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OCR for page 5995
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 5995-6000, May 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Plants and Population: Is There Time?"
held December 5-6, 1998, at the Arnold and Mabel Beckman' Center in Irvine, CA.
Global environmental impacts of agricultural expansion: The need
for sustainable and efficient practices
DAVID TILMAN
Department of Ecology, Evolution, and Behavior, University of Minnesota, 198
ABSTRACT The recent intensification of agriculture, and
the prospects of future intensification, will have major detri-
mental impacts on the nonagricultural terrestrial and aquatic
ecosystems of the world. The doubling of agricultural food
production during the past 35 years was associated with a
6.87-fold increase in nitrogen fertilization, a 3.48-fold increase
in phosphorus fertilization, a 1.68-fold increase in the amount
of irrigated cropland, and a 1.1-fold increase in land in
cultivation. Based on a simple linear extension of past trends,
the anticipated next doubling of global food production would
be associated with approximately 3-fold increases in nitrogen
and phosphorus fertilization rates, a doubling of the irrigated
land area, and an 18% increase in cropland. These projected
changes would have dramatic impacts on the diversity, com-
position, and functioning of the remaining natural ecosystems
of the world, and on their ability to provide society with a
variety of essential ecosystem services. The largest impacts
would be on freshwater and marine ecosystems, which would
be greatly eutrophied by high rates of nitrogen and phospho-
rus release from agricultural fields. Aquatic nutrient eu-
trophication can lead to loss of biodiversity, outbreaks of
nuisance species, shifts in the structure of food chains, and
impairment of fisheries. Because of aerial redistribution of
various forms of nitrogen, agricultural intensification also
would eutrophy many natural terrestrial ecosystems and
contribute to atmospheric accumulation of greenhouse gases.
These detrimental environmental impacts of agriculture can
be minimized only if there is much more efficient use and
recycling of nitrogen and phosphorus in agroecosystems.
The agricultural achievements of the past 35 years have been
impressive. Grain production, mainly from wheat, rice, and
maize, has increased at a rate greater than human population.
This has decreased the number of malnourished people even
as the earth's human population doubled to 5.8 billion. A1-
though the estimates vary widely, world population is projected
to increase about 75% before leveling off at about 10 billion.
In combination with increasing demand for meat in developing
countries and the use of grains as livestock feed, this increased
population density should cause world demand for grain
production to more than double. This raises several important
questions. If it is possible for world food production to double,
again, within the next four or five decades, what impacts would
this doubling have on the functioning of the nonagricultural
ecosystems of the world, and on the services they provide to
humanity? What routes might be used to decrease such
impacts? I explore these questions first by asking what the
global ecological impacts of "more of the same" agriculture
might be, and then by considering practices that might de-
crease such impacts. In particular, insights are sought in the
parallels between natural and agricultural ecosystems, but no
PNAS is available online at www.pnas.org.
Upper Buford Circle, St. Paul, MN 55108
easy answers are uncovered. Rather, a new long-term, multi-
disciplinary research program is needed to develop agricul-
tural methods that can feed a growing world and still preserve
the vital services provided to humanity by the world's natural
ecosystems.
Current agricultural practices involve deliberately maintain-
ing ecosystems in a highly simplified, disturbed, and nutrient-
rich state. To maximize crop yields, crop plant varieties are
carefully selected to match local growing conditions. Limiting
factors, especially water, mineral nitrogen, and mineral phos-
phate, are supplied in excess, and pests are actively controlled.
These three features of modern agriculture-control of crops
and their genetics, of soil fertility via chemical fertilization and
irrigation, and of pests (weeds, insects, and pathogens) via
chemical pesticides are the hallmarks of the green revolution.
They have caused four once-rare plants (barley, maize, rice,
and wheat) to become the dominant plants on earth as humans
became the dominant animal. Indeed, these four annual
grasses now occupy, respectively, 67 million hectares, 140
million hectares, 151 million hectares, and 230 million hect-
ares, each, worldwide, which is 39.8% of global cropland. For
comparison, the total forested area of the United States,
including Alaska, is 298 million hectares. Entire regions of the
world now are dominated by virtual monocultures of a given
crop. These monocultures have replaced natural ecosystems
that once contained hundreds to even thousands of plant
species, thousands of insect species, and many species of
vertebrates. Thus, agriculture has caused a significant simpli-
fication and homogenization of the world's ecosystems.
It is as difficult to predict the future of agriculture now as it
would have been to anticipate, in 1950, the successes and
impacts of the green revolution. However, some insights may
be provided by an analysis of the broad trends that occurred
during the recent doubling of global food production. These
trends may give some insight into the global environmental
impacts that the anticipated second doubling of agricultural
productivity may have. Next, I consider insights that ecology
may offer into the sustainability and stability of agricultural
ecosystems. Finally, I pose the major environmental challenges
that face humanity as global human population and demand
for food continues to increase.
The Ecology of Doubling Crop Production
The Food and Agriculture Organization (FAO) database (1)
provides a wealth of information on agricultural activities for
individual nations, regions, and the world from 1961 to the
present. Using the FAO data, let's look at the pattern of world
food production during this period and the factors that allowed
it to almost double. The majority of the food crops grown on
the arable lands of the earth are cereals (barley, maize, rice,
and wheat), coarse grains, and root crops. For convenience, I
Abbreviation: FAO, Food and Agriculture Organization.
5995
OCR for page 5996
5996 Colloquium Paper: Tilman
will call the sum of these world food production. In 1996,
cereals comprised 57% of this total, coarse grains 25%, and
root crops 18%. By using this measure, world food production,
as estimated from the FAO database (1), almost doubled
(increased 1.97-fold) from 1961 to 1996 (Fig. 1~. Comparable
patterns, and comparable ecological implications, occur if just
cereal production was considered, or if production for just
Europe and the United States, for which better data are
available, was considered.
Many factors contributed to the recent doubling of world
food production. The development of higher-yielding strains of
crops and better agricultural practices were important, as were
increased use of herbicides for weed control and insecticides
and fungicides for pest control. In addition, there were marked
increases in the amounts of nitrogen and phosphorus fertilizers
applied each year worldwide, in the proportion of arable land
that was irrigated, and in the total amount of land that was
cultivated annually worldwide (Fig. 2~. It was the combined
effects of all of these factors, and more, that allowed world
food production to double in 35 years.
The FAO data (1) show that this recent doubling of world
food production was accompanied by 6.87- and 3.48-fold
increases in the global annual rate of nitrogen and phosphorus
fertilization, respectively, by a doubling in the amount of land
that was irrigated, and by a 10% increase in the amount of land
in cultivation (Fig. 2~. What might be required to allow food
production to double again? A simple, naive and optimistic
scenario might assume that, during the next four decades, all
of the relationships of Fig. 2 would remain linear and gains in
crop genetics, weed and pest control, and cultivation practices
would continue at their previous pace. The assumption of
linearity can be used to predict the rates of nitrogen and
phosphorus fertilization and irrigation, and the increase in
amount of cultivated land needed to double food production.
Even this scenario, though, would require, based on the linear
regression of Fig. 2A, that the global rate of application of
nitrogen fertilizer increase from about 75 x 106 metric tons per
year to 235 x 106 metric tons per year. Nitrogen fixed by
legume crops also would need to more than triple. Comparable
calculations, based on the regression of Fig. 2B, predict that
the global annual rate of application of phosphorus fertilizer
would have to increase from about 37 x 106 metric tons per
year to 94 x 106 metric tons per year for food production to
double. Similarly, the worldwide proportion of arable lands
that are irrigated would have to increase from the current 17%
to about 32% (based on extrapolation of Fig. 2C), and the total
amount of land in cultivation would have to increase from
about 1.47 x 109 hectares to 1.73 x 109 hectares (extrapolation
of Fig. ZD). These changes represent a worldwide tripling of
~ 4
o
.O
lo
-
8
IL
1.97-fold increase
in 35 years
3
2
1960 1970 1980
1 990 2000
Year
FIG. 1. Based on FAO data (1), world food production, measured
as the sum of cereals, coarse grains and root crops, almost doubled
from 1961 to 1996. A linear regression, and 95% and 99~o confidence
intervals for the regression, are shown.
Proc. Natl. Acad. Sci. USA 96 (1999'
-
2
3
co
a)
Q 2
cn
o
. _
a)
a'
o
-
O 4
o
IL
o
3
2
6.87-fold increase
in N fertilization I'
. . . . . . . . . . . . .
0 20 40 60 80 100
Nitrogen Fertilization
(1 o6 metric tons per year)
. B.
3.48-fold increase in
phospohorous fertilization ,-'
'~': :<
10 15 20 25 30 35 40
Phosphorous Fertilization
(1 o6 metric tons per year)
1.68-fold increase
0.10 0.12 0.14 0.16 0.18
Proportion of Land Irrigated
1.098 increase in
cultivated land ,'.,
1.341 .361 .381.401 .421 .441 .46 1.48 1.50
Land in Cultivation (109 hectares)
FIG. 2. The relationship between annual global food production
(cereals + coarse grains + root crops) and agricultural inputs, based
on FAO data (1). (A) Global annual nitrogen fertilization rate. (B)
Global annual phosphorus fertilization rate. (C) Proportion of arable
lands that are irrigated. (D) Total land surface in agricultural crop
production.
the annual rates of N and P fertilization, a doubling of the
extent of irrigation, and an 18% increase in the amount of land
farmed.
Such linear projections of yields may be overly optimistic for
a variety of reasons. First, the yield of a crop is a saturating
OCR for page 5997
Colloquium Paper: Tilman
function of the rate of supply of its limiting resource. Adding
fertilizer to already well-fertilized areas, such as productive
croplands in the developed nations that produce the majority
of the world's cereals, will have little impact on yield. Signif-
icant regional gains, though, can be achieved in many devel-
oping countries by appropriately fertilizing croplands not
currently receiving fertilizer. Similar saturating yield curves
occur for phosphorus and irrigation. In total, such saturating
curves imply that it may be difficult to increase yields at a pace
similar to that of the past four decades. Second, the easiest and
greatest gains from crop breeding programs may have already
occurred (2~. Annual gains in yields from breeding programs
are decreasing, and the research costs associated with these
gains are escalating (2~. This is not surprising. Given a fixed
gene pool, the responses to a given selective regime are most
rapid initially and increasingly slower through time. Such yield
gains represent genetic movement on the morphological and
physiology tradeoff surface on which plant species have dif-
ferentiated and evolved. The closer a given variety is to the
optimum point on this tradeoff surface, the less will be the gain
from further selection. Once most of the original genetic
variance preserved within crop landraces and remaining wild
relatives has been exploited, future breeding-based yield gains
are likely to be small or difficult to obtain. Marked yield gains
from crop breeding then would require that plants overcome
major morphological and physiological constraints that no
organisms have overcome during hundreds of millions of years
of evolution. Organisms that greatly overcame such barriers,
perhaps through gene transfers, would be supercompetitive
species that could potentially invade into and change the
structure of nonagricultural ecosystems (3~.
These concerns lead me to wonder if global food production
can be doubled by a continuation of past practices. The other
route for a major increase in food production is a marked
increase in arable land, which the FAO data suggest has played
a modest role in the past 30 years. Because the best land
already has been cultivated, the amount of land dedicated to
agriculture may have to increase disproportionately to the gain
in global food production.
Ecological Impacts of Doubling Global Food Production
If these simple extrapolations of past practices are any indi-
cation, doubling global food production will triple the annual
rates of nitrogen and phosphorus release to the globe. Current
rates of agricultural nitrogen production, via both production
of fertilizer and cultivation of legume crops, already approx-
imately equal the natural (preindustrial) rate of addition of
biologically active nitrogen to the globe (4~. Point-source
releases of phosphorus are tightly regulated in developed
nations because phosphorus is a major limiting nutrient in
aquatic ecosystems and increases in its supply rate harm water
quality and aquatic foodweb structure. A tripling of global
phosphorus supply rates is likely to adversely impact many
aquatic ecosystems, especially those that have significant in-
puts of eroded agricultural soils or phosphorus-rich wastes
from livestock and poultry. Nitrogen is much more motile in
soil than phosphorus because soil bacteria can convert ammo-
nia to nitrate and nitrite, which are readily leached from soil
(5~. Denitrification by bacteria also can convert nitrate into
nitrous oxide, a potent greenhouse gas. In addition, ammonia,
which is both directly applied as fertilizer and created via
bacterial degradation of animal waste and other organic
compounds, is highly volatile. It is transported via air and
deposited on other ecosystems with precipitation. These nu-
merous modes of transport mean that agricultural nitrogen,
less than half of which stays in a field or is harvested with a
crop, impacts both terrestrial and aquatic ecosystems as a
eutrophier, and impacts global climate because of is role as a
greenhouse gas. Indeed, there is a direct and quantitative link
Proc. Natl. Acad. Sci. USA 96 (1999) 5997
between the amounts of nitrogen in the major rivers of the
world and the magnitude of agricultural nitrogen inputs to
their watersheds (6~.
The long-term ecological impacts of increased rates of
agricultural nitrogen and phosphorus input will depend on the
levels to which these nutrients accumulate in various nonag-
ricultural ecosystems. These levels are uncertain because of the
complexities of the global biogeochemistry of nitrogen and
phosphorus. These nutrients accumulate in a variety of forms
in many different sinks (arable soil organic matter, ground-
water, freshwater and marine ecosystems and their sediments,
nonagricultural ecosystems, atmospheric nitrous oxide) after
agricultural application, but the eventual sizes of these pools
will depend on biologically and physically driven rates of
transfer in and out of these pools (5~. For agricultural nitrogen,
one critical step will be the rate and location of denitrification,
especially complete denitrification to N2. The transport of
phosphorus to nonagricultural ecosystems especially will de-
pend on erosion and surface flow. As emphasized by Socolow
(7), a scientific effort comparable to that on the global carbon
cycle will be needed to understand the impacts on global
biogeochemistry of elevated rates of agricultural nitrogen and
phosphorus application.
Nitrogen and phosphorus are the two most important
limiting nutrients of terrestrial, freshwater, and marine eco-
systems (3, 8-11~. The impacts of elevated levels of a major
limiting nutrient are well documented. Nutrient addition
causes dominance by a few, often formerly rare plant and
animal species, and the loss of species diversity (e.g. refs. 3, 9,
12-15~. Both effects are approximately proportional to the
cumulative magnitude of nutrient addition. High rates of
nitrogen deposition caused by intensive, nitrogen-rich agricul-
ture in the Netherlands were a major cause of the conversion
of species-rich native heathlands into monoculture grasslands
and then forest (16~. At high rates of nutrient addition,
nuisance plant species often dominate both terrestrial and
aquatic ecosystems. For instance, high rates of nitrogen addi-
tion cause prairie grasslands to become virtual monocultures
of an otherwise extremely rare nonnative agricultural weed
(17~. Bluegreen algal species, some toxic, often dominate
lakes, rivers and streams that receive high rates of P and N
loading. Similarly, blooms of toxic red algae and of pathogenic
taxa such as Pfisteria occur in nutrient-polluted marine habi-
tats. Anoxic conditions associated with high rates of phospho-
rus and nitrogen loading cause fish die-offs in both freshwater
and marine ecosystems (18~.
Unless its efficiency is increased, a doubling of irrigation
would pose additional environmental problems. Humans al-
ready impact a large portion of the terrestrial hydrologic cycle
(19~. Additional irrigation would divert more water from
aquatic ecosystems and impact groundwaters and surface
waters via additional leaching of agrochemicals.
A conservative estimate, based on the assumption that
future yield gains can match those of the past 35 years, is that
doubling global food production will require 18~o more arable
land. Even this 18~o increase would require the loss of 268
million hectares of nonagricultural ecosystems worldwide,
comparable in size to cultivating all of the currently forested
land of the United States. A doubling of food production may
require a much greater increase in land dedicated to agricul-
ture. The resulting ecosystem destruction would vastly increase
the proportion of the world's species threatened with extinc-
tion. It also would cause a massive release of CO2 from land
clearing and tilling (5~. Because high-diversity ecosystems
generally occur on infertile soils (3, 9, 20), the conversion of
less-fertile ecosystems to agriculture would disproportionately
impact world biodiversity.
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5998 Colloquium Paper: Tilman
Agriculture and the Loss of Ecosystem Services
A doubling of global food production would have major
impacts on the ability of nonagricultural ecosystems to provide
services (21) vital to humanity. Existing nonagricultural eco-
systems provide, at no cost, pure, drinkable water. In contrast,
the groundwater associated with intensive agricultural ecosys-
tems often contains sufficiently high concentrations of nitrite
and nitrates or of pesticides and their residues as to be unfit for
human consumption. Expensive treatment is required to make
it potable. The biodiversity of nonagroecosystems provides
many services to agriculture. For instance, the genetic diversity
of both wild relatives of crop plants and unrelated organisms
is used to increase yields and to reduce impacts of agricultural
pests and pathogens. However, the maintenance of the wild
biodiversity needed for future development of crops and
medicines occurs mainly in nonagricultural ecosystems, the
very ecosystems threatened by agricultural expansion and
nutrient release. Agriculture depends on soil fertility, fertility
created by the ecosystems destroyed when lands are converted
to agriculture. Especially on sandy soils, the best way to regain
soil fertility lost because of tilling is to allow re-establishment
of the native ecosystems. Many agricultural crops depend on
the pollination services provided by insects, birds, or mammals
that live in nearby nonagricultural ecosystems (18~. Similarly,
agricultural crops benefit from biocontrol agents, such as
parasitic and predatory insects, birds, and bats, that live in
neighboring nonagricultural ecosystems and that decrease
outbreaks of agricultural pests. Nonagricultural ecosystems,
such as forests on slopes and wetlands, help meter the release
of water into streams and rivers, and thus help in flood control.
If properly managed, natural ecosystems also can produce a
sustainable supply of goods used by society, including timber
and fiber, fish, and game.
This brief overview of ecosystem services (21) demonstrates
that society, and agriculture, depend on many services pro-
vided by nonagricultural ecosystems. Although it is difficult to
establish economic values for such services (22), it is clear that,
when possible, technological substitutes for lost ecosystem
services can be extremely expensive. This highlights the need
for public policy to consider the short-term and long-term
costs of actions that decrease the ability of nonagricultural
ecosystems to provide vital ecosystem services to society.
More of the Same Will Not Work
The global agricultural enterprise is passing a threshold. It has
gone from being a minor source of off-site environmental
degradation 35 years ago to becoming the major source of
nitrogen and phosphorus loading to terrestrial, freshwater, and
marine ecosystems. If this loading increases as projected here,
agriculture will adversely transform most of the remaining
natural, nonagricultural ecosystems of the world. Because the
global environmental impact of agriculture on natural ecosys-
tems and the services they provide may be as serious a problem
as global climate change, the impacts of agriculture merit more
study.
A "more of the same" approach to the doubling of agricul-
tural production will have significant environmental costs,
costs that could be lowered by processes that increase the
efficiency of fertilizer use, such as precision agriculture (23)
and by incentives for their use. Methods that increase the
nutrient efficiency of the overall agricultural production pro-
cess also are needed. For instance, wastes from large-scale
animal operations are rich in N and P. Unless properly recycled
into arable fields, or subjected to tertiary sewage treatment to
remove nitrogen and phosphorus, such wastes can be a major
source of N and P loading to nonagricultural ecosystems (24~.
However, the regulations that apply to municipal sewage and
factory effluents often have not been applied to large-scale
Proc. Nail. Acad. Sci. USA 96 (1999)
livestock factories or to heavily fertilized fields, even though
these are now major sources of nutrient loading to many
aquatic ecosystems (18~. The development of more nutrient-
efficient crops also could have major environmental benefits.
If crops could be bred to consume a larger proportion of soil
nitrate and ammonium, this would decrease the amount of
unconsumed soil nitrate and ammonium that would be lost via
leaching and volatilization. This would decrease impacts on
off-site ecosystems. Breeding programs that increased crop
yields would decrease some of the future impacts of agriculture
by decreasing the amount of additional land that would have
to be brought into agricultural production.
The ecosystems of the world now are dominated by humans
(25~. The implications of human domination, including im-
pacts from expanding agricultural activities, must be better
understood and incorporated into policy. This will require an
on-going, iterative process in which science and policy regu-
lating agricultural practices advance hand-in-hand, much as is
being done for the climate issue by the Intergovernmental
Panel on Climate Change. This will require predictive, mech-
anistic models of the impacts of agriculture on nonagricultural
ecosystems.
Ecological Insights into Agricultural Impacts and
Sustainability
What might be done to decrease the environmental impacts of
agriculture while maintaining or improving its productivity,
stability, or sustainability? This major challenge will have no
single, easy solution. Partial answers will come from increases
in the precision and efficiency of nutrient and pesticide use,
from advances in crop genetics including advances from bio-
technology, and from a variety of engineering solutions. Some
additional insights may come from a consideration of the
principles that govern the functioning of all ecosystems, in-
cluding agroecosystems. Ecosystem functioning is known to
depend on the traits of the species ecosystem's contain (their
composition), the number of species they contain (their species
diversity), and the physical conditions they experience, espe-
cially disturbance regimes. A consideration of the principles
governing the impacts of composition, diversity, and distur-
bance on ecosystems may suggest ways to decrease impacts of
agriculture or to make it more productive, stable, or sustain-
able. It is critical to realize that these principles apply within
a given ecosystem type. They describe differences in function-
ing of otherwise identical ecosystems that share the same
species pool and differ only in which and how many species
they contain. These principles were not derived from, and do
not apply to, comparisons among different ecosystem types,
such as cattail swamps versus prairies, or mangrove versus
upland forest, or tropical versus temperate forests.
A fundamental principle of epidemiology and ecology is that
the severity and extent of a disease or pest outbreak depends
on the density of the host population. At low host population
densities, there is a low chance of contagious spread. However,
at high host densities, a disease or pest can spread epidemically
throughout the population. An unavoidable effect of high
diversity is that most species have lower densities than in low
diversity communities. For instance, on average a species is
about one-fourth as abundant in a four-species community as
in monoculture. This simple effect caused a variety of plant leaf
fungal diseases to have lower rates of occurrence at higher
plant diversity in a field experiment.
Agriculture has transformed once-rare plants into some of
the most abundant species on earth. Maize, which once
occurred in scattered multispecies mixtures on nutrient-poor
or disturbed soils, now covers 140 million hectares of the earth.
Potential pathogens and pests that never had encountered
maize now do so frequently. Pests and pathogens that formerly
could not have maintained populations on maize now encoun
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Colloquium Paper: Tilman
ter hosts growing at much greater local and regional densities
and with higher tissue nutritional levels. Just as humans have
accumulated diseases as densities increased during the past
2,000 years (26), so, too, will major crops continue to accu-
mulate diseases and pests. Southern corn blight is one such
disease. A strain of western corn rootworm that is newly
adapted to living on both corn and soybeans is an emerging
pest. Wheat head rust is another disease. The latter virtually
eliminated wheat as a major rotation crop from Indiana and
Illinois in the 1920s and now is doing so in western Minnesota
and eastern North and South Dakota. Plant diseases and pests
can have devastating impacts. The American chestnut, once a
dominant tree of eastern U.S. forests, and the American elm
both were virtually eliminated after pathogens, to which no
known resistance occurs, invaded North America. Similarly,
novel pests or pathogens or strains of pathogens could either
greatly reduce the area in which wheat, rice, or maize can be
grown or, perhaps, eliminate these as viable crops. A major
protection against these possibilities is diversity the diversity
of crops deployed in a region, the diversity of substitute crops,
and the diversity of genetic resistances within crops.
All else being equal, the stability of the total rate of plant
production in an ecosystem depends on both the species
diversity of the plant community and its species composition
(e.g. refs. 15 and 27-29~. The stability of primary productivity
is greater for ecosystems containing greater plant diversity (15,
284. This results from three underlying processes. First, the
same statistical averaging process that causes more diverse
portfolios of stocks to be more stable than less diverse port-
folios applies to ecosystems (30-32~. Second, interspecific
competition causes negative covariances in the abundances of
species, and such compensatory effects can act to more greatly
stabilize more diverse ecosystems (15, 32~. Third, the increase
in ecosystem productivity that occurs as diversity increases,
termed overyielding, also tends to stabilize primary produc-
tivity at higher diversity (32~. The greater stability of more
diverse ecosystems means that diversity has an insurance value
by minimizing year-to-year variance in yields. Greater stability
of agricultural yields might be attained by growing, as a single
crop, a mixture of appropriately chosen genotypes of a given
species, such as a mixture of high-yielding hybrid varieties.
The plant species diversity of an ecosystem, and its plant
species composition, influence its primary productivity (33-
38~. Total primary productivity increases about 35-70% as
plant species diversity increases from one to about 20 species.
Such effects have a series of alternative theoretical explana-
tions (27, 32, 39-41~. The two major classes of explanations
are the sampling effect and niche differentiation. The sam-
pling effect implies that the increase in productivity associ-
ated with greater plant diversity is caused by the higher
probability that a more productive species or variety will be
present in a more diverse plot. The niche differentiation
effect is based on complementary use of different limiting
resources by different species. One strain or species may grow
best during the cooler portion of the growing season, and
another during the warmer portion. Or one may better exploit
soil nutrients in deeper soils and another at shallower depths.
Such differing abilities to use limiting resources cause produc-
tivity to increase with diversity (414.
Under conditions typical of high-intensity agriculture (fer-
tilized, irrigated fields in which light limits the growth of all
plants), the sampling effect theory should apply, with maximal
yields provided by the appropriate monoculture. All major
grain crops (corn, wheat, rice, barley, etc.), soybeans, sugar
cane, and most other crops are grown in monoculture. How-
ever hay, some crops harvested for fodder, and grasslands
maintained for grazing often are grown under conditions in
which niche differences could allow benefits from diversity.
Crop diversity also may be of benefit when arable lands are
Proc. Natl. Acad. Sci. USA 96 (1999) 5999
managed to optimize yield in the face of constraints on nutrient
release to the environment.
Recent theory has predicted (35) and recent field experi-
ments have shown (36, 37) that the rates of loss of limiting
nutrients from terrestrial ecosystems are lower at higher plant
diversity, and are equally impacted by species composition.
Cultivation has major effects on soil fertility. Within the first
50 years of tilling, 40-70% of the original store of soil organic
matter (carbon and nitrogen) is lost (42~. For porous sandy
soils, which start with relatively low organic matter and
nitrogen, the loss of fertility during farming can be so great that
the soils cannot be sustainably farmed. Recovery of soil C and
N should be more rapid if abandoned fields are planted with
a high-diversity mixture of appropriate plant species. On the
sandy soils of my research site in central Minnesota, native
warm-season prairie grasses and legumes, combined, signifi-
cantly increase the rate of recovery of soil fertility after
agriculture (43~. Programs designed to restore soil fertility,
such as land set-aside programs, may be more successful if such
lands are planted to high-diversity mixtures of appropriate
species.
Finally, in higher diversity ecosystems, there is more
complete use of limiting resources (36, 37, 41~. The resulting
lower concentrations of unconsumed soil nutrients decreases
the number of other species that invade an ecosystem (324.
Weeds are a major pest of agriculture. In North America,
most weedy species are non-native annuals introduced from
Europe or Asia. The ability of newly introduced weeds to
spread across a landscape will depend on the spatial pattern
of agricultural and native high-diversity ecosystems. Land-
scapes with an appropriate balance of agricultural and
natural ecosystems may be more resistant to invasion by new
weedy species.
Conclusions
A hallmark of modern agriculture is its use of monocultures
grown on fertilized soils. Ecological principles suggest that
such monocultures will be relatively unstable, will have high
leaching loss of nutrients, will be susceptible to invasion by
weedy species, and will have high incidences of diseases and
pests- all of which do occur. Although ecological principles
may predict these problems, they do not seem to offer any easy
solutions to them. Agriculture, and society, seem to be facing
tough tradeoffs. Agricultural ecosystems have become incred-
ibly good at producing food, but these increased yields have
environmental costs that cannot be ignored, especially if the
rates of nitrogen and phosphorus fertilization triple and the
amount of land irrigated doubles. The tradition in agriculture
has been to maximize production and minimize the cost of
food with little regard to impacts on the environment and the
services it provides to society. As the world enters an era in
which global food production is likely to double, it is critical
that agricultural practices be modified to minimize environ-
mental impacts even though many such practices are likely to
increase the costs of production.
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Representative terms from entire chapter:
nonagricultural ecosystems
