxes caused by production and use of fertilizer, burning of wood and other biomass fuels, and clearing of tropical land lead to increasing atmospheric concentrations of nitrous oxide, another potent greenhouse gas that is also involved in the destruction of the stratospheric ozone shield. These and other changes in the nitrogen cycle also result in acid rain and excess nutrient inputs to freshwater systems, estuaries, and coastal marine waters. This nutrient
