Published by the Ecological Society of America
Number 2, Spring 1997
Issues in Ecology
Ecosystem Services: Benefits
Supplied to Human Societies
by Natural Ecosystems
Issues in Ecology
Number 2
Spring 1997
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 Earth’s 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 Earth’s 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
•
•
•
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 sun’s 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
INTRODUCTION
Photo by J. Robert Stottlemeyer/Biological Photo Service
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):
1
2
Figure 1-Aspen (Populus tremuloides)
forest in Colorado, filtering and purifying air and water.
Number 2
Photo by Taylor Ricketts
Issues in Ecology
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 Earth’s 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 Earth’s 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
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.
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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Photo by Taylor Ricketts
Issues in Ecology
Number 2
Spring 1997
erated by sport fishing activities, it raises the total to
a microscope, no human hand has ever typed out a name
$46 billion (Felder and Nickum 1992, cited in Postel and
or description of them, and most human minds have never
Carpenter 1997). The future of these fisheries is in quesspent a moment reflecting on them. Yet the sobering
tion, however, because fish harvests have approached or
fact is, as E. O. Wilson put it: they don’t need us, but we
exceeded sustainable levels virtually everywhere. Nine
need them (Wilson 1987).
of the world’s major marine fishing areas are in decline
due to overfishing, pollution, and habitat destruction.
THE CHARACTER OF ECOSYSTEM SERVICES
(UNFAO 1993; Kaufman and Dayton 1997).
Turning our attention to the land, grasslands are
Moving our attention from the moon back to Earth, let
an important source of marketable goods, including anius look more closely at the services nature performs on
mals used for labor (horses, mules, asses, camels, bulthe only planet we know that is habitable. Ecosystem
locks, etc.) and those whose parts or products are conservices and the systems that supply them are so intersumed (as meat, milk, wool, and leather). Grasslands
connected that any classification of them is necessarily
were also important as the original source habitat for
rather arbitrary. Here we briefly explore a suite of
most domestic animals such as cattle, goats, sheep, and
overarching services that operate in ecosystems worldhorses, as well as many crops, such as wheat, barley,
wide.
rye, oats, and other grasses (Sala and Paruelo 1997). In
a wide variety of terrestrial habitats, people hunt game
Production of Ecosystem Goods
animals such as waterfowl, deer, moose, elk, fox, boar
Humanity obtains from natural ecosystems an
and other wild pigs, rabbits, and even snakes and monarray of ecosystem goods— organisms and their parts
keys. In many countries, game meat forms an important
and products that grow in the wild and that are used
part of local diets and, in many places, hunting is an
directly for human benefit. Many of these, such as fishes
economically and culturally important sport.
and animal products, are commonly traded in economic
Natural ecosystems also produce vegetation used directly
markets. The annual world fish catch, for example,
by humans as food, timber, fuelwood, fiber, pharmaceuamounts to about 100 million metric tons and is valued
ticals and industrial products. Fruits, nuts, mushrooms,
at between $50 billion and $100 billion; it is the leading
honey, other foods, and spices are extracted from many
source of animal protein, with over 20% of the populaforest species. Wood and other plant materials are used
tion in Africa and Asia dependent on fish as their priin the construction of homes and other buildings, as well
mary source of protein (UNFAO 1993). The commercial
as for the manufacture of furniture, farming implements,
harvest of freshwater fish worldwide in 1990 totaled
paper, cloth, thatching, rope, and so on. About 15 perapproximately 14 million tons and was valued at about
cent of the world’s energy consumption is supplied by
$8.2 billion (UNFAO 1994). Interestingly, the value of
fuelwood and other plant material; in developing counthe freshwater sport fishery in the U.S. alone greatly
tries, such “biomass” supplies nearly 40 percent of enexceeds that of the global commercial harvest, with diergy consumption (Hall et al. 1993), although the porrect expenditures in 1991 totaling about $16 billion.
tion of this derived from natural rather than
When this is added to the value of the employment gen-
4
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.
Photo by Taylor Ricketts
Issues in Ecology
Number 2
Spring 1997
human-dominated ecosystems is undocumented. In addiAs described in the previous section, biodiversity
tion, natural products extracted from many hundreds of
is a direct source of ecosystem goods. It also supplies
species contribute diverse inputs to industry: gums and
the genetic and biochemical resources that underpin our
exudates, essential oils and flavorings, resins and oleocurrent agricultural and pharmaceutical enterprises and
resins, dyes, tannins, vegetable fats and waxes, insectimay allow us to adapt these vital enterprises to global
cides, and multitudes of other compounds (Myers 1983;
change. Our ability to increase crop productivity in the
Leung and Foster 1996). The availability of most of
face of new pests, diseases, and other stresses has dethese natural products is in decline due to ongoing habipended heavily upon the transfer to our crops of genes
tat conversion.
from wild crop relatives that confer resistance to these
challenges. Such extractions from biodiversity’s “genetic
Generation and Maintenance of Biodiversity
library” account for annual increases in crop productivBiological diversity, or biodiversity for short, reity of about 1 percent, currently valued at $1 billion (NRC
fers to the variety of life forms at all levels of organiza1992). Biotechnology now makes possible even greater
tion, from the molecular to the landuse of this natural storehouse of gescape level. Biodiversity is generated
netic diversity via the transfer to crops
and maintained in natural ecosystems,
of genes from any kind of organism—
where organisms encounter a wide
not simply crop relatives—and it
variety of living conditions and chance
promises to play a major role in fuevents that shape their evolution in
ture yield increases. By the turn of
unique ways. Out of convenience or
the century, farm-level sales of the
necessity, biodiversity is usually quanproducts of agricultural biotechnoltified in terms of numbers of species,
ogy, just now entering the marketand this perspective has greatly influplace, are expected to reach at least
enced conservation goals. It is im$10 billion per year (World Bank
portant to remember, however, that
1991, cited in Reid et al. 1996).
the benefits that biodiversity supplies
In addition to sustaining the producto humanity are delivered through
tion of conventional crops, the
populations of species residing in livbiodiversity in natural ecosystems may
ing communities within specific physiinclude many potential new foods.
cal settings— in other words, through
Human beings have utilized around
complex ecological systems, or eco7,000 plant species for food over the
systems (Daily and Ehrlich 1995). For
course of history and another 70,000
human beings to realize most of the
plants are known to have edible parts
aesthetic, spiritual, and economic ben(Wilson 1989). Only about 150 food
efits of biodiversity, natural ecosys- Figure 4-Harpoon-whaling in Flores, In- plants have ever been cultivated on a
tems must therefore be accessible. The donesia. The oceans are a key source
large scale, however. Currently, 82
continued existence of coniferous tree of animal protein for the human popu- plant species contribute 90 percent
species somewhere in the world would lation.
of national per-capita supplies of food
not help the inhabitants of a town inplants
(Prescott-Allen
and
undated by flooding because of the clearing of a pine
Prescott-Allen 1990), although a much smaller number
forest upstream. Generally, the flow of ecosystem goods
of these supply the bulk of the calories humans consume.
and services in a region is determined by the type, spaMany other species, however, appear more nutritious or
tial layout, extent, and proximity of the ecosystems supbetter suited to the growing conditions that prevail in
plying them. Because of this, the preservation of only
important regions than the standard crops that domione minimum viable population of each non-human spenate world food supply today. Because of increasing
cies on Earth in zoos, botanical gardens, and the world’s
salinization of irrigated croplands and the potential for
legally protected areas would not sustain life as we know
rapid climate change, for instance, future food security
it. Indeed, such a strategy, taken to extreme, would lead
may come to depend on drought- and salt-tolerant varito collapse of the biosphere, along with its life support
eties that now play comparatively minor roles in agriculservices.
ture.
5
Photo by Paul R. Ehrlich
Issues in Ecology
Number 2
Spring 1997
age 20,000 years ago, for example, much of Europe and
Turning to medicinal resources, a recent survey
North America were covered by mile-thick ice sheets.
showed that of the top 150 prescription drugs used in
While the global climate has been relatively stable since
the United States, 118 are based on natural sources:
the invention of agriculture around 10,000 years ago,
74% on plants, 18% on fungi, 5% on bacteria, and 3%
periodic shifts in climate have affected human activities
on one vertebrate (snake) species. Nine of the top ten
and settlement patterns. Even relatively recently, from
drugs in this list are based on natural plant products
1550-1850, Europe was significantly cooler during a
(Grifo and Rosenthal, in press, as cited in Dobson 1995).
period known as the Little Ice Age. Many of these changes
The commercial value of pharmaceuticals in the develin climate are thought to be caused by alterations in
oped nations exceeds $40 billion per year (Principe 1989).
Earth’s orbital rotation or in the energy output of the
Looking at the global picture, approximately 80% of the
sun, or even by events on the Earth itself—sudden perhuman population relies on traditional medical systems,
turbations such as violent volcanic eruptions and asterand about 85% of traditional medicine involves the use
oid impacts or more gradual tectonic events such as the
of plant extracts (Farnsworth et al. 1985).
uplift of the Himalayas. Remarkably,
Saving only a single populaclimate has been buffered enough
tion of each species could have anthrough all these changes to sustain
other cost. Different populations of
life for at least 3.5 billion years
the same species may produce differ(Schneider and Londer 1984). And
ent types or quantities of defensive
life itself has played a role in this buffchemicals that have potential use as
ering.
pharmaceuticals or pesticides
Climate, of course, plays a
(McCormick et al. 1993); and they
major role in the evolution and distrimay exhibit different tolerances to
bution of life over the planet. Yet most
environmental stresses such as
scientists would agree that life itself
drought or soil salinity. For example,
is a principal factor in the regulation
the development of penicillin as a
of global climate, helping to offset the
therapeutic antibiotic took a full 15
effects of episodic climate oscillations
years after Alexander Fleming’s faby responding in ways that alter the
mous discovery of it in common bread
greenhouse gas concentrations in the
mold. In part, this was because sciatmosphere. For instance, natural ecoentists had great difficulty producing,
systems may have helped to stabilize
extracting, and purifying the subclimate and prevent overheating of the
stance in needed quantities. One key
to obtaining such quantities was the Figure 5-Trapping and releasing butterflies Earth by removing more of the greendiscovery, after a worldwide search, in a mixed-agriculture landscape in Costa house gas carbon dioxide from the
of a population of Fleming’s mold that Rica. Monitoring the impact of human atmosphere as the sun grew brighter
produced more penicillin than the activities on biodiversity and ecosystem over millions of years (Alexander et
original (Dowling 1977). Similarly, services is needed worldwide; butterflies al. 1997). Life may also exert a deplant populations vary in their ability may be useful indicators for monitoring. stabilizing or positive feedback that
reinforces climate change, particularly
to resist pests and disease, traits imduring transitions between interglacial periods and ice
portant in agriculture. Many thousands of varieties of
ages. One example: When climatic cooling leads to drops
rice from different locations were screened to find one
in sea level, continental shelves are exposed to wind and
with resistance to grassy stunt virus, a disease that posed
rain, causing greater nutrient runoff to the oceans. These
a serious threat to the world’s rice crop (Myers 1983).
nutrients may fertilize the growth of phytoplankton, many
Despite numerous examples like these, many of the loof which form calcium carbonate shells. Increasing their
calities that harbor wild relatives of crops remain unpropopulations would remove more carbon dioxide from the
tected and heavily threatened.
oceans and the atmosphere, a mechanism that should
further cool the planet. Living things may also enhance
Climate and Life
warming trends through such activities as speeding up
Earth’s climate has fluctuated tremendously since
microbial decomposition of dead organic matter, thus
humanity came into being. At the peak of the last ice
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Number 2
Spring 1997
soaked up by soils and gradually meted out to plant roots
or into aquifers and surface streams. Thus, the soil itself
slows the rush of water off the land in flash 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
Figure 6-Bark of the Pacific yew tree (Taxus brevifolia),
of nutrients. Downstream costs may include disrupted
which is the source of the new anti-cancer drug, taxol,
or lower quality water supplies; siltation that impairs drainWillamette National Forest, Oregon.
age and maintenance of navigable river channels, harreleasing carbon dioxide to the atmosphere (Schneider
bors, and irrigation systems; increased frequency and
and Boston 1991; Allegre and Schneider 1994). The
severity of floods; and decreased potential for hydroelecrelative influence of life’s stabilizing and destabilizing feedtric power as reservoirs fill with silt (Pimentel et al. 1995).
backs remains uncertain; what is clear is that climate
Worldwide, the replacement cost of reservoir capacity
and natural ecosystems are tightly coupled, and the stalost to siltation is estimated at $6 billion per year.
bility of that coupled system is an important
In addition to protecting soil from erosion, living
ecosystemservice.
vegetation—with its deep roots and above-ground evapoBesides their impact on the atmosphere, ecosysrating surface—also serves as a giant pump, returning
tems also exert direct physical influences that help to
water from the ground into the atmosphere. Clearing of
moderate regional and local weather. For instance, tranplant cover disrupts this link in the water cycle and leads
spiration (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 evapotranspiration—that 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.
Photo by Catherine M. Pringle/Biological Photo Service
Photo by Michael Graybill & Jan Hodder/ Biological Photo Service
Issues in Ecology
Mitigation of Floods and Droughts
An enormous amount of water, about 119,000
cubic kilometers, is rained annually onto the Earth’s land
surface—enough to cover the land to an average depth
of 1 meter (Shiklomanov 1993). Much of this water is
Figure 7-Herbal pharmacist in Dali, Yunnan Province,
China. An estimated 80 percent of the world’s population relies on natural medicinal products.
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Number 2
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Photo by Gretchen C. Daily
Issues in Ecology
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.
8
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.
Services Supplied by Soil
Soil represents an important component of a
nation’s 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 Earth’s 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 nutrients—cations such
as calcium and magnesium—near 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
Photo by L. Evans Roth/Biological Photo Service
Issues in Ecology
Number 2
Spring 1997
recycling of nutrients—the fourth service soils provide—
required by plants. Hydroponic systems supply water
are two aspects of the same process. The fertility of
and nutrients to plants without need of soil, but the marsoils—that is, their ability to supply nutrients to plants—
gin for error is much smaller—even small excesses of
is largely the result of the activities of diverse species of
nutrients applied hydroponically can be lethal to plants.
bacteria, fungi, algae, crustacea, mites, termites, springIndeed, it is a complex undertaking to regulate the nutritails, millipedes, and worms, all of which, as groups, play
ent concentrations, pH, and salinity of the nutrient soluimportant roles. Some bacteria are responsible for “fixtion in hydroponic systems, as well as the air and soluing” nitrogen, a key element in proteins, by drawing it
tion temperature, humidity, light, pests, and plant disout of the atmosphere and converting it to forms usable
eases. Worldwide, the area under hydroponic culture is
by plants and, ultimately, human beings and other anionly a few thousand hectares and is unlikely to grow
mals. Certain types of fungi play extremely important
significantly in the foreseeable future; by contrast, gloroles in supplying nutrients to many kinds of trees. Earthbal cropped area is about 1.4 billion hectares (USDA
worms and ants act as “mechani1993).
cal blenders,” breaking up and mixThird, soil plays a central
ing plant and microbial material and
role in the decomposition of dead
other matter (Jenny 1980). For
organic matter and wastes, and this
example, as much as 10 metric
decomposition process also renders
tonnes of material may pass
harmless many potential human
through the bodies of earthworms
pathogens. People generate a treon a hectare of land each year, remendous amount of waste, includsulting in nutrient rich “casts” that
ing household garbage, industrial
enhance soil stability, aeration, and
waste, crop and forestry residues,
drainage (Lee 1985).
and sewage from their own populaFinally, soils are a key factions and their billions of domestitor in regulating the Earth’s major
cated animals. A rough approximaelement cycles—those of carbon,
tion of the amount of dead organic
nitrogen, and sulfur. The amount
matter and waste (mostly agriculof carbon and nitrogen stored in
tural residues) processed each year
soils dwarfs that in vegetation, for
is 130 billion metric tons, about 30
example. Carbon in soils is nearly
percent of which is associated with
double (1.8 times) that in plant
human activities (derived from
Vitousek et al. 1986). Fortunately, Figure 9-Bacteria (Bradyrhizobium japonicum) matter, and nitrogen in soils is about
there is a wide array of decompos- in a soybean root nodule cell, magnified 3,550 18 times greater (Schlesinger
ing organisms—ranging from vul- times. These bacteria fix atmospheric nitro- 1991). Alterations in the carbon
tures to tiny bacteria—that extract gen into a form that can be utilized by plants. and nitrogen cycles may be costly
over the long term, and in many cases, irreversible on a
energy from the large, complex organic molecules found
time scale of interest to society. Increased fluxes of carin many types of waste. Like assembly-line workers, dibon to the atmosphere, such as occur when land is converse microbial species process the particular compounds
verted to agriculture or when wetlands are drained, conwhose chemical bonds they can cleave and pass along to
tribute to the buildup of key greenhouse gases, namely
other species the end products of their specialized reaccarbon dioxide and methane, in the atmosphere
tions. Many industrial wastes, including soaps, detergents,
(Schlesinger 1991). Changes in nitrogen fluxes caused
pesticides, oil, acids, and paper, are detoxified and deby production and use of fertilizer, burning of wood and
composed by organisms in natural ecosystems if the conother biomass fuels, and clearing of tropical land lead to
centration of waste does not exceed the system’s capacincreasing atmospheric concentrations of nitrous oxide,
ity to transform it. Some modern wastes, however, are
another potent greenhouse gas that is also involved in
virtually indestructible, such as some plastics and the
the destruction of the stratospheric ozone shield. These
breakdown products of the pesticide DDT.
and other changes in the nitrogen cycle also result in
The simple inorganic chemicals that result from
acid rain and excess nutrient inputs to freshwater sysnatural decomposition are eventually returned to plants
tems, estuaries, and coastal marine waters. This nutrient
as nutrients. Thus, the decomposition of wastes and the
9
Photo by Peter J. Bryant/Biological Photo Service
Issues in Ecology
Number 2
Spring 1997
endangered or extinct (Buchmann and Nabhan 1996).
influx causes eutrophication of aquatic ecosystems and
contamination of drinking water sources—both surface
Natural Pest Control Services
and ground water— by high levels of nitrate-nitrogen
Humanity’s competitors for food, timber, cotton,
(Vitousek et al. 1997).
and other fibers are called pests, and they include numerous herbivorous insects, rodents, fungi, snails, nemaPollination
todes, and viruses. These pests destroy an estimated 25
Animal pollination is required for the successful
to 50 percent of the world’s crops, either before or after
reproduction of most flowering plants. About 220,000
harvest (Pimentel et al. 1989). In addition, numerous
out of an estimated 240,000 species of plants for which
weeds compete directly with crops for water, light, and
the mode of pollination has been recorded require an
soil nutrients, further limiting yields.
animal such as a bee or hummingbird to accomplish this
Chemical pesticides, and the strategies by which
vital task. This includes both wild plants and about 70
they are applied to fight crop pests, can have harmful
percent of the agricultural crop species that feed the
unintended consequences. First, pests can develop resisworld. Over 100,000 different animal species—includtance, which means that higher and higher doses of pesing bats, bees, beetles, birds, butterflies, and flies—are
ticides must be applied or new chemicals developed periknown to provide these free pollination services that asodically to achieve the same level of control. Resistance
sure the perpetuation of plants in our croplands, backis now found in more than
yard gardens, rangelands,
500 insect and mite pests,
meadows and forests. In
over 100 weeds, and in
turn, the continued availabilabout 150 plant pathogens
ity of these pollinators de(WRI 1994). Second, popupends on the existence of a
lations of the natural enwide variety of habitat types
emies of pests are decimated
needed for their feeding, sucby heavy pesticide use.
cessful breeding, and
Natural predators are often
completion of their life cycles
more susceptible to synthetic
(Nabhan and Buchmann
poisons than are the pests
1997).
because they have not had
One third of human
the same evolutionary expefood is derived from plants
Figure
10-Sonoran
bumble
bee
(Bombus
sonorus)
pollinatrience with overcoming plant
pollinated by wild pollinators.
chemicals that the pests
Without natural pollination ing a flower.
themselves have had. And natural predators also typiservices, yields of important crops would decline precipically have much smaller population sizes than those of
tously and many wild plant species would become extheir prey. Destruction of predator populations leads to
tinct. In the United States alone, the agricultural value
explosions in prey numbers, not only freeing target pests
of wild, native pollinators—those sustained by natural
from natural controls but often “promoting” other
habitats adjacent to farmlands— is estimated in the bilnon-pest species to pest status. In California in the
lions of dollars per year. Pollination by honey bees, origi1970s, for instance, 24 of the 25 most important agrinally imported from Europe, is extremely important as
cultural pests had been elevated to that status by the
well, but these bees are presently in decline, enhancing
overuse of pesticides (NRC 1989). Third, exposure to
the importance of pollinators from natural ecosystems.
pesticides and herbicides may pose serious health risks
Management of the honey bee in the New World is curto humans and many other types of organisms; the rerently threatened by the movement of, and hybridization
cently discovered declines in human sperm counts may
with, an aggressive African strain of honey bee that was
be attributable in part to such exposure (Colborn et al.
accidentally released in Brazil in 1956. Diseases of honey
1996).
bee colonies are also causing a marked decline in the
Fortunately, an estimated 99 percent of potennumber of managed colonies. Meanwhile, the diversity
tial crop pests are controlled by natural enemies, includof natural pollinators available to both wild and domestiing many birds, spiders, parasitic wasps and flies, lady
cated plants is diminishing: more than 60 genera of polbugs, fungi, viral diseases, and numerous other types of
linators include species now considered to be threatened,
10
Number 2
Spring 1997
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 acSeed Dispersal
tivities. For many, nature is
Once a seed germian unparalleled source of
nates, the resulting plant is
wonderment and inspiration,
Figure
11-Ladybug
larva
(Cycloneda
polita)
eating
an
aphid.
usually rooted in place for
peace and beauty, fulfillment
the rest of its life. For plants, then, movement to new
and rejuvenation (e.g., Kellert and Wilson).
sites beyond the shadow of the parent is usually achieved
through seed dispersal. Many seeds, such as those of
THREATS TO ECOSYSTEM SERVICES
the dandelion, are dispersed by wind. Some are dispersed
by water, the most famous being the seafaring coconut.
Ecosystem services are being impaired and deMany other seeds have evolved ways of getting around
stroyed by a wide variety of human activities. Foremost
by using animals as their dispersal agents. These seeds
among the immediate threats are the continuing destrucmay be packaged in sweet fruit to reward an animal for
tion of natural habitats and the invasion of non-native
its dispersal services; some of these seeds even require
species that often accompanies such disruption; in mapassage through the gut of a bird or mammal before
rine systems, overfishing is a major threat. The most
they can germinate. Others require burial—by, say, a
irreversible of human impacts on ecosystems is the loss
forgetful jay or a squirrel which later leaves its cache
of native biodiversity. A conservative estimate of the
uneaten—for eventual germination. Still others are
rate of species loss is about one per hour, which unfortuequipped with sticky or sharp, spiny surfaces designed
nately exceeds the rate of evolution of new species by a
to catch onto a passing animal and go for a long ride
factor of 10,000 or more (Wilson 1989; Lawton and
before dropping or being rubbed off. Without thousands
May 1995). But complete extinction of species is only
of animal species acting as seed dispersers, many plants
the final act in the process. The rate of loss of local popuwould fail to reproduce successfully. For instance, the
lations of species—the populations that generate ecowhitebark pine (Pinus albicaulis), a tree found in the
system services in specific localities and regions—is orRockies and Sierra Nevada - Cascade Mountains, cannot
ders of magnitude higher (Daily and Ehrlich 1995; Hughes
reproduce successfully without a bird called Clark’s Nutet al., in prep.). Destroying other life forms also disrupts
cracker (Nucifraga columbiana), which chisels pine seeds
the web of interactions that could help us discover the
out of the tightly closed cones and disperses and buries
potential usefulness of specific plants and animals (Ththem; without this service, the cones do not open far
ompson 1994). Once a pollinator or a predacious insect
enough to let the seeds fall out on their own. Animal
is on the brink of extinction, for instance, it would be
seed dispersers play a central role in the structure and
difficult to discover its potential utility to farmers.
regeneration of many pine forests (Lanner 1996). DisOther imminent threats include the alteration of
ruption of these complex services may leave large areas
the Earth’s carbon, nitrogen, and other biogeochemical
of forest devoid of seedlings and younger age classes of
cycles through the burning of fossil fuels and heavy use
trees, and thus unable to recover swiftly from human
of nitrogen fertilizer; degradation of farmland through
impacts such as land clearing.
unsustainable agricultural practices; squandering of freshwater resources; toxification of land and waterways; and
Aesthetic Beauty and Intellectual and Spiritual Stimuoverharvesting of fisheries, managed forests, and other
lation
theoretically renewable systems.
Many human beings have a deep appreciation of
These threats to ecosystem services are driven
natural ecosystems. That is apparent in the art, reliultimately by two broad underlying forces. One is rapid,
gions, and traditions of diverse cultures, as well as in
unsustainable growth in the scale of the human enter11
Photo by Peter J. Bryant/Biological Photo Service
Issues in Ecology
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).
Photo by Gretchen C. Daily
Issues in Ecology
Number 2
Spring 1997
is known as the marginal value. In the case of ecosystem
prise: in population size, in per-capita consumption, and
services, for example, the question that might be posed
also in the environmental impacts that technologies and
would be: By how much would the flow of ecosystem
institutions generate as they produce and supply those
services be augmented (or diminished) with the preservaconsumables (Ehrlich et al. 1977). The other underlying
tion (or destruction) of the next hectare of forest or wetdriver is the frequent mismatch between short-term, inland? Estimation of marginal values is complex (e.g.,
dividual economic incentives and long-term, societal
Bawa and Gadgil 1997; Daily 1997b). Often a qualitawell-being. Ecosystem services are generally greatly untive comparison of relative values is sufficient— that is,
dervalued, for a number of reasons: many are not traded
which is greater, the economic benefits of a particular
or valued in the marketplace; many serve the public good
development project or the benefits supplied by the ecorather than provide direct benefits to individual landownsystem that would be destroyed, measured over a time
ers; private property owners often have no way to benperiod of interest to people concerned about the well-being
efit financially from the ecosystem services supplied to
of their grandchildren?
society by their land; and, in fact, economic subsidies
There are, and will remain, many cases in which
often encourage the conversion of such lands to other,
ecosystem service values are highly uncertain. Yet the
market-valued activities. Thus, people whose activities
pace of destruction of natural ecosystems, and the irredisrupt ecosystem services often do not pay directly for
versibility of most such destruction on a time scale of
the cost of those lost services. Moreover, society often
interest to humanity, warrants substantial caution. Valudoes not compensate landowners and others who do safeing a natural ecosystem, like valuing a human life, is
guard ecosystem services for the economic benefits they
fraught with difficulties. Just as societies have recoglose by foregoing more lucrative but destructive land uses.
nized fundamental human rights, however, it may be pruThere is a critical need for policy measures that address
dent to establish fundamental ecosystem protections even
these driving forces and embed the value of ecosystem
though uncertainty over economic values remains. New
services into decision making frameworks.
institutions and agreements at the international and
subnational level will be needed to encourage fair particiVALUATION OF ECOSYSTEM SERVICES
pation in such protections (see, e.g., Heal 1994).
The tremendous expense and difficulty of repliHuman society would cease to exist in the abcating lost ecosystem services is perhaps best illustrated
sence of ecosystem services. Thus, their immense value
by the results of the first Biosphere 2 “mission,” in which
to humanity is unquestionable. Yet quantifying the value
eight people lived inside a 3.15-acre closed ecosystem
of ecosystem services in specific localities, and measurfor two years. The system featured agricultural land and
ing their worth against that of competing land uses is no
replicas of several natural ecosystems such as forests
simple task. When tradeoffs must be made in the allocaand even a miniature ocean. In spite of an investment of
tion of land and other resources to competing human
more than $200 million in the design, construction, and
activities, the resolution often requires a measure of what
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.
12
Issues in Ecology
Number 2
Spring 1997
• To what extent have varioperation of this model
ous ecosystem services alearth, it proved impossible
ready been impaired? And
to supply the material and
how are impairment and risk
physical needs of the eight
of future impairment distribBiospherians for the inuted in various regions of
tended 2 years. Many unthe globe?
pleasant and unexpected
• How interdependent are
problems arose, including a
different ecosystem serdrop in atmospheric oxygen
vices? How does exploiting
concentration to 14% (the
or damaging one influence
level normally found at an
the functioning of others?
elevation of 17,500 feet),
• To what extent, and over
high spikes in carbon dioxwhat time scale, are ecosyside concentrations, nitrous
tem services amenable to
oxide concentrations high
repair or restoration?
enough to impair the brain,
• How effectively, and at
an extremely high level of
how large a scale, can exextinctions (including 19 of
isting or foreseeable human
25 vertebrate species and all
technologies substitute for
pollinators brought into the
ecosystem services? What
enclosure, which would have
would be the side effects of
ensured the eventual extincsuch substitutions?
tion of most of the plant
• Given the current state
species as well), overgrowth
of aggressive vines and al- Figure 13-The production of this meal benefited from many of technology and the scale
gal mats, and population ecosystem services, including natural pest control, pollina- of the human enterprise,
explosions of crazy ants, tion, maintenance of soil fertility, purification of water, and what proportion and spatial
pattern of land must remain
cockroaches, and katydids. moderation of climate.
relatively undisturbed, loEven heroic personal efforts
cally, regionally, and globally, to sustain the delivery
on the part of the Biospherians did not suffice to make
of essential ecosystem services?
the system viable and sustainable for either humans or
many nonhuman species (Cohen and Tilman 1996).
CONCLUSIONS
MAJOR UNCERTAINTIES
The human economy depends upon the services performed
“for free” by ecosystems. The ecosystem services supSociety would clearly profit by further investigation into
plied annually are worth many trillions of dollars. Ecosome of the following broad research questions so that
nomic development that destroys habitats and impairs
we might avoid on Biosphere 1, the earth, unpleasant
services can create costs to humanity over the long term
surprises like those that plagued the Biosphere 2 project
that may greatly exceed the short-term economic ben(Holdren 1991; Cohen and Tilman 1996; Daily 1997b):
efits of the development. These costs are generally hidden from traditional economic accounting, but are none• What is the relative impact of various human activitheless real and are usually borne by society at large.
ties upon the supply of ecosystem services?
Tragically, a short-term focus in land-use decisions often
• What is the relationship between the condition of an
sets in motion potentially great costs to be borne by
ecosystem—that is, relatively pristine or heavily
future generations. This suggests a need for policies
modified—and the quantity and quality of ecosysthat achieve a balance between sustaining ecosystem
tem services it supplies?
services and pursuing the worthy short-term goals of
• To what extent do ecosystem services depend upon
economic development.
biodiversity at all levels, from genes to species to
landscapes?
13
Issues in Ecology
Number 2
ACKNOWLEDGMENTS
We thank the Packard Foundation and the Pew
Foundation for financial support.
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biodiversity. Scientific American Sept: 108-116.
World Bank. 1991. Agricultural Biotechnology: The Next Green
Revolution? World Bank Technical Paper no. 133. Washington, D.C.
World Resources Institute (WRI). 1994. World Resources: A Guide
to the Global Environment. Oxford University Press, Oxford.
FOR MORE INFORMATION
15
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:
Issues in Ecology
Number 2
Spring 1997
environment. All reports undergo peer review and must
Dr. Gretchen C. Daily, Panel Chair, Department of Biobe approved by the editorial board before publication.
logical Sciences, Stanford University, Stanford, CA 94305
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
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
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 277080340
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
Additional Copies
To receive additional copies of this report or previous Issues in Ecology, please contact:
Dr. David Tilman, Department of Ecology, Evolution and
Behavior, University of Minnesota, St. Paul, MN 55108609
Public Affairs Office
Ecological Society of America
2010 Massachusetts Avenue, NW
Suite 400
Washington, DC 20036
esahq@esa.org
(202) 833-8773
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. Nature’s 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.
Special thanks to the U.S. Environmental Protection Agency Office of Sustainable Ecosystems and Communities for supporting printing and distribution of this
document.
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
Cover photo credits, clockwise from top left:
Nadine Cavender, Nadine Cavender, Nadine Cavender,
Claude Cavender, Jr., Nadine Cavender, unknown, Claude
Cavender, Jr., and unknown.
16
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 nation’s
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 1092-8987