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American Academy of Political and Social Science
The Place of Nature in the City of Man
Author(s): Ian L. McHarg
Source: The Annals of the American Academy of Political and
Social Science, Vol. 352,
Urban Revival: Goals and Standards (Mar., 1964), pp. 1-12
Published by: Sage Publications, Inc. in association with the
American Academy of
Political and Social Science
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The Place of Nature in the City of Man
By IAN L. MCHARG
ABSTRACT: Unparalleled urban growth is pre-empting a
million acres of rural lands each year and transforming these
into the sad emblems of contemporary urbanism. In that
anarchy which constitutes urban growth, wherein the major
prevailing values are short-term economic determinism, the
image of nature is attributed little or no value. In existing
cities, the instincts of eighteenth- and nineteenth-century city
builders, reflected in the pattern of existing urban open space,
have been superseded by a modern process which disdains
nature and seems motivated by a belief in salvation through
stone alone. Yet there is a need and place for nature in the
city of man. An understanding of natural processes should be
reflected in the attribution of value to the constituents of these
natural processes. Such an understanding, reflected in city
building, will provide a major structure for urban and metro-
politan form, an environment capable of supporting physiolog-
ical man, and the basis for an art of city building which will
enhance life and reflect meaning, order, and purpose.
Ian L. McHarg, M.L.A., M.C.P., Philadelphia, Pennsylvania, is

Chairman of the
Department of Landscape Architecture and Professor of City
Planning at the University
of Pennsylvania. He has a private practice in City Planning and
Landscape Architecture
in partnership with Dr. David A. Wallace. His interest in the
subject of values toward
nature and the physical environments which are their products
has been reflected in
many articles, among them "Man and Environment," a chapter
in The Urban Condition,
edited by Leonard Duhl, "The Ecology of the City," published
in the American Institute
of Architects Journal, 1963. On this same subject, he conceived
and moderated a series
of twenty-four television programs entitled "The House We
Live In," initiated by WCAU-
CBS and subsequently shown by National Educational
Television.
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THE ANNALS OF THE AMERICAN ACADEMY
EFORE we convert our rocks and
rills and templed hills into one
spreading mass of low grade urban
tissue under the delusion that because

we accomplish this degradation with
the aid of bulldozers, atomic piles and
electronic computers we are advancing
civilization, we might ask what all this
implies in terms of the historic nature
of man. .. ."-Lewis Mumford.1
The subject of this essay is an inquiry
into the place of nature in the city of
man. The inquiry is neither ironic nor
facetious but of the utmost urgency and
seriousness. Today it is necessary to
justify the presence of nature in the city
of man; the burden of proof lies with
nature, or so it seems. Look at the
modern city, that most human of all
environments, observe what image of
nature exists there-precious little in-
deed and that beleaguered, succumbing
to slow attrition.
William Penn effectively said, Let us
build a fair city between two noble
rivers; let there be five noble squares, let
each house have a fine garden, and let
us reserve territories for farming. But
that was before rivers were discovered
to be convenient repositories for sewage,
parks the best locus for expressways,
squares the appropriate sites for public
monuments, farm land best suited for
buildings, and small parks best trans-
formed into asphalted, fenced play-
grounds.

Charles Eliot once said, in essence,
This is our city, these are our hills,
these are our rivers, these our beaches,
these our farms and forests. I will
make a plan to cherish this beauty and
wealth for all those who do or will live
here. And the plan was good but
largely disdained. So here, as else-
where, man assaulted nature disinter-
1 Lewis Mumford, Man's Role in Changing
the Face of the Earth (Chicago: The Uni-
versity of Chicago, 1956), p. 1142.
estedly, man assaulted man with the
city; nature in the city remains pre-
cariously as residues of accident, rare
acts .of personal conscience, or rarer
testimony to municipal wisdom, the
subject of continuous assault and at-
trition while the countryside recedes
before the annular rings of suburbaniza-
tion, unresponsive to any perception
beyond simple economic determinism.
Once upon a time, nature lay outside
the city gates a fair prospect from the
city walls, but no longer. Climb the
highest office tower in the city, when
atmospheric pollution is only normal,
and nature may be seen as a green rim
on the horizon. But this is hardly a
common condition and so nature lies
outside of workaday experience for most
urban people.

Long ago, homes were built in the
country and remained rural during the
lives of persons and generations. Not
so today, when a country house of
yesterday is within the rural-urban
fringe today, in a suburb tomorrow,
and in a renewal area of the not-too-
distant future.
When the basis for wealth lay in the
heart of the land and the farms upon it,
then the valleys were verdant and beau-
tiful, the farmer steward of the land-
scape, but that was before the American
dream of a single house on a quarter
acre, the automobile, crop surpluses, and
the discovery that a farmer could profit
more by selling land than crops.
Once men in simple cabins saw only
wild nature, silent, implacable, lonely.
They cut down the forests to banish
Indians, animals, and shadows. Today,
Indians, animals, and forests have gone
and wild nature, silence, and loneliness
are hard to find.
When a man's experience was limited
by his home, village, and environs, he
lived with his handiworks. Today, the
automobile permits temporary escapes
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THE PLACE OF NATURE IN THE CITY OF MAN
from urban squalor, and suburbaniza-
tion gives the illusion of permanent
escape.
Once upon a time, when primeval
forests covered Pennsylvania, its original
inhabitants experienced a North Tem-
perate climate, but, when the forests
were felled, the climate became, in
summer, intemperately hot and humid.
Long ago, floods were described as
Acts of God. Today, these are known
quite often to be consequences of the
acts of man.
As long ago, droughts were thought to
be Acts of God, too, but these, it is now
known, are exacerbated by the acts of
man.
In times past, pure air and clean
abundant water were commonplaces.
Today, "pollution" is the word most
often associated with the word "atmos-
phere," drinking water is often a dilute
soup of dead bacteria in a chlorine
solution, and the only peoples who enjoy
pure air and clean water are rural

societies who do not recognize these for
the luxuries they are.
Not more than two hundred years
ago, the city existed in a surround
of farm land, the sustenance of the city.
The farmers tended the lands which
were the garden of the city. Now, the
finest crops are abject fruits compared
to the land values created by the most
scabrous housing, and the farms are
defenseless.
In days gone by, marshes were lonely
and wild, habitat of duck and goose,
heron and egret, muskrat and beaver,
but that was before marshes became the
prime sites for incinerator wastes, rub-
bish, and garbage-marshes are made
to be filled, it is said.
When growth was slow and people
spent a lifetime on a single place, the
flood plains were known and left un-
built. But, now, who knows the flood
plain? Caveat emptor.
Forests and woodlands once had their
own justification as sources of timber
and game, but second-growth timber has
little value today, and the game has
long fled. Who will defend forests and
woods?
Once upon a time, the shad in hun-

dreds of thousands ran strong up the
river to the city. But, today, when they
do so, there is no oxygen, and their
bodies are cast upon the shores.
THE MODERN METROPOLIS
Today, the modern metropolis covers
thousands of square miles, much of the
land is sterilized and waterproofed, the
original animals have long gone, as have
primeval plants, rivers are foul, the
atmosphere is polluted, climate and
microclimate have retrogressed to in-
creased violence, a million acres of land
are transformed annually from farm
land to hot-dog stand, diner, gas
station, rancher and split level, asphalt
and concrete, billboards and sagging
wire, parking lots and car cemeteries,
yet slums accrue faster than new build-
ings, which seek to replace them. The
epidemiologist can speak of urban epi-
demics-heart and arterial disease, renal
disease, cancer, and, not least, neuroses
and psychoses. A serious proposition
has been advanced to the effect that the
modern city would be in serious jeop-
ardy without the safeguards of modern
medicine and social legislation. Lewis
Mumford can describe cities as dys-
genic. There has arisen the recent
specter, described as "pathological to-
getherness," under which density and
social pressure are being linked to the
distribution of disease and limitations

upon reproduction. We record stress
from sensory overload and the response
of negative hallucination to urban an-
archy. When one considers that New
York may well add 1,500 square miles
of new "low-grade tissue" to its perime-
ter in the next twenty years, then one
recalls Loren Eiseley's image and sees
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the cities of man as gray, black, and
brown blemishes upon the green earth
with dynamic tentacles extending from
them and asks: "Are these the evidence
of man, the planetary disease?"
WESTERN VIEWS: MAN AND NATURE
Yet how can nature be justified in the
city? Does one invoke dappled sun-
light filtered through trees of eco-
systems, the shad run or water treat-
ment, the garden in the city or negative
entropy? Although at first glance an
unthinkable necessity, the task of justi-
fying nature in the city of man is, with

prevailing values and process, both
necessary and difficult. The realities
of cities now and the plans for their
renewal and extension offer incontro-
vertible evidence of the absence of
nature present and future. Should
Philadelphia realize the Comprehensive
Plan, then $20 billion and twenty years
later there will be less open space than
there is today. Cities are artifacts be-
coming ever more artificial-as though
medieval views prevailed that nature
was defiled, that living systems shared
original sin with man, that only the
artifice was free of sin. The motto for
the city of man seems to be: salvation
by stone alone.
Of course, the medieval view of nature
as rotten and rotting is only an aspect
of the historic Western anthropocentric-
anthropomorphic tradition in which
nature is relegated to inconsequence.
Judaism and Christianity have been
long concerned with justice and com-
passion for the acts of man to man but
have traditionally assumed nature to be
a mere backdrop for the human play.
Apparently, the literal interpretation of
the creation in Genesis is the tacit text
for Jews and Christians alike-man
exclusively divine, man given dominion
over all life and nonlife, enjoined to

subdue the earth. The cosmos is
thought to be a pyramid erected to
support man upon its pinnacle; reality
exists only because man can perceive it;
indeed, God is made in the image of
man. From origins in Judaism, exten-
sion into classicism, reinforcement in
Christianity, inflation in the Renais-
sance, and absorption into ninteenth-
and twentieth-century thought, the
anthropocentric - anthropomorphic view
has become the tacit Western posture of
man versus nature. The nineteenth-
and twentieth-century city is the most
complete expression of this view. Within
the Western tradition exists a contrary
view of man and nature which has a
close correspondence to the Oriental at-
titude of an aspiration to harmony of
man in nature, a sense of a unitary
and encompassing natural order within
which man exists. Among others, the
naturalist tradition in the West includes
Duns Scotus, Joannes Scotus Erigena,
Francis of Assisi, Wordsworth, Goethe,
Thoreau, Gerald Manley Hopkins, and
the nineteenth- and twentieth-century
naturalists. Their insistence upon nature
being at least the sensible order within
which man exists or a Manifestation of
God demanding deference and reverence
is persuasive to many but not to the

city builders.
Are the statements of scientists likely
to be more persuasive?
David R. Goddard:2
No organism lives without an environ-
ment. As all organisms are depletive, no
organism can survive in an environment of
its exclusive creation.
F. R. Fosberg:3
An ecosystem is a functioning, inter-
acting system composed of one or more
organisms and their effective environment,
2 Transcript, WCAU-TV, "The House We
Live In."
3 F. R. Fosberg, "The Preservation of
Man's Environment," Proceedings of the
Ninth Pacific Science Congress, 1957, Vol. 20,
1958, p. 160.
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both physical and biological. All eco-
systems are open systems. Ecosystems
may be stable or unstable. The stable
system is in a steady state. The entropy
in an unstable system is more likely to
increase than decrease. There is a tend-
ency towards diversity in natural eco-
systems. There is a tendency towards uni-
formity in artificial ecosystems or those
strongly influenced by man.
Paul Sears:4
Any species survives by virtue of its
niche, the opportunity afforded it by
environment. But in occupying this niche,
it also assumes a role in relation to its
surroundings. For further survival it is
necessary that its role at least be not a
disruptive one. Thus, one generally finds
in nature that each component of a highly
organized community serves a constructive
or at any rate, a stabilizing role. The
habitat furnishes the niche, and if any
species breaks up the habitat, the niche
goes with it. ... To persist organic sys-
tems must be able to utilize radiant energy
not merely to perform work, but to main-
tain the working system in reasonably good
order. This requires the presence of
organisms adjusted to the habitat and to
each other so organized to make the fullest
use of the influent radiation and to con-
serve for use and reuse the materials which

the system requires.
Complex creatures consist of billions
of cells, each of which, like any single-
celled creature, is unique, experiences
life, metabolism, reproduction, and
death. The complex animal exists
through the operation of symbiotic rela-
tionships between cells as tissues and
organs integrated as a single organism.
Hans Selye describes this symbiosis as
intercellular altruism, the situation
under which the cell concedes some part
of its autonomy towards the operation
4Paul B. Sears, "The Process of Environ-
mental Change by Man," in Man's Role in
Changing the Face of the Earth, ed. W. L.
Thomas, Jr. (Chicago: University of Chicago
Press, 1956).
of the organism and the organism
responds to cellular processes.
Aldo Leopold has been concerned
with the ethical content of symbiosis:5
Ethics so far studied by philosophers are
actually a process in ecological as well
as philosophical terms. They are also a
process in ecological evolution. An'ethic,
ecologically, is a limitation on freedom of
action in the struggle for existence. An
ethic, philosophically, is a differentiation
of social from anti-social conduct. These

are two definitions of one thing which has
its origin in the tendency of interdependent
individuals and groups to evolve modes of
cooperation. The ecologist calls these
symbioses. There is as yet no ethic deal-
ing with man's relation to the environment
and the animals and plants which grow
upon it. The extension of ethics to in-
clude man's relation to environment is, if
I read the evidence correctly, an evolu-
tionary possibility and an ecological neces-
sity. All ethics so far evolved rest upon
a single premise that the individual is a
member of a community of interdependent
parts. His instincts prompt him to com-
pete for his place in the community, but
his ethics prompt him to cooperate, per-
haps in order that there may be a place to
compete for.
The most important inference from
this body of information is that inter-
dependence, not independence, charac-
terizes natural systems. Thus, man-
nature interdependence presumably
holds true for urban man as for his
rural contemporaries. We await the
discovery of an appropriate physical
and symbolic form for the urban man-
nature relationship.
NATURAL AND ARTIFICIAL
ENVIRONMENTS
From the foregoing statements by

natural scientists, we can examine
certain extreme positions. First, there
5 Aldo Leopold, A Sand County Almanac
(Oxford: Oxford University Press, 1949), pp.
202, 203.
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can be no conception of a completely
"natural" environment. Wild nature,
save a few exceptions, is not a satis-
factory physical environment. Yet the
certainty that man must adapt nature
and himself does not diminish his de-
pendence upon natural, nonhuman proc-
esses. These two observations set limits
upon conceptions of man and nature.
Man must adapt through both biologi-
cal and cultural innovation, but these
adaptations occur within a context of
natural, nonhuman processes. It is not
inevitable that adapting nature to
support human congregations must of
necessity diminish the quality of the
physical environment. Indeed, all of

preindustrial urbanism was based upon
the opposite premise, that only in the
city could the best conjunction of social
and physical environment be achieved.
This major exercise of power to adapt
nature for human ends, the city, need
not be a diminution of physiological,
psychological, and aesthetic experience.
While there can be no completely
natural environments inhabited by man,
completely artificial environments are
equally unlikely. Man in common with
all organisms is a persistent configura-
tion of matter through which the en-
vironment ebbs and flows continuously.
Mechanically, he exchanges his sub-
stance at a very rapid rate while, addi-
tionally, his conceptions of reality are
dependent upon the attribution of mean-
ing to myriads of environmental stimuli
which impinge upon him continuously.
The materials of his being are natural,
as are many of the stimuli which he
perceives; his utilization of the ma-
terials and of many stimuli is involun-
tary. Man makes artifices, but galactic
and solar energy, gases of hydrosphere
and atmosphere, the substance of the
lithosphere, and all organic systems
remain elusive of human artificers.
Yet the necessity to adapt natural en-
vironments to sustain life is common
to many organisms other than man.

Creation of a physical environment by
organisms as individuals and as com-
munities is not exclusively a human
skill. The chambered nautilus, the bee-
hive, the coral formation, to select but
a few examples, are all efforts by organ-
ism to take inert materials and dispose
them to create a physical environment.
In these examples, the environments
created are complementary to the or-
ganisms. They are constructed with
great economy of means; they are
expressive, they have, in human eyes,
great beauty, and they have survived
periods of evolutionary time vastly
longer than the human span.
Simple organisms utilize inert ma-
terials to create physical environments
which sustain life. Man also confronts
this necessity. Man, too, is natural in
that he responds to the same laws as do
all physical and biological systems. He
is a plant parasite, dependent upon
the plant kingdom and its associated
microorganisms, insects, birds, and ani-
mals for all atmospheric oxygen, all
food, all fossil fuel, natural fibers and
cellulose, for the stability of the water
cycle and amelioration of climate and
microclimate. His dependence upon
the plant and photosynthesis establishes
his dependence upon the microorgan-
isms of the soil, particularly the de-
composers which are essential to the

recycling of essential nutrients, the
insects, birds, and animals which are in
turn linked to survival of plant systems.
He is equally dependent upon the
natural process of water purification
by microorganisms. The operation of
these nonhuman physical and bio-
logical processes is essential for human
survival.
Having concluded that there can be
neither a completely artificial nor a
completely natural environment, our
attention is directed to some determi-
nants of optimal proportions. Some
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indication may be inferred from man's
evolutionary history. His physiology
and some significant part of his psy-
chology derive from the billions of
years of his biological history. During
the most recent human phase of a
million or so years, he has been pre-
ponderantly food gatherer, hunter, and,
only recently, farmer. His urban ex-

perience is very recent indeed. Thus,
the overwhelming proportion of his bio-
logical history has involved experience
in vastly more natural environments
than he now experiences. It is to these
that he is physiologically adapted.
According to F. R. Fosberg:6
It is entirely possible that man will not
survive the changed environment that he is
creating, either because of failure of re-
sources, war over their dwindling supply,
or failure of his nervous system to evolve
as rapidly as the change in environment
will require. Or he may only survive in
small numbers, suffering the drastic re-
duction that is periodically the lot of
pioneer species, or he may change beyond
our recognition. . . . Management and
utilization of the environment on a true
sustaining yield basis must be achieved.
And all this must be accomplished without
altering the environment beyond the capac-
ity of the human organism, as we know it,
to live in it.
HUMAN ECOSYSTEMS
There are several examples where eco-
systems, dominated by man, have en-
dured for long periods of time; the
example of traditional Japanese agri-
culture is perhaps the most spectacular.
Here an agriculture of unequaled in-

tensity and productivity has been sus-
tained for over a thousand years, the
land is not impoverished but enriched
by human intervention, the ecosystem,
wild lands, and farm lands are complex,
6F. R. Fosberg, "The Preservation of
Man's Environment," Proceedings of the
Ninth Pacific Science Congress, 1957, Vol. 20,
1958, p. 160.
stable, highly productive, and beautiful.
The pervasive effect of this harmony of
man-nature is reflected in a language
remarkable in its descriptive power of
nature, a poetry succinct yet capable of
the finest shades of meaning, a superb
painting tradition in which nature is
the icon, an architecture and town
building of astonishing skill and beauty,
and, not least, an unparalleled garden
art in which nature and the garden are
the final metaphysical symbol.
In the Western tradition, farming in
Denmark and England has sustained
high productivity for two or more cen-
turies, appears stable, and is very
beautiful; in the United States, com-
parable examples exist in Amish,
Mennonite, and Pennsylvania Dutch
farming.
Understanding of the relationship of
man to nature is more pervasive and
operative among farmers than any other

laymen. The farmer perceives the
source of his food in his crops of cereal,
vegetables, roots, beef, fish, or game.
He understands that, given a soil fer-
tility, his crop is directly related to
inputs of organic material, fertilizer,
water, and sunlight. If he grows cotton
or flax or tends sheep, he is likely to
know the source of the fibers of his
clothes. He recognizes timber, peat,
and hydroelectric power as sources of
fuel; he may well know of the organic
source of coal and petroleum. Experi-
ence has taught him to ensure a func-
tional separation between septic tank
and well, to recognize the process of
erosion, runoff, flood and drought, the
differences of altitude and orientation.
As a consequence of this acuity, the
farmer has developed a formal expres-
sion which reflects an understanding of
the major natural processes. Charac-
teristically, high ground and steep
slopes are given over to forest and
woodland as a source of timber, habitat
for game, element in erosion control,
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and water supply. The more gently
sloping meadows below are planted to
orchards, above the spring frost line, or
in pasture. Here a seep, spring, or well
is often the source of water supply. In
the valley bottom, where floods have
deposited rich alluvium over time, is
the area of intensive cultivation. The
farm buildings are related to conditions
of climate and microclimate, above the
flood plain, sheltered and shaded by
the farm woodland. The septic tank is
located in soils suitable for this purpose
and below the elevation of the water
source.
Here, at the level of the farm, can
be observed the operation of certain
simple, empirical rules and a formal ex-
pression which derives from them. The
land is rich, and we find it beautiful.
Clearly, a comparable set of simple
rules is urgently required for the city
and the metropolis. The city dweller
is commonly unaware of these natural
processes, ignorant of his dependence
upon them. Yet the problem of the
place of nature in the city is more dif-
ficult than that of the farmer. Nature,
as modified in farming, is intrinsic to
the place. The plant community is

relatively immobile, sunlight falls upon
the site as does water, nutrients are
cycled through the system in place.
Animals in ecosystems have circum-
scribed territories, and the conjunction
of plants and animals involves a utiliza-
tion and cycling of energy and ma-
terials in quite limited areas. The
modern city is, in this respect, pro-
foundly different in that major natural
processes which sustain the city, provide
food, raw materials for industry, com-
merce, and construction, resources of
water, and pure air are drawn not from
the city or even its metropolitan area
but from a national and even interna-
tional hinterland. The major natural
processes are not intrinsic to the locus
of the city and cannot be.
NATURE IN THE METROPOLIS
In the process of examining the place
of nature in the city of man, it might
be fruitful to consider the role of
nature in the metropolitan area ini-
tially, as here, in the more rural fringes,
can still be found analogies to the
empiricism of the farmer. Here the
operative principle might be that natu-
ral processes which perform work or
offer protection in their natural form
without human effort should have a

presumption in their favor. Planning
should recognize the values of these
processes in decision-making for pros-
pective land uses.
A more complete understanding of
natural processes and their interactions
must await the development of an eco-
logical model of the metropolis. Such
a model would identify the regional
inventory of material in atmosphere,
hydrosphere, lithosphere, and biosphere,
identify inputs and outputs, and both
describe and quantify the cycling and
recycling of materials in the system.
Such a model would facilitate recogni-
tion of the vital natural processes and
their interdependence which is denied
today. Lacking such a model, it is
necessary to proceed with available
knowledge. On a simpler basis, we can
say that the major inputs in biological
systems are sunlight, oxygen-carbon di-
oxide, food (including nutrients), and
water. The first three are not limiting
in the metropolis; water may well be
limiting both as to quantity and qual-
ity. In addition, there are many other
reasons for isolating and examining
water in process. Water is the single
most specific determinant of a large
number of physical processes and is in-
dispensible to all biological processes.
Water, as the agent of erosion and
sedimentation, is causal to geological
evolution, the realities of physiography.

Mountains, hills, valleys, and plains
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experience variety of climate and
microclimate consequent upon their
physiography; the twin combination of
physiography and climate determines
the incidence and distribution of plants
and animals, their niches, and habitats.
Thus, using water as the point of
departure, we can recognize its impact
on the making of mountains and lakes,
ridges and plains, forests and deserts,
rivers, streams and marshes, the dis-
tribution of plants and animals. Lack-
ing an ecological model, we may well
select water as the best indicator of
natural process. In any watershed, the
uplands represent the majority of the
watershed area. Assuming equal dis-
tribution of precipitation and ground
conditions over the watershed, the maxi-
mum area will produce the maximum
runoff. The profile of watersheds tends
to produce the steeper slopes in the up-
lands with the slope diminishing toward
the outlet. The steeper the slope, the

greater is the water velocity. This
combination of maximum runoff links
maximum volume to maximum velocity
-the two primary conditions of flood
and drought. These two factors in
turn exacerbate erosion, with the conse-
quence of depositing silt in stream
beds, raising flood plains, and increasing
intensity and incidence of floods in
piedmont and estuary.
The natural restraints to flooding and
drought are mainly the presence and
distribution of vegetation, particularly
on the uplands and their steep slopes.
Vegetation absorbs and utilizes consid-
erable quantites of water; the surface
roots, trunks of trees, stems of shrubs
and plants, the litter of forest floor
mechanically retard the movement of
water, facilitating percolation, increasing
evaporation opportunity. A certain
amount of water is removed tempo-
rarily from the system by absorption
into plants, and mechanical retardation
facilitates percolation, reduces velocity,
and thus diminishes erosion. In fact,
vegetation and their soils act as a
sponge restraining extreme runoff, re-
leasing water slowly over longer periods,
diminishing erosion and sedimentation,
in short, diminishing the frequency and
intensity of oscillation between flood
and drought.

Below the uplands of the watershed
are characteristically the more shallow
slopes and broad plains of the piedmont.
Here is the land most often developed
for agriculture. These lands, too, tend
to be favored locations for villages,
towns, and cities. Here, forests are
residues or the products of regeneration
on abandoned farms. Steep slopes in
the piedmont are associated with
streams and rivers. The agricultural
piedmont does not control its own de-
fenses. It is defended from flood and
drought by the vegetation of the up-
lands. The vegetation cover and con-
servation practices in the agricultural
piedmont can either exacerbate or di-
minish flood and drought potential; the
piedmont is particularly vulnerable to
both.
The incidence of flood and drought
is not alone consequent upon the
upland sponge but also upon estuarine
marshes, particularly where these are
tidal. Here at the mouth of the water-
shed at the confluence of important
rivers or of river and sea, the flood
component of confluent streams or the
tidal component of floods assumes great
importance. In the Philadelphia metro-
politan area, the ocean and the estuary
are of prime importance as factors in
flood. A condition of intense precipita-
tion over the region combined with high

tides, full estuary, and strong onshore
winds combines the elements of poten-
tial flood. The relation of environmental
factors of the upland component and
the agricultural piedmont to flood and
drought has been discussed. The estua-
rine marshes and their vegetation con-
9
2017 21:16:15 UTC
stitute the major defense against the
tidal components of floods. These areas
act as enormous storage reservoirs ab-
sorbing mile-feet of potentially destruc-
tive waters, reducing flood potential.
This gross description of water-
related processes offers determinism for
the place of nature in the metropolis.
From this description can be isolated
several discrete and critical phases in
the process. Surface water as rivers,
streams, creeks, lakes, reservoirs, and
ponds would be primary; the particular
form of surface water in marshes would
be another phase; the flood plain as the
area temporarily occupied by water

would be yet another. Two critical
aspects of ground water, the aquifer and
its recharge areas, could be identified.
Agricultural land has been seen to be a
product of alluvial deposition, while
steep slopes and forests play important
roles in the process of runoff. If we
could identify the proscriptions and
permissiveness of these parameters to
other land use, we would have an ef-
fective device for discriminating the
relative importance of different roles
of metropolitan lands. Moreover, if the
major divisions of upland, piedmont,
and estuary and the processes enumer-
ated could be afforded planning recogni-
tion and legislative protection, the met-
ropolitan area would derive its form
from a recognition of natural process.
The place of nature in the metropolis
would be reflected in the distribution of
water and flood plain, marshes, ridges,
forests, and farm land, a matrix of
natural lands performing work or of-
fering protection and recreational op-
portunity distributed throughout the
metropolis.
This conception is still too bald; it
should be elaborated to include areas
of important scenic value, recreational
potential, areas of ecological, botanical,
geological, or historic interest. Yet,
clearly, the conception, analogous to the

empiricism of the farmer, offers oppor-
tunity for determining the place of
nature in the metropolis.
NATURE IN THE CITY
The conception advocated for the
metropolitan area has considerable rele-
vance to the problem of the place of
nature in the city of man. Indeed, in
several cities, the fairest image of nature
exists in these rare. occasions where
river, flood plain, steep slopes and
woodlands have been retained in their
natural condition-the Hudson and
Palisades in New York, the Schuylkill
and Wissahickon in Philadelphia, the
Charles River in Boston and Cam-
bridge. If rivers, flood plains, marshes,
steep slopes, and woodlands in the city
were accorded protection to remain in
their natural condition or were retrieved
and returned to such a condition where
possible, this single device, as an aspect
of water quality, quantity, flood and
drought control, would ensure for many
cities an immeasurable improvement in
the aspect of nature in the city, in ad-
dition to the specific benefits of a
planned watershed. No other device
has such an ameliorative power. Quite
obviously, in addition to benefits of
flood control and water supply, the

benefits of amenity and recreational op-
portunity would be considerable. As
evidence of this, the city of Philadelphia
has a twenty-two mile water front on
the Delaware. The most grandiose re-
quirements for port facilities and water-
related industries require only eight
miles of water front. This entire water
front lies in a flood plain. Levees and
other flood protection devices have
been dismissed as exorbitant. Should
this land be transformed into park,
it would represent an amelioration in
Philadelphia of incomparable scale.
Should this conception of planning
for water and water-related parameters
10
2017 21:16:15 UTC
be effectuated, it would provide the
major framework for the role of nature
in the city of man. The smaller ele-
ments of the face of nature are more
difficult to justify. The garden and
park, unlike house, shop, or factory,
have little "functional" content. They

are, indeed, more metaphysical symbol
than utilitarian function. As such, they
are not amenable to quantification or
the attribution of value. Yet it is fre-
quently the aggregation of these gardens
and spaces which determines the human-
ity of a city. Values they do have.
This is apparent in the flight to the
suburbs for more natural environments
-a self-defeating process of which the
motives are clear. Equally, the.selec-
tion of salubrious housing location in
cities is closely linked to major open
spaces which reflects the same impulse.
The image of nature at this level is
most important, the cell of the home,
the street, and neighborhood. In the
city slum, nature exists in the backyard
ailanthus, sumac, in lice, cockroach, rat,
cat, and mouse; in luxury highrise,
there are potted trees over parking
garages, poodles, and tropical fish. In
the first case, nature reflects "disturb-
ance" to the ecologist; it is somewhat
analogous to the scab on a wound, the
first step of regeneration towards equi-
librium, a sere arrested at the most
primitive level. In the case of the
luxury highrise, nature is a canary in
a cage, surrogate, an artifice, forbidden
even the prospect of an arrested sere.
Three considerations seem operative
at this level of concern. The first is that

the response which nature induces, tran-
quility, calm, introspection, openness to
order, meaning and purpose, the place
of values in the world of facts, is similar
to the evocation from works of art.
Yet nature is, or was, abundant; art
and genius are rare.
The second consideration of some im-
portance is that nature in the city is
very tender. Woodlands, plants, and
animals are very vulnerable to human
erosion. Only expansive dimensions
will support self-perpetuating and self-
cleansing nature. There is a profound
change between such a natural scene
and a created and maintained landscape.
The final point is related to the pre-
ceding. If the dimensions are appropri-
ate, a landscape will perpetuate itself.
Yet, where a site has been sterilized,
built upon, buildings demolished, the
problem of creating a landscape, quite
apart from creating a self-perpetuating
one, is very considerable and the costs
are high. The problems of sustaining
a landscape, once made, are also con-
siderable; the pressure of human erosion
on open space in urban housing and
the inevitable vandalism ensure that
only a small vocabulary of primitive
and hardy plants can survive. These
factors, with abnormal conditions of
ground water, soil air, atmospheric

pollution, stripping, and girdling, limit
nature to a very constricted image.
THE FUTURE
Perhaps, in the future, analysis of
those factors which contribute to stress
disease will induce inquiry into the
values of privacy, shade, silence, the
positive stimulus of natural materials,
and the presence of comprehensible
order, indeed natural beauty. When
young babies lack fondling and mother
love, they sometimes succumb to mo-
ronity and death. The dramatic reversal
of this pattern has followed simple
maternal solicitude. Is the absence of
nature-its trees, water, rocks and
herbs, sun, moon, stars and changing
seasons-a similar type of deprivation?
The solicitude of nature, its essence if
not its image, may be seen to be vital.
Some day, in the future, we may be
able to quantify plant photosynthesis
in the city and the oxygen in the atmos-
11
2017 21:16:15 UTC

phere, the insulation by plants of lead
from automobile exhausts, the role of
diatoms in water purification, the amel-
ioration of climate and microclimate by
city trees and parks, the insurance of
negative ionization by fountains, the
reservoirs of air which, free of combus-
tion, are necessary to relieve inversion
pollution, the nature-space which a bio-
logical inheritance still requires, the
stages in land regeneration and the
plant and animal indicators of such
regeneration, indeed, perhaps, even the
plant and animal indicators of a healthy
environment. We will then be able to
quantify the necessities of a minimum
environment to support physiological
man. Perhaps we may also learn what
forms of nature are necessary to satisfy
the psychological memory of a biological
ancestry.
Today, that place where man and
nature are in closest harmony in the
city is the cemetery. Can we hope for
a city of man, an ecosystem in dynamic
equilibrium, stable and complex? Can
we hope for a city of man, an eco-
system with man dominant, reflecting
natural processes, human and non-
human, in which artifice and nature
conjoin as art and nature, in a natural
urban environment speaking to man

as a natural being and nature as the
environment of man? When we find
the place of nature in the city of man,
we may return to that enduring and
ancient inquiry-the place of man in
nature.
12
2017 21:16:15 UTC
Contentsimage 1image 2image 3image 4image 5image 6image
7image 8image 9image 10image 11image 12Issue Table of
ContentsThe Annals of the American Academy of Political and
Social Science, Vol. 352, Mar., 1964Front Matter [pp.i-
202b]Foreword [p.iv]The Place of Nature in the City of Man
[pp.1-12]Physical and Mental Health in the City [pp.13-
24]Urban Social Differentiation and the Allocation of Resources
[pp.25-32]Culture Change and the Planner [pp.33-38]Urban
Economic Development [pp.39-47]Administrative and Fiscal
Considerations in Urban Development [pp.48-61]The Political
Side of Urban Development and Redevelopment [pp.62-73]The
Urban Pattern [pp.74-83]The Public Art of City Building
[pp.84-94]City Schools [pp.95-106]Housing and Slum
Clearance: Elusive Goals [pp.107-118]Social-Welfare Planning
[pp.119-128]Recreation and Urban Development: A Policy
Perspective [pp.129-140]Urban Transportation Criteria [pp.141-
151]Supplement: Theoretical Economics [pp.152-164]Book
DepartmentOther Books [pp.226-230]Sociologyuntitled [pp.165-
166]untitled [pp.166-167]untitled [pp.167-168]untitled [pp.168-
169]untitled [pp.169-170]untitled [p.170]untitled [pp.170-
174]untitled [p.174]untitled [pp.174-175]untitled [pp.175-
176]Economicsuntitled [pp.176-177]untitled [pp.177-

178]untitled [p.178]untitled [p.179]untitled [pp.179-
183]untitled [p.183]untitled [pp.183-184]untitled [pp.184-
185]untitled [pp.185-186]Politics and Governmentuntitled
[pp.186-187]untitled [pp.187-188]untitled [pp.188-189]untitled
[pp.189-190]untitled [p.190]untitled [pp.190-191]untitled
[pp.191-192]untitled [p.192]untitled [pp.192-193]untitled
[pp.193-194]untitled [pp.194-195]International Relations and
Foreign Policyuntitled [pp.195-196]untitled [pp.196-
200]untitled [pp.200-201]untitled [p.201]untitled [pp.201-
202]untitled [pp.202-203]Asia and Africauntitled [pp.203-
204]untitled [pp.204-205]untitled [pp.205-206]untitled
[p.206]untitled [pp.206-207]untitled [pp.207-208]untitled
[p.211]untitled [pp.211-212]untitled [pp.212-213]untitled
[pp.213-214]untitled [p.214]Europeuntitled [pp.214-
218]untitled [pp.218-219]Historyuntitled [p.219]untitled
[p.222]untitled [pp.222-223]untitled [p.224]untitled [pp.224-
225]untitled [pp.225-226]Back Matter [pp.231-234]
November 2001 / Vol. 51 No. 11 • BioScience 933
Articles
The tapestry of life on Earth is unraveling as
humansincreasingly dominate and transform natural ecosys-
tems. Scarce resources and dwindling time force conserva-
tionists to target their actions to stem the loss of biodiversity—
a pragmatic approach, given the highly uneven distribution
of species and threats (Soulé and Kohm 1989, Olson and
Dinerstein 1998, Mace et al. 2000, Myers et al. 2000). Unfor-

tunately, the ability to focus strategically is hindered by the ab-
sence of a global biodiversity map with sufficient biogeo-
graphic resolution to accurately reflect the complex
distribution of the Earth’s natural communities. Without
such a map, many distinctive biotas remain unrecognized. In
this article, we address the disparity in resolution between
maps currently available for global conservation planning
and the reality of the Earth’s intricate patterns of life. We
have developed a detailed map of the terrestrial ecoregions of
the world that is better suited to identify areas of outstand-
ing biodiversity and representative communities (Noss 1992).
We define ecoregions as relatively large units of land
containing
a distinct assemblage of natural communities and species, with
boundaries that approximate the original extent of natural
communities prior to major land-use change.
Our ecoregion map offers features that enhance its utility
for conservation planning at global and regional scales: com-
prehensive coverage, a classification framework that builds on
existing biogeographic knowledge, and a detailed level of
biogeographic resolution. Ecoregions reflect the distribu-
tions of a broad range of fauna and flora across the entire
planet, from the vast Sahara Desert to the diminutive Clip-
perton Island (eastern Pacific Ocean). They are classified
within a system familiar to all biologists—biogeographic
realms and biomes. Ecoregions, representing distinct biotas
(Dasmann 1973, 1974, Udvardy 1975), are nested within the
biomes and realms and, together, these provide a framework
for comparisons among units and the identification of rep-
resentative habitats and species assemblages.
Although our ecoregions are intended primarily as units
for conservation action, they are built on the foundations of
classical biogeography and reflect extensive collaboration
with over 1000 biogeographers, taxonomists, conservation bi-

ologists, and ecologists from around the world. Consequently,
ecoregions are likely to reflect the distribution of species and
communities more accurately than do units based on global
and regional models derived from gross biophysical features,
such as rainfall and temperature (Holdridge 1967, Walter
and Box 1976, Schulz 1995, Bailey 1998), vegetation structure
(UNESCO 1969, deLaubenfels 1975, Schmidthüsen 1976), or
David Olson (e-mail [email protected]), Eric Dinerstein, Eric
Wikra-
manayake, Neil Burgess, George Powell, Jennifer D’Amico,
Holly
Strand, John Morrison, Colby Loucks, Thomas Allnutt, John
Lamoreux,
Wesley Wettengel, and Kenneth Kassem are conservation
scientists
in the Conservation Science Program at World Wildlife Fund–
US, Wash-
ington, DC 20037. Emma Underwood is a doctoral candidate in
the
Graduate Group in Ecology, Information Center for the
Environment,
University of California, Davis, CA 95616. Illanga Itoua is a
conser-
vation biologist, 78230 Le Pecq, France. Taylor Ricketts is a
post-
doctoral researcher at the Center for Conservation Biology,

Depart-
ment of Biological Sciences, Stanford University, Palo Alto, CA
94305. Yumiko Kura is a conservation specialist with the World
Re-
sources Institute, Washington, DC 20002. Prashant Hedao is a
conservation GIS specialist with Environmental Systems
Research
Institute, Inc., Redlands, CA 92373. © 2001 American Institute
of
Biological Sciences
Terrestrial Ecoregions of
the World: A New Map of
Life on Earth
DAVID M. OLSON, ERIC DINERSTEIN, ERIC D.
WIKRAMANAYAKE, NEIL D. BURGESS, GEORGE V. N.
POWELL,
EMMA C. UNDERWOOD, JENNIFER A. D’AMICO,
ILLANGA ITOUA, HOLLY E. STRAND, JOHN C.
MORRISON, COLBY
J. LOUCKS, THOMAS F. ALLNUTT, TAYLOR H. RICKETTS,
YUMIKO KURA, JOHN F. LAMOREUX, WESLEY W.
WETTENGEL, PRASHANT HEDAO, AND KENNETH R.
KASSEM
A NEW GLOBAL MAP OF TERRESTRIAL
ECOREGIONS PROVIDES AN INNOVATIVE
TOOL FOR CONSERVING BIODIVERSITY

934 BioScience • November 2001 / Vol. 51 No. 11
Articles
spectral signatures from remote-sensing data (Defries et al.
1995, Loveland and Belward 1997). None of these other ap-
proaches emphasizes the importance of endemic genera and
families (higher taxa), distinct assemblages of species, or the
imprint of geological history, such as the strong influence of
past glaciations or Pleistocene land bridges, on the distribu-
tion of plants and animals.
Existing maps of global biodiversity have been ineffective
planning tools because they divide the Earth into extremely
coarse biodiversity units. These units are typically well beyond
the size of landscapes tractable for designing networks of
conservation areas, the largest of protected areas, or the
50,000 km2 threshold for restricted-range species (Stattersfield
et al. 1998) that are of particular concern (Stuart Pimm
[Center for Environmental Research and Conservation, Co-
lumbia University, NY], personal communication, 2000).
The average size of our ecoregions is roughly 150,000 km2
(median 56,300 km2), whereas the biotic provinces of Udvardy
(1975) have an approximate mean of 740,000 km2 (median
306,000 km2) and the biodiversity hotspots of Myers et al.
(2000), which represent threatened regions with high con-
centrations of endemic species, have an approximate mean
of 787,760 km2 (median 324,000 km2).
We subdivided the terrestrial world into 14 biomes and
eight biogeographic realms (Figure 1). Nested within these are
867 ecoregions (Figure 2). This is roughly a fourfold increase

in resolution over that of the 198 biotic provinces of Dasmann
(1974) and the 193 units of Udvardy (1975). The increased
resolution is most apparent in the tropics (between the Trop-
ics of Cancer and Capricorn) where Dasmann (1974) and Ud-
vardy (1975) identify 115 and 117 units, respectively, compared
with 463 found in the ecoregion map. Biodiversity assessments
that employ large biotic provinces or hotspots often fail to dis-
cern smaller but highly distinctive areas, which may result in
these areas receiving insufficient conservation attention. The
island of New Guinea is illustrative. Dasmann and Udvardy
treat the island as a single unit, whereas the new terrestrial map
distinguishes 12 ecoregions: four lowland and four montane
broadleaf forests, one alpine scrub ecoregion along the cen-
tral cordillera, a mangrove forest, a freshwater swamp forest,
and a savanna–grassland, all with distinct biotas and ecolog-
ical conditions.
The delineation of ecoregions
We began by accepting the biogeographic realms of Pielou
(1979) and Udvardy (1975) and modifying the biome systems
of Dinerstein et al. (1995) and Ricketts et al. (1999) (Figure
1). We then consulted existing global maps of floristic or
zoogeographic provinces (e.g., Rübel 1930, Gleason and
Cronquist 1964, Good 1964), global and regional maps of
units based on the distribution of selected groups of plants
and animals (e.g., Hagmeier 1966), the world’s biotic province
maps (Dasmann 1973, 1974, Udvardy 1975), and global maps
of broad vegetation types (e.g., UNESCO 1969, deLaubenfels
1975, Schmidthüsen 1976). These were useful for evaluating
the extent of realms and biomes, the first two tiers in our hi-
erarchical classification. We then identified published re-
gional classification systems to be used as a baseline for ecore-
gion boundaries. Data and consultations from regional experts
were also important for final ecoregion delineations.

The use of widely recognized biogeographic maps as a
basis for ecoregions enhances the utility of the map as a plan-
ning tool in different regions.
For example, White’s (1983)
phytogeographic regions serve
as the basis for the ecoregions
of the Afrotropics. The Aus-
tralian ecoregions are derived
from Thackway and Cresswell’s
(1995) biogeographic region-
alization. Nearctic ecoregions
are adapted from the ecoregion
systems of Omernik (1995),
Gallant et al. (1995), Wiken et
al. (1989), and Rzedowski
(1978). A more diverse set of
sources was used for the
Neotropics, including habitat
classifications for Brazil from
the Instituto Brasilero de Ge-
ografia Estatística (IBGE 1993),
the vegetation maps of Huber
and Alarcon (1988) and Hu-
ber et al. (1995) for Venezuela
and Guyana, and Holdridge’s
(1977) life zones for Central
America. The western Palearc-
Figure 1. The ecoregions are categorized within 14 biomes and
eight biogeographic realms
to facilitate representation analyses.
tic ecoregions (except Africa) were developed in concert with

the DMEER (2000) project. The ecoregions of Russia are
adapted from Kurnaev (1990) and Isachenko and colleagues
(1988), Japan from Miyawaki (1975), China from the systems
developed by the Chinese Vegetation Map Compilation Com-
mittee (1979) and the Changchun Institute of Geography and
Chinese Academy of Sciences (1990), and Southwest Asia
from Zohary (1973). The major divisions for Indo-Malayan
ecoregions are based on the MacKinnon (1997) units that
build upon Dasmann’s and Udvardy’s biotic provinces. A key
to the terrestrial ecoregions of the world map (Figure 2), the
sources for ecoregions, technical descriptions, and digital data
are available at the Web site www.worldwildlife.org/science.
Most existing systems required that units be aggregated or
divided, or that boundaries be modified, to achieve three
goals: (1) match recognized biogeographic divisions inade-
quately reflected in that system, (2) achieve a similar level of
biogeographic resolution of units, and (3) match units and
boundaries in adjacent systems, when necessary. Where widely
accepted biogeographic maps were unavailable, we relied
first on landforms and second on vegetation to inform the bi-
otic divisions. For example, montane and lowland habitats
support distinct biotic communities and dynamics. These were
separated where they occurred over extensive areas. Detailed
vegetation maps were then consulted. Vegetation is an im-
portant proxy for both plants and invertebrates, which together
constitute the vast majority of species. Most invertebrates, and
to some extent vertebrates, are associated with different plant
communities, particularly where ecoclimatic differences are
strong (e.g., tropical wet forest versus tropical dry forest).
The appropriate delineation of ecoregions was obvious in
many cases. The sand pine scrubs of central Florida, for ex-
ample, support many endemic species and higher taxa, and
one can confidently discern the distinctiveness of its biota as
well as its geographic extent. Other ecoregions required closer

scrutiny to discern the influ-
ence of historic events on
present-day distributions. For
example, the effects of changes
in sea level and land bridges in
the Philippines archipelago
during the Pleistocene have re-
sulted in several island ecore-
gions in close proximity har-
boring many unique taxa
(Heaney 1986, 1991). Delin-
eation of ecoregions varied
slightly in boreal and polar
habitats, where species assem-
blages are relatively homoge-
neous across large regions.
Thus, dynamics and processes
were emphasized, such as ma-
jor variations in climate, fire
disturbance regimes, and large
vertebrate migrations (Ricketts
et al. 1999).
Three caveats are appropriate for all biogeographic map-
ping approaches. First, no single biogeographic framework is
optimal for all taxa. Ecoregions reflect the best compromise
for as many taxa as possible. Second, ecoregion boundaries
rarely form abrupt edges; rather, ecotones and mosaic habi-
tats bound them. Third, most ecoregions contain habitats that
differ from their assigned biome. For example, rainforest
ecoregions in Amazonia often contain small edaphic savan-
nas. More detailed biogeographic analyses should map the less
dominant habitat types that occur within the larger ecoregions,
and ecoregion conservation strategies should address their re-
quirements.

Ecoregions as a tool for conservation
How can a map of the world’s ecoregions contribute to con-
serving biodiversity? Our ecoregion map has already been used
as a biogeographic framework to highlight those areas of the
world that are most distinctive or have high representation
value and are therefore worthy of greater attention (Olson and
Dinerstein 1998, Ricketts et al. 1999, Wikramanayake et al.
2001). Ecoregions were ranked by the distinctiveness of their
biodiversity features—species endemism, the rarity of higher
taxa, species richness, unusual ecological or evolutionary
phenomena, and global rarity of their habitat type (e.g.,
Mediterranean-climate woodlands and scrub and temperate
rainforests). Ecoregions can also be ranked by threats to bio-
diversity, the status of their natural habitats and species, and
degree of protection (Dinerstein et al. 1995, Olson and Din-
erstein 1998, Ricketts et al. 1999, Wikramanayake et al. 2001).
Using this framework, biologists can examine one of the
most interesting biological problems: the concordance and
mismatches in patterns of richness and endemism for indi-
cator taxa, often birds and mammals, used in conservation pri-
ority setting (Stattersfield et al. 1998, Fonseca et al. 2000,
Articles
Figure 2. The map of terrestrial ecoregions of the world
recognizes 867 distinct units,
roughly a fourfold increase in biogeographic discrimination
over that of the 193 units of
Udvardy (1975). Maps of freshwater and marine ecoregions are
similarly needed for
conservation planning.

Mace et al. 2000). As an illus-
tration, patterns of richness and
endemism by ecoregion for the
world’s 4,600+ terrestrial mam-
mal species reveal some major
differences. The three richest
mammal assemblages are in the
northern Indochina subtropi-
cal forests, the southwestern
Amazon moist forests, and the
central Zambezian miombo
woodlands (Figure 3), whereas
the ecoregions with the highest
number of endemic mammals
are the Central Range montane
forests of New Guinea, the Al-
bertine Rift montane forests of
Central Africa, and the Sulawesi
montane forests (Figure 4).
Similar analyses for birds, her-
petofauna, and vascular plants
are under way, to be incorpo-
rated into a database that can be continually improved as new
data are acquired. This ecoregion–species database will com-
plement emerging grid-based species datasets by providing in-
sights into the biogeographic relationship among cells (Brooks
et al. 2001).
The ecoregion map complements global priority-setting
analyses, such as Global 200 (Olson and Dinerstein 1998) and
Hotspots (Myers et al. 2000), by providing an even finer level
of resolution to assess biodiversity features. For example, the
25 terrestrial hotspots identified by Myers et al. (2000) amal-
gamate 414 of the 867 ecoregions of the world, and the 237

units of Global 200 contain 402 terrestrial ecoregions. On our
map, the Indo–Burma hotspot
(Mittermeier et al. 1999, Myers
et al. 2000) covers 37 terrestrial
ecoregions and Global 200’s
eastern Himalayan forests (Ol-
son and Dinerstein 1998) en-
compass four terrestrial ecore-
gions. The rich mosaic of the
map’s ecoregions calls atten-
tion to the importance of global
biodiversity, including those
ecoregions that lie outside the
species-rich tropics.
New ways of looking at bio-
diversity loss and global
threats—from climate change
to oil exploration, mining, road
development, and logging—
are facilitated by this detailed
map of ecoregions. Currently, a
consortium of conservation or-
ganizations, museums, and
herbaria are using this base map to frame discussions with log-
ging companies and wood product retailers about reducing
the loss of forest biodiversity. It is also being used as a strate-
gic tool to determine conservation investments for the World
Bank, the US Agency for International Development, the
World Wildlife Fund, the World Resources Institute, The Na-
ture Conservancy, and several foundations (Dinerstein et al.
1995, Roca et al. 1997, Olson and Dinerstein 1998).
Conservation strategies that consider biogeographic units
at the scale of ecoregions are ideal for protecting a full range

of representative areas, conserving special elements, and en-
suring the persistence of populations and ecological processes,
Articles
Figure 3. The relative richness of terrestrial mammal species by
ecoregion is depicted.
Warmer colors denote ecoregions containing richer
assemblages.
Figure 4. The level of species endemism for terrestrial mammals
shows different patterns
than that of richness. Warmer colors denote ecoregions
containing more endemic species.
particularly those that require the largest areas or are most sen-
sitive to anthropogenic alterations (Noss et al. 1999, Soulé and
Terborgh 1999, Groves et al. 2000, Margules and Pressey
2000). Some of the most promising tools for designing net-
works of conservation areas—gap analysis, equal-area grid
analyses, complementarity analyses, and other reserve selec-
tion algorithms (Kiester et al. 1996, Margules and Pressey 2000,
Williams et al. 1997, 2000)—will be more robust if con-
ducted within the context of biologically defined units such
as ecoregions, as the distribution of species and communities
rarely coincides with political units. An ecoregion perspective
can also help identify whether conservation areas are redun-
dant or complementary across political boundaries.
Ecoregions approximate the dynamic arena within which
ecological processes most strongly interact (Orians 1993).
This critical component of the ecoregion concept allows us

to expand the scope of factors considered in conservation plan-
ning to include ecological phenomena as well as distributions
of species. Preserving the migrations in East Africa, large
predator–prey interactions in the South Asian jungles (Joshi
et al. 2001), or sufficient forest cover in the Amazon Basin to
maintain rainfall patterns requires conservation efforts across
entire ecoregions.
Fortunately, conducting conservation assessments within
the framework of larger biogeographic units is an approach
that is gaining support in all of the major international con-
servation organizations and in many government agencies
(Groves et al. 2000, Johnson et al. 1999, Mittermeier et al.
1999,
Ricketts et al. 1999). Ecoregion-level strategies are receiving
increased funding from major conservation donors. This
growing interest offers encouragement that ecoregion maps
and analyses can heighten awareness about the urgency of bio-
diversity loss and play an important role in conserving the ex-
traordinary variety of life on Earth.
Acknowledgments
We greatly appreciate the thousand-plus regional experts
who provided invaluable knowledge and assistance in the
development of this map. We appreciate the support of the
World Bank; the Commission for Environmental Coopera-
tion; Comisión Nacional Para el Conocimiento y Uso de la
Biodiversidad; Instituto Nacional de Estatística, Geografia e
Informática; the US Agency for International Development;
the Digital Map of European Ecological Regions working
group; the European Union; the Fitzpatrick Institute; the
Biodiversity Support Program; the World Resources Institute;
The Nature Conservancy; and the World Wildlife Fund Net-
work. We thank J. Leape, D. Wood, J. Martin-Jones, G. Hem-
ley, A. Carroll, R. Sayre, J. Soberón, E. Iñigo, K. Redford, J.
Robinson, U. Bohn, O. Ostermann, P. Regato, R. Thackway,

M. McKnight, M. Taye, and T. Green for their support and ex-
pertise. We thank A. Balmford, S. Pimm, G. Orians, L. Farley,
S. O’Connor, and S. Osofsky for reviewing earlier drafts of this
paper. We would also like to thank the National Geographic
Society, in collaboration with World Wildlife Fund, which has
provided 10 copies of the ecoregion map and teacher’s guides
to all public and private schools in the United States to pro-
mote biogeographic literacy and a better understanding of
global biodiversity. We are indebted to all of the individuals
who helped develop the World Wildlife Fund/National Ge-
ographic Society/Environmental Systems Research Institute,
Inc., educational Web site that uses this map to present pat-
terns of biodiversity from around the world (www.world-
wildlife.org/wildworld).
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Articles
REVIEW
Taking the “Waste” Out of
“Wastewater” for Human Water
Security and Ecosystem Sustainability
Stanley B. Grant,1,2* Jean-Daniel Saphores,1,3 David L.
Feldman,3 Andrew J. Hamilton,4

Tim D. Fletcher,5 Perran L. M. Cook,6 Michael Stewardson,2
Brett F. Sanders,1 Lisa A. Levin,7
Richard F. Ambrose,8 Ana Deletic,9 Rebekah Brown,10 Sunny
C. Jiang,1 Diego Rosso,1
William J. Cooper,1 Ivan Marusic11
Humans create vast quantities of wastewater through
inefficiencies and poor management of
water systems. The wasting of water poses sustainability
challenges, depletes energy reserves, and
undermines human water security and ecosystem health. Here
we review emerging approaches
for reusing wastewater and minimizing its generation. These
complementary options make
the most of scarce freshwater resources, serve the varying water
needs of both developed and
developing countries, and confer a variety of environmental
benefits. Their widespread adoption
will require changing how freshwater is sourced, used,
managed, and priced.
M
ore than 4 billion people live in parts
of the world where freshwater scarcity
directly threatens human water secu-
rity or river biodiversity (1). Threats to human
water security can be overcome by building cen-
tralized infrastructure that harvests, stores, treats,
and transports water for agricultural, industrial,
and municipal uses. For countries that can af-
ford it, this approach has greatly benefited hu-
man health and economic development, but it is

often energy-intensive and comes at a steep ec-
ological price. In the developing world, on the
other hand, an estimated 1 billion people lack
access to safe affordable drinking water, 2.7 bil-
lion lack access to sanitation, and many millions
die each year from preventable waterborne dis-
eases (2). Thus, developed and developing coun-
tries face separate but overlapping challenges.
In developed countries, existing water infrastruc-
ture needs reengineering to sustain a high stan-
dard of living while reducing its environmental
footprint and sustaining or restoring biodiver-
sity. In developing countries, affordable infra-
structure is needed to satisfy the water needs of
humans and to preserve aquatic ecosystems (1).
Meeting these twin challenges will require strik-
ing a balance between delivering new sources
of water and using water more productively
through pricing, conservation, and wastewater
reuse.
How Is Water Used and Wasted?
Water use can be classified as consumptive or
nonconsumptive, depending on how readily the
used water can be reused. Consumptive use con-
verts water into a form that cannot be reused. A
portion of the water used for irrigation, for ex-
ample, is evaporated, transpired, and incorpo-
rated into plant biomass. This consumed water is
unavailable for reuse in the watershed over time
scales of practical interest. In contrast, after non-
consumptive use, water can be captured, treated,
and reused. If a nonconsumptive use degrades
the quality of the water (for example, by adding
contaminants), it is said to generate wastewater.

An example of nonconsumptive use is the flush-
ing of a toilet, which converts drinking water
into domestic wastewater. In principle, domestic
wastewater can be collected, treated to remove
human pathogens and other contaminants, and
then reused for potable or nonpotable purposes.
Globally, the largest consumptive use of water is
for agriculture, whereas the largest nonconsump-
tive use of water is for industrial and municipal
supplies (3).
What Is Water Productivity and How
Can It Be Improved?
Addressing threats to human water security and
biodiversity will require getting the most out of
locally available water resources. But what does
that mean in practice? One way to evaluate wa-
ter use is to consider its “productivity,” defined
as the value of goods and services produced per
unit of water used. By improving water produc-
tivity, communities can enjoy the same goods
and services, generate less wastewater, and leave
more freshwater in streams, rivers, lakes, and
coastal estuaries to support biodiversity. Because
less water is harvested, treated, and transported,
fossil fuel consumption and greenhouse gas emis-
sions are reduced. Although water productivity
has steadily improved in the United States since
the mid-1970s, additional gains are possible both
here and around the world (4). In this Review,
we focus on three general strategies for improving
water productivity (Fig. 1): substituting higher-
quality water with lower-quality water where ap-
propriate, regenerating higher-quality water from
lower-quality water by treatment, and reducing
the volume of higher-quality water used to gen-

erate goods and services.
What Are the Opportunities for Substituting?
Many municipal, industrial, and agricultural uses
can be satisfied by lower-quality water. For ex-
ample, treated domestic wastewater that would
not be suitable for municipal water supplies may
be perfectly suitable for industrial cooling and
landscape irrigation, to name a few (5). Although
the use of treated wastewater in the United States
is currently limited (<5% of municipal supply), it
could be expanded to 17 teraliters per year (Tl
year–1) (~27% of municipal supply), providing a
new drought-resistant source of water in coastal
areas where treated wastewater is currently dis-
charged to the sea (6). Large-scale (centralized)
wastewater treatment and potable substitution
schemes can reduce overall energy consumption
and reduce greenhouse gas emissions. In southern
California, substituting potable water with treated
wastewater consumes less energy and generates
fewer greenhouse gases as compared to interbasin
transfers of water or desalination of seawater or
brackish groundwater (7).
Treated domestic wastewater is not the only
lower-quality water that can be exploited in po-
table substitution schemes. Hong Kong’s dual wa-
ter system, which has been in operation for over
50 years, supplies seawater for toilet flushing to
80% of its 7 million residents, cutting municipal
water use in the city by 20% (8). A triple-water
distribution system at Hong Kong’s International
Airport, consisting of freshwater, seawater, and
treated graywater from sinks and aircraft wash-
down, cuts municipal water use by over 50% (8).

Potable substitution can also be implemented
at neighborhood and single-home scales (Fig.
2). Rainwater (from roofs) and graywater (from
SPECIALSECTION
1Department of Civil and Environmental Engineering, E4130
Engineering Gateway, University of California, Irvine, CA
92697-2175, USA. 2Department of Infrastructure Engineer-
ing, Melbourne School of Engineering, Engineering Block D,
University of Melbourne, Parkville 3010, Victoria, Australia.
3Department of Planning, Policy, and Design, 202 Social Ecol-
ogy I, University of California, Irvine, CA 92697-7075, USA.
4Department of Agriculture and Food Systems, University of
Melbourne, 940 Dookie-Nalinga Road, Dookie College, Victoria
3647, Australia. 5Melbourne School of Land and Environment,
University of Melbourne, Burnley Campus, 500 Yarra Boule-
vard, Richmond, Victoria 3121, Australia. 6Water Studies
Centre,
School of Chemistry, Monash University, Victoria 3800,
Australia.
7Center for Marine Biodiversity and Conservation, Scripps
Insti-
tution of Oceanography, La Jolla, CA 92093-0218, USA. 8En-
vironmental Science and Engineering Program, University of
California, Los Angeles, CA 90095-1772, USA. 9Monash Water
for Liveability, Department of Civil Engineering, Building 60,
Monash University, Victoria 3800, Australia. 10Monash Water
for Liveability, School of Geography and Environmental Sci-
ence, Monash University, Clayton, Victoria 3800, Australia.
11Department of Mechanical Engineering, Melbourne School
of Engineering, Engineering Block E, University of Melbourne,
Parkville 3010, Victoria, Australia.
*To whom correspondence should be addressed. E-mail:

[email protected]
www.sciencemag.org SCIENCE VOL 337 10 AUGUST 2012
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laundry, dishwashing, and bathing) can be used
in place of drinking water for a variety of activ-
ities. The reuse of graywater for toilet flushing
and yard irrigation can cut household municipal
water use by 50% or more (9). The energy cost,
water savings, and reliability associated with rain-
water harvesting depend on engineering consid-
erations (e.g., contributing roof area and storage
tank volume), local climate, connected end uses
(e.g., toilet, laundry, and hot water), and temporal
patterns (10). In a case study of a model home in
Melbourne, Australia, the use of rainwater tanks
to supply water for laundry, dishwashing, toilets,
and an outside garden reduced household munic-

ipal water use by 40% (9). How-
ever, even in Melbourne, where
rainwater-harvesting schemes are
commonplace, they contribute a
modest 5 gigaliters (Gl) year−1
to the city’s overall water bud-
get, which represents 1.2% of the
city’s total water use and 1.4% of
its municipal supply (11).
Stormwater runoff from roads
and other impermeable surfaces
is another locally available source
of water, but here the challenge
is harvesting and storing the run-
off (which can be generated over
very short periods of time) and
adequately removing contaminants
(pathogens, metals, and organic
pollutants). These challenges can
be overcome through the integra-
tion of natural treatment systems
into the urban landscape, includ-
ing green roofs, rain gardens, bio-
filters, and constructed wetlands
(12). Processes responsible for
pollutant removal in natural treat-
ment systems include (12–15)
gravitational sedimentation of
large particles, pathogen remov-
al by solar ultraviolet (UV) inac-
tivation and predation, filtration
of colloidal contaminants, oxi-
dation of labile organics by hy-
drolysis and sunlight-generated

reactive oxygen species, precip-
itation of metals, and nitrogen removal by bacte-
rially mediated nitrification and denitrification in
sediments. Plants play a key role, taking up excess
nutrients and serving as both a source of organic
carbon to fuel denitrification, and a source of
oxygen through their root systems to fuel nitri-
fication. As runoff moves through natural treat-
ment systems, a portion of the water returns to
the atmosphere (evapotranspiration); a portion
infiltrates into the subsurface (groundwater re-
charge); and the rest can be harvested, stored,
and ultimately used for nonpotable purposes.
In Melbourne, stormwater harvesting is a rela-
tively minor component (5 Gl year−1 or 1.4% of
municipal water use) of the city’s water budget
(11), but including stormwater reuse schemes in
new greenfield and brownfield developments
until 2050 could result in a sevenfold increase in
nonpotable water availability for the city (35 Gl
year−1 or 9.8% of municipal water use) (16).
Integrating natural treatment systems into
urban landscapes confers many benefits beyond
improving human water security. In warmer cli-
mates, the evapotranspiration of runoff moder-
ates the urban heat island effect (17), whereas
infiltration recharges the groundwater and pro-
vides environmental water for local wetlands
and riparian zones (12). The construction of new
wetlands or reinvigoration of existing wetlands
creates habitats for resident and migratory spe-
cies and sustains biodiversity by enhancing habitat
heterogeneity, connectivity, and food web sup-

port (18). When storm water is locally detained
and retained throughout the catchment, less run-
off enters rivers and streams, pollutant loads are
reduced, and flow regimes more closely resem-
ble predevelopment conditions (19). As a result,
streams are less likely to overtop their banks and
cause flooding (20), and the negative effects of
urbanization on stream health and function, col-
lectively known as the “urban stream syndrome”
(21), can be mitigated (22).
What Are the Opportunities for Regeneration?
With adequate treatment, higher-quality water
can be regenerated from wastewater. Because
additional goods and services are produced every
time a parcel of water is recycled, regeneration
has the potential to significantly increase water
productivity. A prime example of regeneration
is potable reuse, in which wastewater is treated
with conventional and advanced methods and
then added back to the water supply either
directly (direct potable reuse) or indirectly, by
holding the water for a time in groundwater
or surface-water reservoirs (indirect potable
reuse) (5, 6).
Apart from a few small-scale facilities, direct
potable reuse is not practiced in the United States.
However, several indirect potable reuse facilities
are operational. The world’s largest is the Ground-
water Replenishment System (GWRS) in Foun-
tain Valley, California, which treats up to 97 Gl
year−1 of domestic wastewater using conventional
(primary and secondary sewage treatment) and ad-
vanced (microfiltration, reverse osmosis, and UV
disinfection) techniques (23). Water produced by

the GWRS provides approximately 20% of the
water needed to maintain the local groundwater
aquifer in Orange County, a primary source of mu-
nicipal supply for more than 2 million residents.
Substitution
Regeneration
Reduction
Higher-quality water
Lower-quality water
Treated water
Percent increase in water productivity
b Melbourne rainwater harvesting
b Melbourne stormwater harvesting
0 20 40 60 80 100
c H.K. triple water system
a
b
c
d
e
Nation

City
International airport
Groundwater basin
Residence
e Graywater reuse
e Rainwater harvesting
a U.S. wastewater reuse (potential)
b H.K. dual water system
b Melbourne stormwater harvesting (potential)
a U.S. wastewater reuse
b Windhoek direct potable reuse
d Fountain Valley GWRS
a Israel agricultural wastewater reuse
b Melbourne wastewater reuse (WSP)
a Singapore wastewater reuse
b Eliminate non-revenue water (poorly run utility)
b Dual-flush toilets in Florianopolis (best case)
b Dual-flush toilets in Florianopolis (worst case)
e High-efficiency toilet
e High-efficiency shower head
b Elimination of non-revenue water (well-run utility)
e High-efficiency clothes washer

Scale of interest
Fig. 1. (Left) Three complementary approaches for improving
the productivity of higher-quality water. The water level
in each glass shows how much water is used in producing a
fixed value of goods and services. Substitution uses lower-
quality water in place of higher-quality water for some
activities. Regeneration transforms lower-quality water into
higher-quality water by treatment. Reduction achieves the same
value of goods and services using less higher-quality
water. In these hypothetical examples, each option cuts by half
the use of higher-quality water and therefore doubles its
productivity. (Right) Percent increase in water productivity
associated with the 21 case studies described in the text (51).
These productivity improvements are illustrative only and will
vary substantially in practice. The scale at which a
particular water-saving intervention was implemented is
indicated. The bars are color-coded to match the three general
approaches for improving water productivity.
10 AUGUST 2012 VOL 337 SCIENCE www.sciencemag.org682
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Internationally, the longest-running example
of direct potable reuse is in Windhoek, Namibia,
where recycled wastewater (mostly domestic sew-
age) has been added to the potable water distribu-
tion system more or less continuously since the
late 1960s with no obvious adverse health effects
among the population of several hundred thou-
sand (24). The current facility produces enough
water (7.7 Gl year−1) to meet approximately 35%
of the city’s municipal water needs.
Among the centralized options for augment-
ing potable water supplies, potable reuse is pref-
erable to interbasin water transfers for several
reasons (25): (i) Interbasin water transfers re-
duce the water available at the source for critical
ecosystems and agricultural production; (ii) trans-
porting water over long distances can be energy-
and carbon-footprint–intensive; and (iii) the water
transmission systems are vulnerable to disruption
by natural and human-made disasters, such as
earthquakes and acts of terrorism. All three prob-
lems are evident in California, where the southern
part of the state has long relied on water imported
from sources located hundreds of kilometers to
the east and north. In 2001, an estimated 4% of
the electric power consumption in California was
used for water supply and treatment (largely trans-
portation) for urban and agricultural users; this
estimate increases to 7% if end uses in agricul-
ture (which are mainly related to pumping) are

included (26). The depletion of source waters in
the state has led to habitat deterioration, the de-
cline and extinction of native fish species, the
near-collapse of the Sacramento–San Joaquin
River Delta ecosystem (27), and the desiccation
of Owens Lake, whose dry lake bed is arguably
the single largest source of asthma- and cancer-
inducing respirable suspended particles in the
United States (28). Potable reuse also has advan-
tages relative to the desalination of seawater. By
one estimate, potable reuse consumes less than
one-half the energy [~1000 to 1500 kilowatt-hours
per megaliter (kWh Ml−1)] beyond conventional
treatment) required for the desalination of sea-
water (~3400 to 4000 kWh Ml−1) (25).
Relative to the classification scheme presented
in Fig. 1, some nonpotable wastewater reuse is
best described as regeneration, provided that the
treated effluent replaces water of equal or lower
quality, such as river diversions (Fig. 2). For exam-
ple, 73% of Israel’s municipal sewage is treated
and reused for agricultural irrigation, which is
equal to roughly 5% of the country’s total water use
(29) and 13% of its municipal supply. In Singa-
pore, 27 Gl year−1 of highly treated domestic waste-
water is used primarily for industrial applications,
which is equal to 5% of its total water use and 9%
of its municipal supply (30).
Relatively low-energy centralized approaches
for nonpotable wastewater reuse are also availa-
ble, such as waste stabilization ponds (WSPs), in
which sewage is directed through a series of open-
air shallow ponds where physical processes (floc-

culation and gravitational sedimentation), microbial
processes (algal growth, aerobic and anaerobic
heterotrophic metabolism, nitrification, and deni-
trification), and exposure to sunlight jointly remove
pathogens, organic contaminants, and nitrogen (31).
Effluent from WSPs can irrigate crops (Fig. 2)
or recharge groundwater aquifers, and the ponds
themselves may provide a much needed quasi-
wetland habitat for waterbird conservation (18).
The world’s largest WSP system, the Western
Treatment Plant in Melbourne, produces 40 Gl
year−1 of treated wastewater, equivalent to 11%
A
A
A
B
C C
DWTP
C
C
Biofilter
WWTP
WSP
C

Fig. 2. Practical examples of substitution (A), regeneration (B),
and reduction
(C) at the household scale. Substitution includes watering a
garden with rain-
water from a rainwater tank and flushing toilets and washing
laundry with treated
stormwater effluent from a biofilter. For regeneration, a waste
stabilization pond
(WSP) transforms sewage from the house into high-quality
water used for irri-
gating an orchard. Reduction includes repairing leaks in the
water distribution
system, drip irrigation, a dual-flush toilet, a low-flow shower
rose, and a front-
loading clothes washer. Other water infrastructure elements
shown include a
conventional drinking water plant (DWTP); a conventional
wastewater treat-
ment plant (WWTP); and a river diversion (supplying the
orchard).
www.sciencemag.org SCIENCE VOL 337 10 AUGUST 2012
683
SPECIALSECTION
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of Melbourne’s municipal supply, and uses ap-
proximately 500 kWh Ml−1 less energy than con-
ventional wastewater treatment (32). Recycled
water from the Western Treatment Plant is used
for a variety of nonpotable applications, includ-
ing in-plant uses and dual pipe schemes for the
irrigation of agricultural crops, gardens, golf
courses, and conservation areas.
Primary concerns associated with wastewater
reuse include the buildup of contaminants and
salts in soils (in the case of wastewater irriga-
tion) and the possibility that incomplete removal
of chemical or microbiological hazards during
treatment may cause disease in an exposed pop-
ulation (6). Disease risk can be evaluated on a
case-by-case basis using a statistical framework,
such as quantitative microbial risk assessment,
that predicts a population’s disease burden, given
the types and concentrations of pathogens that
are likely to be present in the water, as well as
particular exposure scenarios (33).
What Are the Opportunities for Reduction?
Water productivity can also be improved by re-
ducing the volume of water used to produce a

fixed value of goods and services. A modeling
study of the water supply system in Florianopolis,
Brazil, concluded that replacing single-flush
toilets with dual-flush toilets would reduce mu-
nicipal water use in the city by 14 to 28% and
reduce energy use at upstream (drinking water)
and downstream (wastewater) treatment plants by
4 GWh year–1—enough energy to supply 1000
additional households (34). An analysis of 96
owner-occupied single-family homes in Califor-
nia, Washington, and Florida concluded that the
installation of high-efficiency showerheads, toi-
lets, and clothes washers reduced household use
of municipal water by 10.9, 13.3, and 14.5%, re-
spectively (35). Because water is not technically
required for bathroom waste disposal, the instal-
lation of composting toilets and waterless urinals
can reduce municipal water use even further (36).
Agriculture accounts for the majority of glob-
al freshwater withdrawals (37), and thus even
small improvements in water productivity in
this sector can result in substantial water savings.
Water savings can be achieved by switching to
less–water-consuming crops, laser-leveling of
fields, reducing nonproductive evaporation of
water from soil or supply canals, changing irri-
gation scheduling, and adopting more efficient
sprinkler systems, including microirrigation tech-
niques (drip irrigation and microsprinklers) that
precisely deliver water to plant roots (37). These
approaches could help mitigate escalating water
demand associated with growing energy crops,
such as corn, particularly if projected increases
in U.S. biofuel production are realized (38).

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