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umans have extensively altered the global
environment, changing global
biogeochemical cycles, transforming land and
enhancing the mobility of biota. Fossil-fuel
combustion and deforestation have increased
the concentration of atmospheric carbon dioxide (CO
by 30% in the past three centuries (with more than half of
this increase occurring in the past 40 years). We have
more than doubled the concentration of methane and
increased concentrations of other gases that contribute to
climate warming. In the next century these greenhouse
gases are likely to cause the most rapid climate change that
the Earth has experienced since the end of the last
glaciation 18,000 years ago and perhaps a much longer
time. Industrial fixation of nitrogen for fertilizer and other
human activities has more than doubled the rates of
terrestrial fixation of gaseous nitrogen into biologically
available forms. Run off of nutrients from agricultural and
urban systems has increased several-fold in the developed
river basins of the Earth, causing major ecological changes
in estuaries and coastal zones. Humans have transformed
40–50% of the ice-free land surface, changing prairies,
forests and wetlands into agricultural and urban systems.
We dominate (directly or indirectly) about one-third of
the net primary productivity on land and harvest fish that
use 8% of ocean productivity. We use 54% of the available
fresh water, with use projected to increase to 70% by
. Finally, the mobility of people has transported
organisms across geographical barriers that long kept the
biotic regions of the Earth separated, so that many of the
ecologically important plant and animal species of many
areas have been introduced in historic time
Together these changes have altered the biological diver-
sity of the Earth (Fig. 1). Many species have been eliminated
from areas dominated by human influences. Even in
preserves, native species are often out-competed or con-
sumed by organisms introduced from elsewhere. Extinction
is a natural process, but it is occurring at an unnaturally rapid
rate as a consequence of human activities. Already we have
caused the extinction of 5–20% of the species in many groups
of organisms (Fig. 2), and current rates of extinction are esti-
mated to be 100–1,000 times greater than pre-human rates
In the absence of major changes in policy and human
behaviour, our effects on the environment will continue to
alter biodiversity. Land-use change is projected to have the
largest global impact on biodiversity by the year 2100,
followed by climate change, nitrogen deposition, species
introductions and changing concentrations of atmospheric
(ref. 6). Land-use change is expected to be of particular
importance in the tropics, climatic change is likely to be
important at high latitudes, and a multitude of interacting
causes will affect other biomes (Fig. 3)
. What are the ecolog-
ical and societal consequences of current and projected
effects of human activity on biological diversity?
Ecosystem consequences of altered diversity
Diversity at all organizational levels, ranging from genetic
diversity within populations to the diversity of ecosystems in
landscapes, contributes to global biodiversity. Here we focus
on species diversity, because the causes, patterns and conse-
quences of changes in diversity at this level are relatively well
documented. Species diversity has functional consequences
because the number and kinds of species present determine
the organismal traits that influence ecosystem processes.
Species traits may mediate energy and material fluxes direct-
ly or may alter abiotic conditions (for example, limiting
resources, disturbance and climate) that regulate process
rates. The components of species diversity that determine
this expression of traits include the number of species
present (species richness), their relative abundances (species
Consequences of changing
F. Stuart Chapin III*, Erika S. Zavaleta†, Valerie T. Eviner§, Rosamond L. Naylor‡, Peter M. Vitousek†,
Heather L. Reynolds||, David U. Hooper¶, Sandra Lavorel#, Osvaldo E. Sala✩, Sarah E. Hobbie**,
Michelle C. Mack* & Sandra Díaz††
*Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99775, USA (e-mail: firstname.lastname@example.org)
†Department of Biological Sciences and ‡Institute for International Studies, Stanford University, Stanford, California 94305, USA
§Department of Integrative Biology, University of California, Berkeley, California 94720, USA
||Department of Biology, Kalamazoo College, Kalamazoo, Michigan 49006, USA
¶Department of Biology, Western Washington University, Bellingham, Washington 98225, USA
#Centre d’Ecologie Fonctionnelle et Evolutive, CNRS UPR 9056, 34293 Montpellier Cedex 05, France
✩Cátedra de Ecología and Instituto de Fisiología y Ecología Vinculadas a la Agricultura, Faculty of Agronomy, University of Buenos Aires, Ave
San Martín 4453, Buenos Aires C1417DSE, Argentina
**Department of Ecology, Evolution, and Behavior, University of Minnesota, St Paul, Minnesota 55108, USA
††Instituto Multidisciplinario de Biología Vegetal, Universidad Nacional de Córdoba, FCEFyN, Casilla de Correo 495, 5000 Córdoba, Argentina
Human alteration of the global environment has triggered the sixth major extinction event in the history of
life and caused widespread changes in the global distribution of organisms. These changes in biodiversity
alter ecosystem processes and change the resilience of ecosystems to environmental change. This has
profound consequences for services that humans derive from ecosystems. The large ecological and societal
consequences of changing biodiversity should be minimized to preserve options for future solutions to global
© 2000 Macmillan Magazines Ltd
evenness), the particular species present (species composition), the
interactions among species (non-additive effects), and the temporal
and spatial variation in these properties. In addition to its effects on
current functioning of ecosystems, species diversity influences the
resilience and resistance of ecosystems to environmental change.
Species richness and evenness
Most theoretical and empirical work on the functional consequences
of changing biodiversity has focused on the relationship between
species richness and ecosystem functioning. Theoretical possibilities
include positive linear and asymptotic relationships between rich-
ness and rates of ecosystem processes, or the lack of a simple statistical
(Box 1). In experiments, species richness correlates
with rates of ecosystem processes most clearly at low numbers of
species. We know much less about the impact of species richness in
species-rich, natural ecosystems. Several studies using experimental
species assemblages have shown that annual rates of primary produc-
tivity and nutrient retention increase with increasing plant species
richness, but saturate at a rather low number of species
mycorrhizal species richness also seems to enhance plant production
in an asymptotic fashion, although phosphorus uptake was
enhanced in a linear fashion from 1 to 14 species of fungi
richness can lead to increased decomposition of organic matter
contrast, no consistent statistical relationship has been observed
between plant species richness of litter inputs and decomposition
. Thus, in experimental communities (which typically focus on
only one or two trophic levels), there seems to be no universal
relationship between species richness and ecosystem functioning,
perhaps because processes differ in their sensitivity to species rich-
ness compared with other components of diversity (such as evenness,
composition or interactions). The absence of a simple relationship
between species richness and ecosystem processes is likely when one
or a few species have strong ecosystem effects.
Although the relationship of species richness to ecosystem func-
tioning has attracted considerable theoretical and experimental
attention because of the irreversibility of species extinction, human
activities influence the relative abundances of species more frequent-
ly than the presence or absence of species. Changes in species
evenness warrant increased attention, because they usually respond
more rapidly to human activities than do changes in species richness
and because they have important consequences to ecosystems long
before a species is threatened by extinction.
Particular species can have strong effects on ecosystem processes by
directly mediating energy and material fluxes or by altering abiotic
conditions that regulate the rates of these processes (Fig. 4)
Species’ alteration of the availability of limiting resources, the distur-
bance regime, and the climate can have particularly strong effects on
ecosystem processes. Such effects are most visible when introduced
species alter previous patterns of ecosystem processes. For example,
the introduction of the nitrogen-fixing tree Myrica faya to nitrogen-
limited ecosystems in Hawaii led to a fivefold increase in nitrogen
inputs to the ecosystem, which in turn changed most of the function-
al and structural properties of native forests
. Introduction of the
deep-rooted salt cedar (Tamarix sp.) to the Mojave and Sonoran
Deserts of North America increased the water and soil solutes
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11 MAY 2000
Figure 1 The role of biodiversity
in global change. Human
activities that are motivated by
economic, cultural, intellectual,
aesthetic and spiritual goals (1)
are now causing environmental
and ecological changes of
global significance (2). By a
variety of mechanisms, these
global changes contribute to
changing biodiversity, and
changing biodiversity feeds
back on susceptibility to species
invasions (3, purple arrows; see
text). Changes in biodiversity,
through changes in species
traits, can have direct
consequences for ecosystem
services and, as a result,
human economic and social
activities (4). In addition,
changes in biodiversity
can influence ecosystem
processes (5). Altered ecosystem processes can thereby influence ecosystem services that benefit humanity (6) and feedback to further alter biodiversity (7, red arrow). Global
changes may also directly affect ecosystem processes (8, blue arrows). Depending on the circumstances, the direct effects of global change may be either stronger or weaker than
effects mediated by changes in diversity. We argue that the costs of loss of biotic diversity, although traditionally considered to be ‘outside the box’ of human welfare, must be
recognized in our accounting of the costs and benefits of human activities.
(percentage of global species)
Figure 2 Proportion of the global number of species of birds, mammals, fish and
plants that are currently threatened with extinction
© 2000 Macmillan Magazines Ltd
accessed by vegetation, enhanced productivity, and increased surface
litter and salts. This inhibited the regeneration of many native
species, leading to a general reduction in biodiversity
. The perenni-
al tussock grass, Agropyron cristatum, which was widely introduced
to the northern Great Plains of North America after the 1930s
‘dustbowl’, has substantially lower allocation to roots compared with
native prairie grasses. Soil under A. cristatum has lower levels of
available nitrogen and ~25% less total carbon than native prairie soil,
so the introduction of this species resulted in an equivalent reduction
of 480 2 10
g carbon stored in soils
. Soil invertebrates, such as
earthworms and termites, also alter turnover of organic matter and
nutrient supply, thereby influencing the species composition of the
aboveground flora and fauna
Species can also influence disturbance regime. For example,
several species of nutritious but flammable grasses were introduced
to the Hawaiian Islands to support cattle grazing. Some of these
grasses spread into protected woodlands, where they caused a 300-
fold increase in the extent of fire. Most of the woody plants, including
some endangered species, are eliminated by fire, whereas grasses
. Similar increases in the ecological role of fire
resulting from grass invasions have been widely observed in the
Americas, Australia and elsewhere in Oceania. The invasion of cheat-
grass (Bromus tectorum) into western North America is one of the
most extensive of these invasions. Cheatgrass has increased fire fre-
quency by a factor of more than ten in the >40 million hectares
(1 ha = 10
) that it now dominates
Species-induced changes in microclimate can be just as impor-
tant as the direct impacts of environmental change. For example, in
late-successional boreal forests, where soil temperatures have a
strong influence on nutrient supply and productivity, the presence of
moss, which reduces heat flux into the soil, contributes to the stability
of permafrost (frozen soils) and the characteristically low rates of
. As fire frequency increases in response to high-lat-
itude warming, moss biomass declines, permafrost becomes less sta-
ble, the nutrient supply increases, and the species composition of
forests is altered. Plant traits can also influence climate at larger
scales. Simulations with general circulation models indicate that
widespread replacement of deep-rooted tropical trees by shallow-
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11 MAY 2000
Figure 3 Scenarios of change in species diversity in selected biomes by
the year 2100. The values are the projected change in diversity for each
biome relative to the biome with greatest projected diversity change
Biomes are: tropical forests (T), grasslands (G), Mediterranean (M),
desert (D), north temperate forests (N), boreal forests (B) and arctic (A).
Projected change in species diversity is calculated assuming three
alternative scenarios of interactions among the causes of diversity
change. Scenario 1 assumes no interaction among causes of diversity
change, so that the total change in diversity is the sum of the changes
caused by each driver of diversity change. Scenario 2 assumes that only
the factor with the greatest impact on diversity influences diversity
change. Scenario 3 assumes that factors causing change in biodiversity
interact multiplicatively to determine diversity change. For scenarios 1
and 2, we show the relative importance of the major causes of projected
change in diversity. These causes are climatic change, change in land
use, introduction of exotic species, and changes in atmospheric CO
and/or nitrogen deposition (labelled ‘other’). The graph shows that all
biomes are projected to experience substantial change in species
diversity by 2100, that the most important causes of diversity change
differ among biomes, and that the patterns of diversity change depend
on assumptions about the nature of interactions among the causes of
diversity change. Projected biodiversity change is most similar among
biomes if causes of diversity change do not interact (scenario 1) and
differ most strongly among biomes if the causes of biodiversity change
interact multiplicatively (scenario 3).
TGMDNBA TGMDNBA TGMDNBA
Relative diversity change
(proportion of maximum)
Scenario 1 Scenario 2 Scenario 3
There has been substantial debate over both the form of the relationship between species richness and ecosystem processes and the
mechanisms underlying these relationships
. Theoretically, rates of ecosystem processes might increase linearly with species richness if all
species contribute substantially and in unique ways to a given process — that is, have complementary niches. This relationship is likely to saturate
as niche overlap, or ‘redundancy’, increases at higher levels of diversity
. Several experiments indicate such an asymptotic relationship of
ecosystem process rates with species richness. An asymptotic relationship between richness and process rates could, however, arise from a
‘sampling effect’ of increased probability of including a species with strong ecosystem effects, as species richness increases
. The sampling
effect has at least two interpretations. It might be an important biological property of communities that influences process rates in natural
, or it might be an artefact of species-richness experiments in which species are randomly assigned to treatments, rather than
following community assembly rules that might occur in nature
. Finally, ecosystem process rates may show no simple correlation with species
richness. However, the lack of a simple statistical relationship between species richness and an ecosystem process may mask important
functional relationships. This could occur, for example, if process rates depend strongly on the traits of certain species or if species interactions
determine the species traits that are expressed (the ‘idiosyncratic hypothesis’)
. This mechanistic debate is important scientifically for
understanding the functioning of ecosystems and effective management of their biotic resources. Regardless of the outcome of the debate,
conserving biodiversity is essential because we rarely know a priori which species are critical to current functioning or provide resilience and
resistance to environmental changes.
Species richness and ecosystem functioning
© 2000 Macmillan Magazines Ltd
rooted pasture grasses would reduce evapotranspiration and lead to a
warmer, drier climate
. At high latitudes, the replacement of
snow-covered tundra by a dark conifer canopy will probably increase
energy absorption sufficiently to act as a powerful positive feedback
to regional warming
Most ecosystem processes are non-additive functions of the traits of
two or more species, because interactions among species, rather than
simple presence or absence of species, determine ecosystem charac-
teristics (Fig. 5). Species interactions, including mutualism, trophic
interactions (predation, parasitism and herbivory), and competition
may affect ecosystem processes directly by modifying pathways of
energy and material flow
or indirectly by modifying the abundances
or traits of species with strong ecosystem effects
Mutualistic species interactions contribute directly to many
essential ecosystem processes. For example, nitrogen inputs to terres-
trial ecosystems are mediated primarily by mutualistic associations
between plants and nitrogen-fixing microorganisms. Mycorrhizal
associations between plant roots and fungi greatly aid plant
nutrient uptake from soil, increase primary production and speed
. Highly integrated communities (consortia) of soil
microorganisms, in which each species contributes a distinct set of
enzymes, speeds the decomposition of organic matter
. Many of
these interactions have a high degree of specificity, which increases
the probability that loss of a given species will have cascading effects
on the rest of the system.
Trophic interactions can have large effects on ecosystem process-
es either by directly modifying fluxes of energy and materials, or by
influencing the abundances of species that control those fluxes.
When top predators are removed, prey populations sometimes
explode and deplete their food resources, leading to a cascade of
ecological effects. For example, removal of sea otters by Russian
fur traders allowed a population explosion of sea urchins that
(Fig. 6a). Recent over-fishing in the North Pacific
may have triggered similar outbreaks of sea urchin, as killer whales
moved closer to shore and switched to sea otters as an alternate
. In the absence of dense populations of sea urchins, kelp
provides the physical structure for diverse subtidal communities
and attenuates waves that otherwise augment coastal erosion and
. Removing bass from lakes that were fertilized with
phosphorus caused an increase in minnows, which depleted the
biomass of phytoplankton grazers and caused algal blooms
(Fig. 6b). The algal blooms turned the lake from a net source to a net
sink of CO
. Thus, biotic change and altered nutrient cycles can
interact to influence whole-system carbon balance. The zebra
mussel (Dreissena polymorpha) is a bottom-dwelling invasive
species that, through its filter feeding, markedly reduces phyto-
plankton while increasing water clarity and phosphorus availabili-
. Introduction of this species shifts the controlling interactions of
the food web from the water column to the sediments. Trophic
interactions are also important in terrestrial ecosystems. At the
micro scale, predation on bacteria by protozoan grazers speeds
nitrogen cycling near plant roots, enhancing nitrogen availability to
. At the regional scale, an improvement in hunting technolo-
gy at the end of the Pleistocene may have contributed to the loss of
the Pleistocene megafauna and the widespread change from steppe
grassland to tundra that occurred in Siberia 10,000–18,000 years
. The resulting increase in mosses insulated the soil and led to
cooler soils, less decomposition and greater sequestration of carbon
in peat. Today, human harvest of animals continues to have a
pronounced effect of the functioning of ecosystems.
Competition, mutualisms and trophic interactions frequently
lead to secondary interactions among other species, often with
strong ecosystem effects (Fig. 5). For example, soil microbial com-
position can modify the outcome of competition among plant
, and plants modify the microbial community of their
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Figure 4 Mechanisms by which
species traits affect ecosystem
processes. Changes in biodiversity
alter the functional traits of species
in an ecosystem in ways that directly
influence ecosystem goods and
services (1) either positively (for
example, increased agricultural or
forestry production) or negatively
(for example, loss of harvestable
species or species with strong
aesthetic/cultural value). Changes in
species traits affect ecosystem
processes directly through changes
in biotic controls (2) and indirectly
through changes in abiotic controls,
such as availability of limiting
resources (3a), disturbance regime
(3b), or micro- or macroclimate
variables (3c). Illustrations of these
effects include: reduction in river
flow due to invasion of deep-rooted
desert trees (3a; photo by E.
Zavaleta); increased fire frequency
resulting from grass invasion that
destroys native trees and shrubs in
Hawaii (3b, photo by C. D’Antonio);
and insulation of soils by mosses in
arctic tundra, contributing to
conditions that allow for permafrost (3c; photo by D. Hooper). Altered processes can then influence the availability of ecosystem goods and services directly (4) or indirectly by further
altering biodiversity (5), resulting in loss of useful species or increases in noxious species.
Global changes Human benefits
3a 3b 3c
© 2000 Macmillan Magazines Ltd
neighbours, which, in turn, affects nitrogen supply and plant
. Stream predatory invertebrates alter the behaviour of their
prey, making them more vulnerable to fish predation, which leads to
an increase in the weight gain of fish
. In the terrestrial realm, graz-
ers can reduce grass cover to the point that avian predators keep vole
populations at low densities, allowing the persistence of Erodium
botrys, a preferred food of voles
. The presence of E. botrys increases
and increases soil moisture
, which often limits produc-
tion and nutrient cycling in dry grasslands. These examples clearly
indicate that all types of organisms — plants, animals and microor-
ganisms — must be considered in understanding the effects of
biodiversity on ecosystem functioning. Although each of these
examples is unique to a particular ecosystem, the ubiquitous nature
of species interactions with strong ecosystem effects makes these
interactions a general feature of ecosystem functioning. In many
cases, changes in these interactions alter the traits that are expressed
by species and therefore the effects of species on ecosystem process-
es. Consequently, simply knowing that a species is present or absent
is insufficient to predict its impact on ecosystems.
Many global changes alter the nature or timing of species interac-
. For example, the timing of plant flowering and the emergence
of pollinating insects differ in their responses to warming, with
potentially large effects on ecosystems and communities
Plant–herbivore interactions in diverse communities are less likely
to be disrupted by elevated CO
(ref. 43) than in simple systems
involving one specialist herbivore and its host plant
Resistance and resilience to change
The diversity–stability hypothesis suggests that diversity provides a
general insurance policy that minimizes the chance of large ecosys-
tem changes in response to global environmental change
microcosm experiments show less variability in ecosystem processes
in communities with greater species richness
, perhaps because
every species has a slightly different response to its physical and biotic
environment. The larger the number of functionally similar species
in a community, the greater is the probability that at least some of
these species will survive stochastic or directional changes in envi-
ronment and maintain the current properties of the ecosystem
This stability of processes has societal relevance. Many traditional
farmers plant diverse crops, not to maximize productivity in a given
year, but to decrease the chances of crop failure in a bad year
the loss of rare species may jeopardize the resilience of ecosystems.
For example, in rangeland ecosystems, rare species that are function-
ally similar to abundant ones become more common when grazing
reduces their abundant counterparts. This compensation in
response to release from competition minimizes the changes in
Species diversity also reduces the probability of outbreaks by ‘pest’
species by diluting the availability of their hosts. This decreases host-
, plant-feeding nematodes
and consumption of
preferred plant species
. In soils, microbial diversity decreases fungal
diseases owing to competition and interference among microbes
Resistance to invasions
Biodiversity can influence the ability of exotic species to invade com-
munities through either the influence of traits of resident species or
some cumulative effect of species richness. Early theoretical models
and observations of invasions on islands indicated that species-poor
communities would be more vulnerable to invasions because they
offered more empty niches
. However, studies of intact ecosystems
find both negative
correlations between species rich-
ness and invasions. This occurs in part because the underlying factors
that generate differences in diversity (for example, propagule supply,
disturbance regime and soil fertility) cannot be controlled and may
themselves be responsible for differences in invasibility
diversity effects on invasibility are scale-dependent in some cases. For
example, at the plot scale, where competitive interactions might exert
their effect, increased plant diversity correlated with lower vulnera-
bility to invasion in Central Plains grasslands of the United States.
Across landscape scales, however, ecological factors that promote
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Figure 5 Mechanisms by which species interactions affect ecosystem processes. Global environmental change affects species interactions (mutualism, competition and trophic
interactions) both directly (1) and through its effects on altered biodiversity. Species interactions may directly affect key traits (for example, the inhibition of microbial nitrogen fixation
by plant secondary metabolites) in ecosystem processes (2) or may alter the abundances of species with key traits (3). Examples of these species interactions include (a) mutualistic
consortia of microorganisms, each of which produces only some of the enzymes required to break down organic matter (photo by M. Klug), (b) altered abundances of native California
forbs due to competition from introduced European grasses (photo by H. Reynolds), and (c) alteration of algal biomass due to presence or absence of grazing minnows
(photo by M.
Power). Changes in species interactions and the resulting changes in community composition (3) may feedback to cause a cascade of further effects on species interactions (4).
© 2000 Macmillan Magazines Ltd
native plant diversity (for example, soil type and disturbance regime)
also promote species invasions
Experimental studies with plants
or soil microorganisms
show that vulnerability to invasion is governed more strongly by the
traits of resident and invading species than by species richness per se.
Both competition and trophic interactions contribute to these effects
of community composition on invasibility. For example, in its native
range, the Argentine ant (Linepithaema humile) is attacked by
species-specific parasitoids that modify its behaviour and reduce its
ability to dominate food resources and competitively exclude other
. These parasitoids are absent from the introduced range
of Argentine ants, which may explain their success at eliminating
native ant communities in North America
. Observational and
experimental studies together indicate that the effect of species
diversity on vulnerability to invasion depends on the components of
diversity involved (richness, evenness, composition and species
interactions) and their interactions with other ecological factors such
as disturbance regime, resource supply and rate of propagule arrival.
Humans significantly affect all of these factors (Figs 1, 4), thereby
dramatically increasing the incidence of invasions worldwide.
Societal consequences of altered diversity
Biodiversity and its links to ecosystem properties have cultural,
intellectual, aesthetic and spiritual values that are important to
society. In addition, changes in biodiversity that alter ecosystem func-
tioning have economic impacts through the provision of ecosystem
goods and services to society (Fig. 1 and Box 2). Changes in diversity
can directly reduce sources of food, fuel, structural materials, medici-
nals or genetic resources. These changes can also alter the abundance
of other species that control ecosystem processes, leading to further
changes in community composition and vulnerability to invasion.
Introduction of exotic species or changes in community composition
can affect ecosystem goods or services either by directly reducing
abundances of useful species (by predation or competition), or by
altering controls on critical ecosystem processes (Fig. 4).
These impacts can be wide-ranging and costly. For example, the
introduction of deep-rooted species in arid regions reduces supplies
and increases costs of water for human use. Marginal water losses to
the invasive star thistle, Centaurea solstitialis, in the Sacramento River
valley, California, have been valued at US$16–56 million per year (J. D.
Gerlach, unpublished results) (Fig. 7). In South Africa’s Cape region,
the presence of rapidly transpiring exotic pines raises the unit cost of
water procurement by nearly 30% (ref. 62). Increased evapotranspi-
ration due to the invasion of Tamarix in the United States costs an
estimated $65–180 million per year in reduced municipal and agricul-
tural water supplies
. In addition to raising water costs, the presence
of sediment-trapping Tamarix stands has narrowed river channels
and obstructed over-bank flows throughout the western United
States, increasing flood damages by as much as $50 million annually
Those species changes that have greatest ecological impact
frequently incur high societal costs. Changes in traits maintaining
constitute an ecosystem service whose value in
tropical forests has been estimated at $220 ha
(ref. 64). The loss
or addition of species that alter disturbance regimes can also be
costly. The increased fire frequency resulting from the cheatgrass
invasion in the western United States has reduced rangeland values
and air quality and led to increased expenditures on fire suppres-
. The disruption of key species interactions can also have large
societal and ecological consequences. Large populations of passenger
pigeons (Ectopistes migratorius) in the northeastern United States
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Figure 6 Trophic interactions can affect ecosystem
processes by influencing species’ abundances.
a, Removal of sea otters by Russian fur traders
caused an explosion in the population of sea urchins
that overgrazed kelp. (Photographs courtesy of M.
Sewell/Still Pictures and J. Rotman/BBC Natural History
Unit.) b, Similarly, changes in the species balance and
the abundance of fish can deplete phytoplankton grazers
and cause algal blooms. (Photograph courtesy of J.
Foott/BBC Natural History Unit.)
© 2000 Macmillan Magazines Ltd
may once have controlled Lyme tick-bearing mice by out-competing
them for food
. The loss of the passenger pigeon to nineteenth-
century over-hunting may, therefore, have contributed to the rise of
Lyme disease in humans in the twentieth century. The economic
impacts of invasions of novel species are particularly well document-
ed. The introduction and spread of single pests such as the golden
apple snail (Pomacea canaliculata) and the European corn borer
(Ostrinia nubilalis) have had major impacts on food production and
. Estimates of the overall cost of invasions by exotic
species in the United States range widely from $1.1 to $137 billion
. In Australia, plant invasions alone entail an annual cost
of US$2.1 billion
The provision of tangible ecosystem goods and services by
natural systems depends not only on species’ presence or absence
but also on their abundance. Large populations of the white-footed
mouse (Peromyscus leucopus) in the northeastern United States
control outbreaks of gypsy moth (Lymatria dispar) but spread
Lyme disease, whereas small populations of the mouse decrease the
incidence of Lyme disease but allow gypsy moth defoliation
analysis of the costs of changes in biodiversity thus involves more
than just analysis of extinctions and invasions. The loss of a species
to extinction is of special societal concern, however, because it is
irreversible. Future opportunities to learn and derive newly recog-
nized benefits from an extinct species are lost forever. Preventing
such a loss preserves an ‘option value’ for society — the value of
attaining more knowledge about species and their contribution to
human well being in order to make informed decisions in the
. For example, significant value ($230–330 million) has
been attributed to genetic information gained from preventing
land conversion in Jalisco, Mexico, in an area containing a wild
grass, teosinte (Euchlaena mexicana), that can be used to develop
viral-resistant strains of perennial corn
. If this land had been con-
verted to agriculture or human settlements, the societal benefits of
development would have come at the expense of an irreversible loss
in genetic material that could be used for breeding viral resistance
in one of the most widely consumed cereal crops in the world. The
perceived costs of diversity loss in this situation might have been
small — especially relative to the development benefits — whereas
the actual (unrecognized) costs of losing genetic diversity would
have been significant (Fig. 8). Decisions to preserve land to gain
further information about the societal value of species diversity or
ecosystem function typically involve a large degree of uncertainty,
which often leads to myopic decisions regarding land use.
Global environmental changes have the potential to exacerbate
the ecological and societal impacts of changes in biodiversity
many regions, land conversion forces declining populations towards
the edges of their species range, where they become increasingly
vulnerable to collapse if exposed to further human impact
ing allows the poleward spread of exotics and pathogens, such as
dengue- and malaria-transmitting mosquitoes (Aedes and Anopheles
and pests of key food crops, such as corn-boring insects
Warming can also exacerbate the impacts of water-consuming
invasive plant species in water-scarce areas by increasing regional
water losses. The Tamarix-invaded Colorado River in the United
States currently has a mean annual flow that is 10% less than regional
water allocations for human use
. Warming by 4˚C would reduce the
flow of the Colorado River by more than 20%, further increasing the
marginal costs of water losses to Tamarix
. Similar impacts of global
change in regions such as Sahelian Africa, which have less water and
less well developed distribution mechanisms, might directly affect
human survival. In many cases, accelerated biodiversity loss is
already jeopardizing the livelihoods of traditional peoples
The combination of irreversible species losses and positive
feedbacks between biodiversity changes and ecosystem processes are
likely to cause nonlinear cost increases to society in the future, partic-
ularly when thresholds of ecosystem resilience are exceeded
example, Imperata cylindrica, an aggressive indigenous grass,
colonizes forest lands of Asia that are cleared for slash-and-burn
agriculture, forming a monoculture grassland with no vascular plant
diversity and many fewer mammalian species than the native forest.
The total area of Imperatain Asia is currently about 35 million ha (4%
of land area)
. Once in place, Imperata is difficult and costly to
remove and enhances fire, which promotes the spread of the grass.
The annual cost of reversing this conversion in Indonesia, where 4%
of the nation’s area (8.6 million ha) is now in Imperata grasslands,
would be over $400 million if herbicides are used, and $1.2 billion if
labour is used to remove the grass manually. Farmers typically burn
the fields because herbicides and labour are too expensive. Burning
these grasslands, however, increases losses of soil nitrogen and
carbon, which erode agricultural productivity, and enhances regen-
eration of Imperata. This positive feedback with nonlinear changes in
land cover will probably continue in the future as lands are deforested
insight review articles
11 MAY 2000
Figure 7 Water losses to
the invasive, deep-rooted
provides an example of
the financial impacts of
introducing exotic species
courtesy of P. Collins/A-Z
Ecosystem services are defined as the processes and conditions of
natural ecosystems that support human activity and sustain human
life. Such services include the maintenance of soil fertility, climate
regulation and natural pest control, and provide flows of ecosystem
goods such as food, timber and fresh water. They also provide
intangible benefits such as aesthetic and cultural values
Ecosystem services are generated by the biodiversity present in
natural ecosystems. Ecologists and economists have begun to
quantify the impacts of changes in biodiversity on the delivery of
ecosystem services and to attach monetary value to these changes.
Techniques used to attach value to biodiversity change range from
direct valuation based on market prices to estimates of what
individuals are willing to pay to protect endangered wildlife
Although there are estimates of the global values of ecosystem
, valuation of the marginal losses that accompany specific
biodiversity changes are most relevant to policy decisions.
Predicting the value of such losses involves uncertainty, because
ecological and societal systems interact in nonlinear ways and
because human preferences change through time. Assumptions
today about future values may underestimate the values placed on
natural systems by future generations
. Therefore, minimizing loss
of biodiversity offers a conservative strategy for maintaining this
© 2000 Macmillan Magazines Ltd
for timber and agricultural purposes, causing further declines in
Uncertainty related to positive feedbacks and nonlinear changes
in land cover and biodiversity make social adaptation to change more
difficult and costly (Fig. 8). It may be more important from an
economic perspective to understand the nature and timing of rapid
or nonlinear changes in societal costs caused by loss of biodiversity
and associated ecosystem services than it is to predict average conse-
quences of current trends of species decline. By analogy, economic
models of ecological ‘surprises’ in response to climatic change show
that the information about the nonlinearities in damage from warm-
ing is worth up to six times more than information about current
trends in damage levels
. In the Imperata example, the costs of
replacing the original ecosystem goods and services from the forest
— including timber products, fire stability and soil nutrients — rise
sharply as Imperata spreads. If these nonlinearities in the ecological
and economic effects of this conversion had been anticipated,
policies could have been implemented to encourage agroforestry
instead of rice production or to reduce migration and settlement in
the most vulnerable areas
In sum, these examples indicate a tight coupling between altered
species diversity, ecosystem function and societal costs. A pressing
task for ecologists, land managers and environmental policy makers is
to determine where and when such tight couplings exist. Policies to
safeguard ecosystem services must be able to respond dynamically to
new knowledge, the rapidly changing global environment, and evolv-
ing societal needs. Nonlinearity, uncertainty and irreversibility call for
a more aggressive approach to mitigating changes in biodiversity than
is now being pursued so that future options are not foreclosed.
We are in the midst of one of the largest experiments in the history of
the Earth. Human effects on climate, biogeochemical cycles, land use
and mobility of organisms have changed the local and global diversi-
ty of the planet, with important ecosystem and societal consequences
(Fig. 1). The most important causes of altered biodiversity are factors
that can be regulated by changes in policy: emissions of greenhouse
gases, land-use change and species introductions. In the past, the
international community has moved to reduce detrimental human
impacts with unambiguous societal consequences. For example, the
Montreal Protocol prohibited release of chlorofluorocarbons in
response to evidence that these chemicals caused loss of ozone and
increased levels of cancer-producing UV-B radiation. Strong
evidence for changes in biodiversity and its ecosystem and societal
consequences calls for similar international actions. We urge the
following blueprint for action.
● The scientific community should intensify its efforts to identify
the causes of nonlinearities and thresholds in the response of
ecosystem and social processes to changes in biodiversity.
● The scientific community and informed citizens should become
engaged in conveying to the public, policy-makers and land man-
agers the enormity and irreversibility of current rapid changes in
biodiversity. Despite convincing scientific evidence, there is a
general lack of public awareness that change in biodiversity is a
global change with important ecological and societal impacts and
that these changes are not amenable to mitigation after they have
● Managers should consider the ecological and social consequences
of biodiversity change at all stages in land-use planning. For
example, environmental impact assessments should consider
both the current costs of ecosystem services that will be lost and
the risk of nonlinear future change. Managed landscapes can
support a large proportion of regional biodiversity with proper
planning, management and adaptive responses.
● Scientists and other citizens should collaborate with governmen-
tal organizations, from local to national levels, in developing and
implementing policies and regulations that reduce environmen-
tal deterioration and changes in biodiversity. For example, more
stringent restrictions on the import of biotic materials could curb
the rate of biotic invasions, and improved land and watershed
management could reduce their rates of spread.
● A new international body that would be comparable to the Inter-
governmental Panel on Climate Change (IPCC) should assess
changes in biodiversity and their consequences as an integral
component of the assessment of the societal impacts of global
● International bodies should establish and implement agreements
such as the Convention on Biological Diversity that institute
mechanisms for reducing activities that drive the changes in
biodiversity. These activities include fossil-fuel emissions,
land-use change and biotic introductions.
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