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### About Issues in Ecology

Issues in Ecology is designed to report, in language understandable by non-scientists, the consensus of a panel of scientific experts on issues relevant to the environment. Issues in Ecology is supported by the Pew Scholars in Conservation Biology program and by the Ecological Society of America. It is published at irregular intervals, as reports are completed. All reports undergo peer review and must be approved by the Editorial Board before publication.

Issues in Ecology is an official publication of the Ecological Society of America, the nations leading professional society of ecologists. Founded in 1915, ESA seeks to promote the responsible application of ecological principles to the solution of environmental problems.For more information, contact the Ecological Society of America, 2010 Massachusetts Avenue, NW, Suite 400, Washington, DC, 20036. ISSN 1092-8987Published by the Ecological Society of America Number 2, Spring 1997 

# Ecosystem Services: Benefits Supplied to Human Societies by Natural EcosystemsIssues in EcologyIssues in Ecology Number 2 Spring 1997

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## 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.1 2 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 forgranted, 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 thatthey 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 widerange 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 Service3 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 focuson 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, humanitywill 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 irreparablydisrupt 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 hadalready 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 needto 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 considerwhich species are crucial to supporting those used directly: the bacteria, fungi, and invertebrates that help make soil fertile and break down wastes and organicmatter; the insects, bats, and birds that pollinate flowers; and the grasses, herbs, and trees that hold soil inplace, regulate the water cycle, and supply food for animals. The clear message of this exercise is that no oneknows 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 transferof 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 nearly12 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-dwellingspecies have never been subjected to even cursory inspection: no human eye has ever blinked at them through4 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. Ecosystemservices and the systems that supply them are so interconnected that any classification of them is necessarily rather arbitrary. Here we briefly explore a suite ofoverarching 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 leadingsource 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 greatlyexceeds 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 andCarpenter 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 arean 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, boarand 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 directlyby 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 byfuelwood 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 thanPhoto by Taylor RickettsAs 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 theface 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 geneticlibrary account for annual increases in crop productivity of about 1 percent, currently valued at $1 billion (NRC 1992). Biotechnology now makes possible even greateruse of this natural storehouse of genetic diversity via the transfer to crops of genes from any kind of organism not simply crop relativesand itpromises to play a major role in future yield increases. By the turn of the century, farm-level sales of theproducts 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 inimportant 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 securitymay come to depend on drought- and salt-tolerant varieties that now play comparatively minor roles in agriculture.

5 Issues in Ecology Number 2 Spring 1997Photo by Taylor Ricketts

Figure 4 -Harpoon-whaling in Flores, Indonesia. The oceans are a key sourceof 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 andexudates, 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 ofthese natural products is in decline due to ongoing habitat conversion.

Generation and Maintenance of BiodiversityBiological 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 ornecessity, 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 throughpopulations of species residing in living communities within specific physical settings in other words, throughcomplex ecological systems, or ecosystems (Daily and Ehrlich 1995). For human beings to realize most of theaesthetic, spiritual, and economic benefits of biodiversity, natural ecosystems must therefore be accessible. The

continued existence of coniferous tree species somewhere in the world wouldnot help the inhabitants of a town inundated by flooding because of the clearing of a pine forest upstream. Generally, the flow of ecosystem goodsand 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 onlyone 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.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 ofthe 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 15years after Alexander Flemings famous discovery of it in common breadmold. 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 abilityto 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 thesun, 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). Andlife itself has played a role in this buffering. Climate, of course, plays amajor 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 theatmosphere. For instance, natural ecosystems may have helped to stabilize climate and prevent overheating of theEarth by removing more of the greenhouse gas carbon dioxide from the atmosphere as the sun grew brighter over millions of years (Alexander etal. 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, thus7 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). Therelative influence of lifes stabilizing and destabilizing feedbacks remains uncertain; what is clear is that climateand 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 tomoderate 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 (Salati1987). 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 andalso 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 holdit 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 disruptedor lower quality water supplies; siltation that impairs drainage and maintenance of navigable river channels, harbors, and irrigation systems; increased frequency andseverity 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, livingvegetationwith 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 leadsServices 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 (Dailyet 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 costsof 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 toplants. 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 suchas 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 andto 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 amuch 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 frequencyand 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 andhigh 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 integrityof 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 havelargely prevented the severe flooding along the Mississippi River in 1993.9 Issues in Ecology Number 2 Spring 1997

Figure 9 -Bacteria ( Bradyrhizobium japonicum ) in a soybean root nodule cell, magnified 3,550times. 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 waterand 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 growsignificantly 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 humanpathogens. 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 organicmatter 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 foundin many types of waste. Like assembly-line workers, diverse microbial species process the particular compounds whose chemical bonds they can cleave and pass along toother 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 ofbacteria, fungi, algae, crustacea, mites, termites, springtails, millipedes, and worms, all of which, as groups, playimportant 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 usableby plants and, ultimately, human beings and other animals. Certain types of fungi play extremely importantroles 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 earthwormson 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 atime 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 inacid rain and excess nutrient inputs to freshwater systems, estuaries, and coastal marine waters. This nutrient10 Figure 10 -Sonoran bumble bee ( Bombus sonorus ) pollinating a flower.Photo by Peter J. Bryant/Biological Photo Service Issues in Ecology Number 2 Spring 1997 influx causes eutrophication of aquatic ecosystems and contamination of drinking water sourcesboth surface and ground water by high levels of nitrate-nitrogen (Vitousek et al. 1997).

Pollination Animal pollination is required for the successful reproduction of most flowering plants. About 220,000 out of an estimated 240,000 species of plants for which the mode of pollination has been recorded require an animal such as a bee or hummingbird to accomplish this vital task. This includes both wild plants and about 70 percent of the agricultural crop species that feed theworld. Over 100,000 different animal speciesincluding bats, bees, beetles, birds, butterflies, and fliesareknown to provide these free pollination services that assure the perpetuation of plants in our croplands, backyard gardens, rangelands,

meadows and forests. In
turn, the continued availability of these pollinators depends on the existence of a wide variety of habitat typesneeded for their feeding, successful breeding, and completion of their life cycles (Nabhan and Buchmann 1997).

One third of human food is derived from plants pollinated by wild pollinators. Without natural pollinationservices, yields of important crops would decline precipitously and many wild plant species would become extinct. In the United States alone, the agricultural value of wild, native pollinatorsthose sustained by naturalhabitats adjacent to farmlands is estimated in the billions of dollars per year. Pollination by honey bees, originally imported from Europe, is extremely important as well, but these bees are presently in decline, enhancing the importance of pollinators from natural ecosystems.Management of the honey bee in the New World is currently threatened by the movement of, and hybridization with, an aggressive African strain of honey bee that was accidentally released in Brazil in 1956. Diseases of honey bee colonies are also causing a marked decline in the number of managed colonies. Meanwhile, the diversityof natural pollinators available to both wild and domesticated plants is diminishing: more than 60 genera of pollinators include species now considered to be threatened, endangered or extinct (Buchmann and Nabhan 1996).

Natural Pest Control Services Humanitys competitors for food, timber, cotton,and other fibers are called pests, and they include numerous herbivorous insects, rodents, fungi, snails, nematodes, and viruses. These pests destroy an estimated 25 to 50 percent of the worlds crops, either before or after harvest (Pimentel et al. 1989). In addition, numerous weeds compete directly with crops for water, light, and soil nutrients, further limiting yields. Chemical pesticides, and the strategies by which they are applied to fight crop pests, can have harmfulunintended consequences. First, pests can develop resistance, which means that higher and higher doses of pesticides must be applied or new chemicals developed periodically to achieve the same level of control. Resistance

is now found in more than 500 insect and mite pests, over 100 weeds, and in about 150 plant pathogens(WRI 1994). Second, populations of the natural enemies of pests are decimated by heavy pesticide use. Natural predators are often more susceptible to synthetic poisons than are the pests because they have not hadthe same evolutionary experience with overcoming plant chemicals that the peststhemselves have had. And natural predators also typically have much smaller population sizes than those of their prey. Destruction of predator populations leads to explosions in prey numbers, not only freeing target pests from natural controls but often promoting other non-pest species to pest status. In California in the1970s, for instance, 24 of the 25 most important agricultural pests had been elevated to that status by the overuse of pesticides (NRC 1989). Third, exposure to pesticides and herbicides may pose serious health risksto humans and many other types of organisms; the recently discovered declines in human sperm counts may be attributable in part to such exposure (Colborn et al. 1996). Fortunately, an estimated 99 percent of potential crop pests are controlled by natural enemies, including many birds, spiders, parasitic wasps and flies, lady bugs, fungi, viral diseases, and numerous other types ofactivities such as gardeningand pet-keeping, nature photography and film-making, bird feeding and watching, hiking and camping,ecotouring and mountaineering, river-rafting and boating, fishing and hunting, andin a wide range of other activities. For many, nature is an unparalleled source of wonderment and inspiration, peace and beauty, fulfillment and rejuvenation (e.g., Kellert and Wilson).

THREATS TO ECOSYSTEM SERVICES

Ecosystem services are being impaired and destroyed by a wide variety of human activities. Foremostamong the immediate threats are the continuing destruction of natural habitats and the invasion of non-nativespecies that often accompanies such disruption; in marine systems, overfishing is a major threat. The most irreversible of human impacts on ecosystems is the loss of native biodiversity. A conservative estimate of therate of species loss is about one per hour, which unfortunately exceeds the rate of evolution of new species by a factor of 10,000 or more (Wilson 1989; Lawton and May 1995). But complete extinction of species is onlythe final act in the process. The rate of loss of local populations of speciesthe populations that generate ecosystem services in specific localities and regionsis orders of magnitude higher (Daily and Ehrlich 1995; Hughes et al., in prep.). Destroying other life forms also disrupts the web of interactions that could help us discover thepotential usefulness of specific plants and animals (Thompson 1994). Once a pollinator or a predacious insect
is on the brink of extinction, for instance, it would be difficult to discover its potential utility to farmers.
Other imminent threats include the alteration of the Earths carbon, nitrogen, and other biogeochemical cycles through the burning of fossil fuels and heavy use of nitrogen fertilizer; degradation of farmland throughunsustainable agricultural practices; squandering of freshwater resources; toxification of land and waterways; and overharvesting of fisheries, managed forests, and other theoretically renewable systems. These threats to ecosystem services are driven ultimately by two broad underlying forces. One is rapid,unsustainable growth in the scale of the human enter11 Issues in Ecology Number 2 Spring 1997

Figure 11 -Ladybug larva ( Cycloneda polita ) eating an aphid.Photo by Peter J. Bryant/Biological Photo Service organisms (DeBach 1974). These natural biological control agents save farmers billions of dollars annuallyby protecting crops and reducing the need for chemical control (Naylor and Ehrlich 1997).

Seed DispersalOnce a seed germinates, the resulting plant is usually rooted in place for the rest of its life. For plants, then, movement to new sites beyond the shadow of the parent is usually achieved through seed dispersal. Many seeds, such as those of the dandelion, are dispersed by wind. Some are dispersed by water, the most famous being the seafaring coconut. Many other seeds have evolved ways of getting around by using animals as their dispersal agents. These seeds may be packaged in sweet fruit to reward an animal for its dispersal services; some of these seeds even require passage through the gut of a bird or mammal before they can germinate. Others require burialby, say, a forgetful jay or a squirrel which later leaves its cache uneatenfor eventual germination. Still others are equipped with sticky or sharp, spiny surfaces designed to catch onto a passing animal and go for a long ride before dropping or being rubbed off. Without thousands of animal species acting as seed dispersers, many plants would fail to reproduce successfully. For instance, the whitebark pine (Pinus albicaulis), a tree found in the
Rockies and Sierra Nevada - Cascade Mountains, cannotreproduce successfully without a bird called Clarks Nutcracker (Nucifraga columbiana), which chisels pine seeds out of the tightly closed cones and disperses and buries them; without this service, the cones do not open far enough to let the seeds fall out on their own. Animal seed dispersers play a central role in the structure andregeneration of many pine forests (Lanner 1996). Disruption of these complex services may leave large areas of forest devoid of seedlings and younger age classes of trees, and thus unable to recover swiftly from human impacts such as land clearing.

Aesthetic Beauty and Intellectual and Spiritual Stimulation Many human beings have a deep appreciation ofnatural ecosystems. That is apparent in the art, religions, and traditions of diverse cultures, as well as in12 Issues in Ecology Number 2 Spring 1997

Figure 12 -The goods and services supplied by this badly deforested and eroded region of Madagascar are all but gone and would be difficult to restore.Photo by Gretchen C. Daily prise: in population size, in per-capita consumption, and also in the environmental impacts that technologies and institutions generate as they produce and supply those consumables (Ehrlich et al. 1977). The other underlyingdriver is the frequent mismatch between short-term, individual economic incentives and long-term, societalwell-being. Ecosystem services are generally greatly undervalued, for a number of reasons: many are not traded or valued in the marketplace; many serve the public goodrather than provide direct benefits to individual landowners; private property owners often have no way to benefit financially from the ecosystem services supplied to society by their land; and, in fact, economic subsidies often encourage the conversion of such lands to other, market-valued activities. Thus, people whose activities disrupt ecosystem services often do not pay directly for the cost of those lost services. Moreover, society oftendoes not compensate landowners and others who do safeguard ecosystem services for the economic benefits they lose by foregoing more lucrative but destructive land uses. There is a critical need for policy measures that address these driving forces and embed the value of ecosystem services into decision making frameworks.

VALUATION OF ECOSYSTEM SERVICES

Human society would cease to exist in the absence of ecosystem services. Thus, their immense value to humanity is unquestionable. Yet quantifying the valueof ecosystem services in specific localities, and measuring their worth against that of competing land uses is nosimple task. When tradeoffs must be made in the allocation of land and other resources to competing human activities, the resolution often requires a measure of what is known as the marginal value. In the case of ecosystem services, for example, the question that might be posed would be: By how much would the flow of ecosystemservices be augmented (or diminished) with the preservation (or destruction) of the next hectare of forest or wetland? Estimation of marginal values is complex (e.g.,Bawa and Gadgil 1997; Daily 1997b). Often a qualitative comparison of relative values is sufficient that is, which is greater, the economic benefits of a particulardevelopment project or the benefits supplied by the ecosystem that would be destroyed, measured over a time period of interest to people concerned about the well-being of their grandchildren? There are, and will remain, many cases in which ecosystem service values are highly uncertain. Yet thepace of destruction of natural ecosystems, and the irreversibility of most such destruction on a time scale ofinterest to humanity, warrants substantial caution. Valuing a natural ecosystem, like valuing a human life, isfraught with difficulties. Just as societies have recognized fundamental human rights, however, it may be prudent to establish fundamental ecosystem protections even though uncertainty over economic values remains. New institutions and agreements at the international andsubnational level will be needed to encourage fair participation in such protections (see, e.g., Heal 1994).The tremendous expense and difficulty of replicating lost ecosystem services is perhaps best illustrated by the results of the first Biosphere 2 mission, in which eight people lived inside a 3.15-acre closed ecosystem for two years. The system featured agricultural land and replicas of several natural ecosystems such as forests and even a miniature ocean. In spite of an investment of more than $200 million in the design, construction, and13 Issues in Ecology Number 2 Spring 1997 operation of this model earth, it proved impossible to supply the material and physical needs of the eightBiospherians for the intended 2 years. Many unpleasant and unexpected problems arose, including a drop in atmospheric oxygen concentration to 14% (the level normally found at an elevation of 17,500 feet),high spikes in carbon dioxide concentrations, nitrous oxide concentrations high enough to impair the brain, an extremely high level of extinctions (including 19 of 25 vertebrate species and all pollinators brought into the
enclosure, which would have

ensured the eventual extinction of most of the plant species as well), overgrowthof aggressive vines and algal mats, and population explosions of crazy ants, cockroaches, and katydids. Even heroic personal efforts on the part of the Biospherians did not suffice to make the system viable and sustainable for either humans or many nonhuman species (Cohen and Tilman 1996).

MAJOR UNCERTAINTIES

Society would clearly profit by further investigation into some of the following broad research questions so that we might avoid on Biosphere 1, the earth, unpleasant surprises like those that plagued the Biosphere 2 project (Holdren 1991; Cohen and Tilman 1996; Daily 1997b):

• What is the relative impact of various human activities upon the supply of ecosystem services?

• What is the relationship between the condition of an ecosystemthat is, relatively pristine or heavilymodifiedand the quantity and quality of ecosystem services it supplies?

• To what extent do ecosystem services depend upon biodiversity at all levels, from genes to species to landscapes? • To what extent have various ecosystem services already been impaired? And how are impairment and riskof future impairment distributed in various regions of the globe?

• How interdependent aredifferent ecosystem services? How does exploiting or damaging one influence the functioning of others?

• To what extent, and overwhat time scale, are ecosystem services amenable to repair or restoration?

• How effectively, and athow large a scale, can existing or foreseeable human technologies substitute for ecosystem services? What

would be the side effects of

such substitutions?

• Given the current state of technology and the scale of the human enterprise, what proportion and spatial pattern of land must remainrelatively undisturbed, locally, regionally, and globally, to sustain the delivery of essential ecosystem services?

CONCLUSIONS

The human economy depends upon the services performedfor free by ecosystems. The ecosystem services supplied annually are worth many trillions of dollars. Economic development that destroys habitats and impairs services can create costs to humanity over the long termthat may greatly exceed the short-term economic benefits of the development. These costs are generally hidden from traditional economic accounting, but are nonetheless real and are usually borne by society at large. Tragically, a short-term focus in land-use decisions often sets in motion potentially great costs to be borne by future generations. This suggests a need for policies that achieve a balance between sustaining ecosystem services and pursuing the worthy short-term goals of economic development. Figure 13 -The production of this meal benefited from manyecosystem services, including natural pest control, pollination, maintenance of soil fertility, purification of water, and

moderation of climate.14 Issues in Ecology Number 2 Spring 1997

ACKNOWLEDGMENTS

We thank the Packard Foundation and the Pew Foundation for financial support.

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About the Panel of Scientists This report presents the consensus reached by a panel of 11 scientists chosen to include a broad array of expertise in this area. This report underwent peer review and was approved by the Board of Editors of Issues in Ecology . The affiliations of the members of the panel of

scientists are:environment. All reports undergo peer review and must be approved by the editorial board before publication.

Editorial Board of Issues in EcologyDr. David Tilman, Editor-in-Chief, Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN 55108-6097. E-mail: tilman@lter.umn.edu

Board members Dr. Stephen Carpenter, Center for Limnology, University
of Wisconsin, Madison, WI 53706 Dr. Deborah Jensen, The Nature Conservancy, 1815 North Lynn Street, Arlington, VA 22209 Dr. Simon Levin, Department of Ecology & Evolutionary Biology, Princeton University, Princeton, NJ

08544-1003 Dr. Jane Lubchenco, Department of Zoology, Oregon State University, Corvallis, OR 97331-2914 Dr. Judy L. Meyer, Institute of Ecology, The University of Georgia, Athens, GA 30602-2202 Dr. Gordon Orians, Department of Zoology, University of Washington, Seattle, WA 98195 Dr. Lou Pitelka, Appalachian Environmental Laboratory, Gunter Hall, Frostburg, MD 21532 Dr. William Schlesinger, Departments of Botany and Geology, Duke University, Durham, NC 27708-

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Cover photo credits, clockwise from top left:
Nadine Cavender, Nadine Cavender, Nadine Cavender,
Claude Cavender, Jr., Nadine Cavender, unknown, Claude
Cavender, Jr., and unknown. Issues in Ecology Number 2 Spring 1997

16Dr. Gretchen C. Daily, Panel Chair, Department of Biological Sciences, Stanford University, Stanford, CA 94305

Dr. Susan Alexander, Earth Systems Science and Policy, California State University, Monterey Bay, 100 Campus
Center, Seaside, CA 93955

Dr. Paul R. Ehrlich, Department of Biological Sciences, Stanford University, Stanford, CA 94305

Dr. Larry Goulder, Department of Economics, Stanford University, Stanford, CA 94305

Dr. Jane Lubchenco, Department of Zoology, Oregon State University, Corvallis, OR 97331

Dr. Pamela A. Matson, Environmental Science Policy and Management, University of California, Berkeley, CA

94720

Dr. Harold A. Mooney, Department of Biological Sciences, Stanford University, Stanford, CA 94305

Dr. Sandra Postel, Global Water Policy Project, 107 Larkspur Drive, Amherst, MA 01002

Dr. Stephen H. Schneider, Department of Biological Sciences, Stanford University, Stanford, CA 94305

Dr. David Tilman, Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN 55108-

609

Dr. George M. Woodwell, Woods Hole Research Center, P.O. Box 296, Woods Hole, MA 02543

Much of the information in this report was derived from G. Daily, editor. 1997. Natures Services: Societal Dependence on Natural Ecosystems. Island Press, Washington, D.C.

About the Science Writer
Yvonne Baskin, a science writer, edited the report of the panel of scientists to allow it to more effectively communicate its findings with non-scientists.

About Issues in Ecology Issues in Ecology is designed to report, in language understandable by non-scientists, the consensus of a panel of scientific experts on issues relevant to the