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Issues in Ecology is designed to report, in language understandable by non-scientists, the

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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

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.
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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
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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
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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
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.

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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.
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 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
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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
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

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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.
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 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
10

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 the

world. Over 100,000 different animal speciesincluding bats, bees, beetles, birds, butterflies, and fliesare

known 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 types

needed 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 pollination

services, 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 natural

habitats 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 diversity

of 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 harmful

unintended 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 had

the same evolutionary experience with overcoming plant

chemicals that the pests

themselves 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 the

1970s, 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 risks

to 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 of
activities such as gardening

and pet-keeping, nature photography and film-making,

bird feeding and watching,

hiking and camping,

ecotouring and mountaineering, river-rafting and boating, fishing and hunting, and

in 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. Foremost

among the immediate threats are the continuing destruction of natural habitats and the invasion of non-native

species 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 the

rate 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 only

the 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 the

potential 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 through

unsustainable 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 annually

by protecting crops and reducing the need for chemical control (Naylor and

Ehrlich 1997).

Seed Dispersal

Once 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, cannot

reproduce 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 and

regeneration 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 of

natural ecosystems. That is apparent in the art, religions, and traditions of diverse cultures, as well as in
12

 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 underlying

driver is the frequent mismatch between short-term, individual economic incentives and long-term, societal

well-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 good

rather 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 often

does 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 value

of ecosystem services in specific localities, and measuring their worth against that of competing land uses is no

simple 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 ecosystem

services 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 particular

development 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 the

pace of destruction of natural ecosystems, and the irreversibility of most such destruction on a time scale of

interest to humanity, warrants substantial caution. Valuing a natural ecosystem, like valuing a human life, is

fraught 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 and

subnational 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, and
13

Issues in Ecology Number 2 Spring 1997

operation of this model

earth, it proved impossible

to supply the material and

physical needs of the eight

Biospherians 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), overgrowth

of 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 heavily

modifiedand 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 risk

of future impairment distributed in various regions of

the globe?

• How interdependent are

different ecosystem services? How does exploiting

or damaging one influence

the functioning of others?

• To what extent, and over

what time scale, are ecosystem services amenable to

repair or restoration?

• How effectively, and at

how 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 remain

relatively undisturbed, locally, regionally, and globally, to sustain the delivery

of essential ecosystem services?

CONCLUSIONS

The human economy depends upon the services performed

for 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 term

that 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 many

ecosystem 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

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Editorial Board of Issues in Ecology

Dr. 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-

0340

Additional Copies

To receive additional copies of this report or previous Issues in Ecology, please contact:

Public Affairs Office

Ecological Society of America

2010 Massachusetts Avenue, NW

Suite 400

Washington, DC 20036

esahq@esa.org

(202) 833-8773

Special thanks to the U.S. Environmental Protection Agency Office of Sustainable Ecosystems and Communities for supporting printing and distribution of this

document.

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

16

Dr. 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 onNatural 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