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OCR for page 16
4
The Panel's Advice to the USGCRP
The USGCRP's research program has emphasized climate change and vari-
ability (NSTC ~ 997~; three of its four main foci are related to climate, and the
fourth—changes in land cover and terrestrial and aquatic ecosystems is
strongly oriented toward how those changes interact with climate change. The
Pathways report (NRC ~ 999a) reflects that emphasis, although it makes strong
recommendations that more attention be paid to biological diversity, and its
recommendation for more research on carbon and nitrogen cycles includes a
strong recommendation for research on how those cycles directly affect
ecosystems. The Ecosystems Pane] fully endorses the emphasis on climate
change. Climate change is a global phenomenon and has the potential to have
large effects that are irreversible over many decades, perhaps centuries.
Understanding climate variability and change over periods from months to
centuries could well lead to the solution of related scientific questions and
could lead to breakthroughs in understanding. Climate change itself has the
potential to produce surprising effects, something that the Pathways report
identified as important to understand and be prepared for. Understanding
climate variability and change requires large, national and international
cooperative research programs, and requires a large and expensive scientific
infrastructure (e.g., satellites and supercomputers).
However, the model shown in Figure 3-1 makes clear that a focus on
climate variability and change is not sufficient. Global change has many
facets that do not act through the climate system. Some of these have already
had profound economic, sociological, biological, and political effects. For
example, biotic mixing has had much larger economic effects already than
climate change has or is likely to have over the next decade, even according
16
OCR for page 17
THE PANEL S ADVICE TO THE USGCRP
17
to the most pessimistic climate and economic models. The General Account-
ing Office reported that federal departments spent more than $630 million in
fiscal 2000 for invasive species activities and state governments spent more
than $230 million (GAO 2000~. Those expenditures do not include most
losses to nongovernment entities and are only for the United States. Many
species have become extinct as a result of biotic mixing. As another example,
the concentration of people and their infrastructure in urban areas has changed
the local climate (mainly temperature) ofthose areas faster than even the most
pessimistic climate models predict for global means over the next decade; as
a result, a significant proportion of the Earth's human population has already
experienced significant climate changes through anongIobal mechanism. The
transformation of landscapes from natural ecosystems to agricultural ones-
and to deserts in many places has had profound effects on people and biota,
as have the spread and evolution of resistance of many infectious diseases.
Changes in biogeochemical cycles in particular carbon and nitrogen have
effected many aspects ofthe Earth's environments, as have changes in hv~ro-
~ ~ _
logical cycles, in ways not mediated through climate.
Many of the most important environmental problems of the coming
decades are likely to reflect the combined action of several driving forces,
acting at a range of different spatial end temporal scales. Both the experimen-
tal evidence and the theory for understanding these responses are in the early
stages of development, leaving many unanswered questions.
Some ofthe problems will involve related causes. For example, elevated
carbon dioxide (CO2) is likely to occur in a context that is simultaneously
altered by warming and increased or decreased precipitation. But changes
will not be limited to climate and CO2 only. Atmospheric CO2 is changing in
the context of a system that includes nitrogen deposition, a history of chang-
ing land use, and widespread biological invasions. Some sites will mix
responses to these factors with contamination from pollutants, diminished
biological diversity, and altered frequency of disturbance.
For these reasons, the pane] recommends the development of a new
research initiative that emphasizes Cycles, Habitat (land cover and use),
Invasive species (biotic mixing), and Ecosystem Functioning (CHIEF), as
described in more detail below. This initiative is complementary to (and
partially overlaps) the four Research Imperatives in Chapter 2 ofthe Pathways
report (land surface and climate, biogeochemistry, multiple stresses, and
biodiversity). It is similar in many aspects to the research recommendations
of the Ecological Society of America's Sustainable Biosphere Initiative
(Lubchenco et al. 1991) and complements a recent National Science Board
report's recommendations (National Science Board 2000~. It is also based on
the panel's conclusion that much of the current ecosystem science research
OCR for page 18
18
GLOBAL CHANGE ECOSYSTEMS RESEARCH
agenda is applicable to understanding global change: to understand change,
we need to understand key processes before the change occurs. Ecosystem
functioning and biological diversity are related to cycles, habitat, and invasive
species, and they are discussed under each of those major headings in the
CHTEF initiative below.
Mounting a large, coordinated, and sustained effort to understand those
aspects of global change not emphasized in the current climate-related re-
search agenda is a significant undertaking but it is essential to understand,
predict, and deal with the major global changes that have already occurred and
whose pace seems likely to accelerate. Research on CHTEF issues would add
value to the current USGCRP by providing insight into what is at risk and
how risk might be reduced, much as medical and public health research adds
value.
The panel recommends that the USGCRP be broadened to include the
CHIEF issues raised in this report. Many productive reoptimizations of global
change research will result from including these ecological research areas.
Because the subject areas of CHTEF and those of the current USGCRP com-
plement one another, work on CHTEF will strengthen the USGCRP.
CYCLES
Nitrogen, carbon, phosphorus, and hydrologic cycles are critically
important to the nature and quality of life on Earth. Human activities have
pervasively affected those cycles, resulting in significant global changes. All
of these cycles affect nonhuman living organisms but they also are affected
by them. Because the cycles and humans' effects on them are not completely
understood, the panel recommends a substantial research effort on them. The
carbon cycle receives much attention, especially as it relates to climate. Much
better understanding of nitrogen and phosphorus cycles is needed, including
the role of the oceans in the cycles, the response of natural ecosystems to
nitrogen and phosphorus, and the interactions between them.
Hydrologic cycles have been profoundly altered and aquatic ecosystems
have been altered or destroyed by human activities (Data! 1990, Postel et al.
1996, 1999). Rivers have been regulated and channeled; in some cases, rivers
that used to flow year-round have become intermittent. Groundwater has been
mined andpolluted end recharge has oftenbeenreduced, affecting springs and
the quantity and quality of surface water in general; for example, Trautman
(1981) describes dramatic changes in Ohio's surface waters over the past 200
years. Much information and better understanding is needed ofthe details and
consequences of these processes on regional and global scales (Posse! et al.
1996, 1999).
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THE PANEL S ADVICE TO THE USGCRP
19
These cycles operate through a variety of mechanisms, some of which
are global in scale and others, although not global in scale individually, are
cumulative, and thus produce effects at global scales. For example, both
water and nitrogen are volatilized into gases (e.g., water vapor, ammonia) and
can be spread globally by atmospheric circulation. Nitrogen and phosphorus
fertilizers are applied over large areas of the Earth and the sum of regional
excesses and runoff in rivers produces global effects. Excesses of phospho-
rus, nitrogen, and other nutrients in the ocean can also be spread globally,
especially because they, like atmospheric nitrogen and water, are added to the
environment in many areas over the globe.
Changes in cycles of nitrogen and phosphorus are important because
they often are limiting nutrients for terrestrial and aquatic plants, which have
evolved in environments where the available forms of those nutrients are not
abundant. When their concentrations are increased by human activities, plants
are freed from these limitations and grow faster. In aquatic and marine
ecosystems, a process called eutrophication results: species compositions
change, the water often becomes less transparent, and dissolved oxygen often
decreases. In terrestrial systems, plants often store more carbon energy and
biomass in the presence of increased phosphorus and nitrogen, especially
when atmospheric CO2 is increased as well.
Nitrogen Deposition and Global Changes
Of the major global changes, human impacts on the nitrogen cycle are
among the most significant. Human actions more than double the natural
inputs to terrestrial ecosystems. Manufactured fertilizer, fossil-fuel combus-
tion, and cultivation of legumes with nitrogen-fixing symbioses are the
dominant anthropogenic sources (Vitousek et al. 1997a,b).
Nitrogen deposition plays a role in a diverse array of global changes,
many of which link to an even broader array of effects through interactions
with other factors. Nitrogen fixation for fertilizers has increased sharply since
the ~ 940s (Vitousek et al. ~ 997b). The surplus accumulates in soils, erodes,
and leaches to groundwaters (Vitousek et al. ~ 997b) and runs off into surface
waters (Carpenter et al. 1998~. From fresh waters, it arrives in estuaries and
coastal marine ecosystems. Some of the added nitrogen moves to the atmo-
sphere through volatilization of ammonia (NH3) (Schlesinger and Hartley
19921. Much of the nitrogen volatilized to the atmosphere re-enters aquatic
and terrestrial ecosystems when it is deposited from the atmosphere (Howarth
et al. ~ 996~. Nitrogen in the atmosphere as oxides of nitrogen (NOX) is a key
factor in atmospheric chemistry, especially ozone and photochemical smog.
It also contributes, along with sulfur, to acid precipitation. Nitrogen in the
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20
GLOBAL CHANGE ECOSYSTEMS RESEARCH
atmosphere as nitrous oxide (N2O) is a major greenhouse gas, accounting for
approximately 10°/O ofthe greenhouse effect of anthropogenic gases (Schimel
et al. 1996~. The mechanisms through which ecosystems regulate nitrogen
emission are incompletely understood, though agricultural practices can have
a large impact on fluxes as can land-use change, including the conversion of
tropical forest to pasture (Matson et al. 1997~.
Elevated CO2 and Global Changes
Observed and projected increases in atmospheric CO2 will have direct
physiological effects on plants but not on animals. Carbon dioxide is a nutri-
ent for plants and increases growth, especially in the presence of other nutri-
ents. Changes in plant performance can range from changes in species com-
position to changes in tissue chemistry and are likely to have significant
effects on the relationships between plants and their herbivores and among
plants and soil organisms. These effects are likely to be felt by animals at
higher trophic levels as well.
Most of the research on ecosystem responses to elevated CO2 has fo-
cused on the prospect for increased carbon storage as a consequence of
increased photosynthesis (Mooney et al. 1999~. The large body of research on
this topic has uncovered leads about a wide range of other kinds of responses,
many with the potential for major impacts on ecosystem structure and func-
tioning. Interactions between the carbon and nitrogen cycles have been
identified as important. Elevated CO2 is likely to intensify nitrogen limitation
(Pan et al. ~ 998~. In at least some cases, this leads to decreased emissions of
nitrogen trace gases (Hungate et al. ~ 997) or increased success of plants with
symbiotic nitrogen fixation (Hebeisen et al. 1997~.
Understanding the balance between sources of carbon and sinks is
important. The so-called "missing sink" is an important object of research,
because the known sources of carbon emissions—mainly CO2—are greater
than the known sinks, but our models have not accounted for the fate of the
excess carbon until recently, when evidence was presented that accumulation
of carbon in terrestrial forests or reforestation in the Northern Hemisphere
accounted for at least a substantial portion of it (Tans et al. 1990; Nabuurs et
al. ~997; Schime} et al. 20004. Complicating matters is the discovery that the
carbon budget is affected by nitrogen (e.g., Curtis et al. 2000; Pregitzer et al.
2000; Zak et al. 2000a,b). Increasing availability of soil nitrogen increases the
photosynthetic rate of some plants and hence their rate of storage of atmo-
spheric CO2 as carbon. Thus nitrogen pollution, a cause of eutrophication in
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THE PANEL S ADVICE TO THE USGCRP
21
many freshwater and marine ecosystems, perhaps helps to reduce or counter-
act some effects of CO2 emissions. These interactions in ecosystems need
much additional research.
Indirect effects of elevated CO2 can also be mediated through effects on
the water budget. Several empirical and modeling studies provide evidence
for local warming as a result of the tendency for elevated CO2 to decrease
water loss (Kimball et al. ~ 994, Bremer et al. ~ 996~. This affects temperature
because decreased water loss entails a decreased use of energy for evaporating
water, which leaves more energy to heat the air. In some simulations, the
warming caused by this mechanism is, at least in some locations, 25-50% of
the warming caused by solar radiation (Sellers et al. ~ 996~. This is an exam-
ple of a class of ecosystem impacts on climate that is very poorly understood
but potentially important. Such effects clearly deserve increased research
emphasis, including dedicated efforts to address interdisciplinary issues.
Other indirect effects of elevated CO2 can lead to changes in species
composition, though current capability to predict the nature and consequences
of these changes is quite limited. One of the pervasive ecosystem changes in
the last century the expanding dominance of woody plants in grassland and
savanna ecosystems worldwide may be at least in part a response to in-
creased CO2 (Polley et al. 1996~. Most of the woody plants invading grass-
lands use the C3 photosynthesis pathway (the terms C3 and C4 refer to specific
sequences of biochemical reactions to fix carbon). Photosynthesis in C3 plants
increases strongly under elevated CO2, while that of the grasses using the C4
photosynthesis pathway changes only slightly, if at all. Some of the woody
species expanding in grasslands are legumes, suggesting the possibility that
symbiotic nitrogen fixation further enhances these species' competence as
competitors. Of course, most terrestrial ecosystems have been subjected to
diverse human impacts over the past century, and it is possible, even likely,
that changes in fire regime, grazing management, and climate interact with the
elevated CQ to modulate the changing composition of grasslands.
Biogeochemical and Hydrologic Cycles and Potentially Harmful
Substances: Interactions with Biological Diversity and
Ecosystem Functioning
These cTimate-related issues are addressed in Chapter 2 ofthe Pathways
report, here the emphasis is on the ecosystem structure and functioning rather
than on the carbon budget. These factors affect biological diversity and
functioning primarily at local end regional scales but occur globally. The time
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22
GLOBAL CHANGE ECOSYSTEMS RESEARCH
scales of the processes and responses vary from instantaneous to cumulative
and Tong-term (century or greater) impacts.
Biogeochemical processes emphasized here relate to cycling of nutrients
significant for enhancement or limitation of primary production, those aspects
of the hydrologic cycle affecting the biological diversity of ecosystems and
the consequences of potentially ha~ful substances (e.g., toxic substances and
wastes) released into the habitat. Phosphorus is of particular concern as a
pollutant that causes eutrophication and significant changes in biotic composi-
tion of freshwater ecosystems (Carpenter et al. 1998~. Phosphorus is also
significant as a limiting resource to primary producers in the ocean as well as
in fresh waters (Vitousek and Howarth 1991; Tyrrell 1999~. Nitrogen also is
a concern for both marine and freshwater ecosystems as discussed below.
For most terrestrial ecosystems, the availability of freshwater is the
limiting resource for net ecosystem production and for delivery of ecosystem
services for human use. Freshwater resources are predicted ultimately to
become the limiting factor for expanding agricultural production, creating
competition between supplies for human services and natural resources. This
has already occurred in some arid regions (e.g., NRC 1999c).
The availability of water, its abundance, seasonal distribution, and
predictability exert significant controls on the composition, structure, and
biological diversity of ecosystems. The composition and structure of terres-
trial ecosystems feeds back on the hydrologic cycle through ecosystem func-
tioning, including patterns of evapotranspiration, and through canopy inter-
ception, infiltration, storage, and runoff. Biological diversity is frequently
greatest at soil moistures midway between soil drought and saturated condi-
tions, when there is little seasonal pattern in moisture distributions, and when
predictability of soil moisture is high. Either extreme of water resources
results in ecosystems having fewer species and Tower complexity.
The availability of water in the environment affects the rates of soil
weathering and decomposition and availability of nutrients for vegetative
growth, factors affecting the composition and diversity of ecosystems.
Differential growth responses of species to nutrient limitations and inputs
ultimately will influence how specific nutrients affect ecosystems. Changes
in availability of nutrients will affect competitive and trophic relationships,
potentially causing significant shifts in biodiversity. Under excessive water
inputs, soil nutrients may become leached from the soil and become deficient
(e.g., in the wet tropics). With most nutrients sequestered in tropical plant
canopies, ecosystem production and biodiversity depends on the quantity and
rate of decomposition for recycling of essential nutrients. Under water-
limited conditions, essential nutrients may be limiting due to low water
uptake; salts may accumulate in the soil to toxic conditions. Under saturated
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THE PANEL S ADVICE TO THE USGCRP
23
conditions, pH, electrical conductivity, and solubility of heavy metals may
increase. Relatively few species tolerate high accumulations of soil ions. The
recent NRC report on the hydrologic science content of the USGCRP (NRC
~ 9996) has research recommendations that complement the ones in this report
and also emphasize the coupling of hydrologic and biogeochemical cycles
through ecosystems.
Human activities have caused significant transiocation of soil nutrients
to groundwater and surface runoff, in some systems through poor irrigation
management, irrigation with high salinity water sources, and excessive
fertilizer inputs, which cause additions and enhancements of nutrients to
downsiope and downstream systems. For example, the hypoxic zone in the
Gulf of Mexico is presumed to be brought about by agriculture in the Missis-
sippi River watershed (Moffat ~ 998; Mississippi River/GulfofMexico Water-
shed Nutrient Task Force web site available at http://www.nos.noaa.gov/
products/pubs_hypox.htmI). A region defined by a dissolved-oxygen concen-
tration of less than 2 mg/L that is unable to sustain most forms of multicellular
organisms, this hypoxic zone forms along the bottom ofthe Gulfeach summer
as the seawater stratifies. Planktonic organisms die and sink, and the dis-
solved oxygen at lower depths is consumed as decomposition occurs. The
area of the hypoxic zone has been about ~ 8,000 km2 in recent years. There is
evidence that plankton growth in the Gulf is stimulated by nitrogen runoff
from fertilized fields and livestock in the Mississippi River Basin.
Human activities also affect water quality though the addition of a wide
range of potentially harmful substances and chemicals, e.g., herbicides,
pesticides, and industrial and household substances and through enhanced soil
erosion and sediment transport. Cumulative local problems have global
impact. Because of differential species growth and toxicity responses to
individual substances, it is difficult to generalize these impacts. In some
cases, primary producers may show little sensitivity, but devastating impacts
may occur at other trophic levels. Clearly, the time scales related to impacts
of toxic substances can vary from nearly immediate gradual, such as the
leaching of substances into groundwater, resulting in serious but delayed
impacts on ecosystems over decades or even centuries.
Requirements for Sustainable Production
Where biological production is actively managed, as in agriculture and
forestry, much remains to be learned about the amounts of various nutrients
and micronutrients that must be returned (recycled) to the land if sustainable
production and sustainable ecological functioning are to be achieved. The
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24
GEOBAE CHANGE ECOSYSTEMS RESEARCH
results of such research are required for the management of crop and forestry
wastes and to minimize the use of fertilizers and other external inputs.
Excess nitrogen that ends up in water has a range of impacts. It can help
stimulate blooms of harmful algae or enhance the success ofthe heterotrophs
that lead to dead zones deprived of oxygen. Extra nitrogen can also stimulate
other kinds of less dramatic but potentially important changes in the composi-
tion and function of aquatic ecosystems.
In terrestrial ecosystems, nitrogen deposition leads to diverse effects.
Some of these are biogeochemical, for example long-term decreases in the
availability of calcium (Likens et al. ~ 996~. Others are ecological, for exam-
ple, the replacement of heathiand vegetation adapted to nutrient-poor sites
with vegetation that is dramatically different in form and function (Berendse
and Elberse 19901. Nitrogen addition is a powerful cause of vegetation
changes, altering the balance of competition in many habitats (Tilman and
Downing ~ 994~. Especially in nutrient poor sites, the species that profit most
from nitrogen additions are often nonnative, providing additional pressure for
an expanding influence of biotic mixing (e.g., Lauenroth et al. 1978~.
Elevated CO2, especially in combination with other global changes,
could lead to a variety of other kinds of ecosystem responses. One hypothesis
clearly deserving more study involves water resources. When elevated CO2
leads to decreased transpiration, possible responses include increased runoff
and success of water-demanding species (Field et al. ~ 995~. When the species
that succeed under elevated CO2 are nonnative invaders, control becomes
more difficult, and negative impacts on native species become more likely
(Dukes and Mooney 1999~.
Many experimental and modeling studies document the impacts of
elevated CO2 on photosynthesis, transpiration, nutrient dynamics, and growth
of individual species. Yet it is very difficult to use these results as a founda-
tion for predicting changes in ecosystem structure and functioning, because
of our still-limited ability to model changes in species composition. These
challenges expand markedly when the context shifts from only elevated CO2
to a broader array of global changes. For example, developing the capacity
to predict changes in disturbance regimes, especially fire, presents a suite of
challenges different from those posed by elevated CO2.
To address these gaps in our knowledge, three kinds of approaches are
critical. First, experimental studies must run for long enough and on a large
enough spatial scale that investigators can observe species dynamics and
disturbance responses. Second, it is important to develop and explore mode!
systems where the full range of interactions and dynamics can be observed in
a tractable framework. Third, it is critical to take advantage of long-term
natural experiments, as in areas surrounding naturally occurring CO2 springs,
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THE PANEL S ADVICE TO THE USGCRP
25
to understand the effects of elevated CO2 on plants and biological communi-
ties (e.g., Cook et al. 1998~. Finally, the community must work to integrate
information from these three approaches, so it all contributes to answering a
single set of questions.
Much remains to be learned about the transport and fate of nitrogen
nutrients, about the rate at which anthropogenically fixed nitrogen is denitri-
fied in terrestrial and aquatic systems, and about the ecological consequences
of increased inputs of fixed nitrogen to aquatic and terrestrial ecosystems
(Schnoor et al. 1995; Vitousek et al. 1997; Galloway 1998; Socolow 1999~.
The fates and effects of anthropogenic nitrogen in the oceans are also of great
importance, and we know little about them. For example, Lapointe et al. (in
press) have argued that excess nitrogen loading has adversely affected corals
and other marine organisms in Florida Bay. Because nitrogen enhancement
through atmospheric pathways is global (Howarth et al. 1996; Vitousek et al.
1997a,b), open-ocean ecosystems could be affected, as coastal systems have
been (Vitousek et al. 1997a,b; NRC 2000~.
Key Questions
1. How will elevated atmospheric CO2 and anthropogenic acceleration
of the nitrogen and phosphorus cycles interact with other elements of global
change, especially climate change and biotic mixing, to alter the future
trajectory of primary production and the species composition of terrestrial,
aquatic, and marine ecosystems?
2. What are the impacts of freshwater diversion and contamination on
the resilience of terrestrial and aquatic ecosystems (i.e., their ability to cope
with global changes)?
3. What are the two-way interactions between changes in biological
diversity (at all scales) and changes in the global cycles of water, carbon, and
nutrients?
4. How will global changes in climate, land-use, and the composition
of the atmosphere alter frequency and intensity of disturbances like fire,
disease, andpest outbreaks, end how will these disturbances affect ecosystems
over the Tong term?
HABITAT: LAND USE AND LAND COVER
Among the most important global changes are the alteration of land-
scapes and degradation of habitats. Land cover is the ecological state and
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26
GEOBAE CHANGE ECOSYSTEMS RESEARCH
physical appearance of the land surface (e.g., closed forests, open forests,
grasslands) (Meyer and Turner 1994~. Change in land cover converts land of
one type to another, regardless of its use. Landt use refers to the purpose for
which the land is used by people (e.g., forests used for timber, recreation, or
pasture; industrial areas; human settlements) (Meyer and Turner 1994~.
Change in land use may cause a significant change in land cover, but need not.
For example, change from selectively harvested forests to protected forests
will not cause much discernible cover change in the short term, but change to
cultivated land will cause a large change in land cover.
The accumulation of small local changes poses a substantial long-term
challenge for research and land management. Individual changes in land use
may appear to have only local significance, but the large number of local
changes is transforming the surface of the earth. Gradual but widespread
change leads to significant impacts on vegetative cover, wildlife habitat, soils,
and water quality. Land-cover changes have profound global effects because
they are so pervasive and have such strong influences on biological diversity
and ecosystem functioning.
Human activities have strong effects on land use and land cover. As
global human population and the per-capita resource use have increased, there
have been changes in the area, type, and intensity of land use and land man-
agement (Perlin 1989; Turner et al. 1990; Dale et al. 2000~. Forests and
grasslands have undergone extensive changes as agriculture and managed
forest lands have expanded (Houghton 19954. Lands in some temperate
regions have been reforested during the past century as agricultural lands were
abandoned. Desertification provides an extreme example of land-cover
change that can result from land use (e.g., broad-scare Toss of tropical forest
can disrupt local hydrological regimes) (Shukla et al. ~ 990~. Patterns of land
use and land cover in the terrestrial landscape also affect aquatic systems,
directly (e.g., construction of dams and impoundments, drainage of wetlands
for agriculture, construction of aquaculture ponds in coastal areas) and indi-
rectly (e.g., through the nutrient-laden runoff that enters aquatic systems).
Land-cover patterns are also generated by disturbance regimes. There-
fore, changes in the frequency, intensity and extent (size) of natural distur-
bances have important implications for spatial pattern of land cover (Pickett
and White 1985~. For example, the reduction of natural fire frequency in
much of the southwestern United States has changed many forests from a
mosaic of young and old forests to forests dominated by mature late-succes-
sional trees, which often have dense fuel loads (Cooper 1960~. Instead of
frequent surface fires, the new pattern of land cover fosters more severe crown
fires that bum larger areas and create greater expanses of similar cover across
the landscape. These landscape changes alter the amount and distribution of
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THE PANEL S ADVICE TO THE USGCRP
27
cover and forage available for animals and thus affect faunal diversity as well.
As another example, alterations to flood regimes can influence the structure
of riparian habitat, modifying the composition and spatial and temporal
distribution of species that use those places and changing ecosystem processes
(Neiman and DeCamps ~ 997~. If the current path of land-use changes contin-
ues, continued Toss of wildlife and vegetation, erosion of soils, and nonpoint
pollution of groundwater and surface water is likely (Turner et al. ~ 9981.
Comprehensive global and accurate data on land-use and land-cover
types is an urgent data need, and the panel strongly endorses the USGCRP's
effort to obtain detailed, precise, and comprehensive data on land cover and
land use. Much of that effort is properly devoted to obtaining remotely sensed
data from satellites and aircraft. Information on land-cover patterns and the
location and rates of land-cover change needs to be global, but regional and
local data are also important because such changes occur at finer spatial
scales. The panel recommends that increased effort be devoted to ground-
based studies, especially outside the United States and Western Europe. In the
less-industrialized countries, the changes are faster and greater, and more
information is needed on the mechanisms of change and factors that influence
rates of change. In addition, the prediction of trends in land cover must
include consideration of socioeconomic factors that drive land use and of
changes in disturbance regimes that may result from changes in climate
(especially variation in extreme events, such as drought) and land use.
This report focuses on the ecological aspects of the USGCRP, and so the
panel does not provide specific advice on how to incorporate research on
social and economic drivers here. However, such interdisciplinary work is
essential, and we recommend that USGCRP encourage collaborations between
the Human Dimensions part of USGCRP and the research proposed here.
Biological Diversity
Worldwide, land cover today is altered principally by direct human use:
by agriculture, raising of livestock, forest harvesting, and construction (Meyer
1995~. Changes in land use and land cover can result in Toss and fragmenta-
tion of natural habitats as well as the introduction of new types of habitat.
Each of these changes can influence biodiversity. indeed, global land-use
change can be considered as an enormous uncontrolled experiment in how
habitat changes influence the biota and ecosystem functioning.
Land-use patterns have important influences on biodiversity for several
reasons (Turner et al. 1998~. First, land-use activities may alter the relative
abundances of natural habitats and result in the establishment of new land-
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28
GLOBAL CHANGE ECOSYSTEMS RESEARCH
cover types. The introduction of new cover types can increase the variety of
species by providing a greater diversity of habitats, but natural habitats are
often reduced, leaving less area available for native species. Nonnative
species also may gain a foothold and outcompete the native species (see next
section, Changed Biotic Mix). For example, the presence of livestock facili-
tates the spread of Eurasian grasses that can survive trampling. The livestock
defoliate areas, reduce the abundance of native grasses that cannot survive
trampling, and thus allow for the spread of nonnative grasses.
Second, the spatial pattern of habitats may be altered, often resulting in
the fragmentation of once-continuous habitat. The effects of habitat fragmen-
tation on animals, plants, and their habitats are numerous (see summaries by
Saunders et al. 1991 andNoss and Csuti 1994), and the biodiversity of native
species is almost always reduced. Habitat fragmentation may disrupt biologi-
cal processes, such as location of mates, predation, herbivory, dispersal of
juveniles, and migration, that are necessary for persistence of a species. For
example, the fragmentation of forests of the eastern United States into mosa-
ics of forested and open areas has led to numerous edge-related effects such
as increased nest parasitism by cow birds (Molothrus ater) on neotropical
migrant birds there. Fragmentation of Midwestern forests into smeller pieces,
along with overhunting, caused the disappearance of many wide-ranging
mammals by 1860 (especially animals at high trophic levels with large home
ranges, e.g., black bear [Ursus americanus], gray wolf [Cants /lupus], and
mountain lion Delis concolor]) (Reeves 1976~. Interestingly, increased
connectivity of habitat for iris species has resulted in hybridization among
species and changes in mating systems (Arnold and Bennett 1993~. Even
though the fragmentation of habitats is local or regional in scale, it is so
pervasive that there is a global Toss of large, continuous habitat types as a
result. Although a topic of much research, many questions remain about the
effects of habitat fragmentation because it affects different species in different
ways (Robinson et al. 1992~.
Third, land-use activities may change the natural pattern of environmen-
tal variation, especially by causing changes in natural patterns of disturbance.
For example, the environment may be changed directly when fire control and
logging after the frequency and extent of natural fires. Natural disturbances
create and maintain biodiversity by creating a mosaic of habitats; in general,
the chances of losing native species and disrupting ecological functioning
increase when the patterns of natural habitats are altered (Turner et al. 1998~.
The importance of understanding the legacy of past land use as a factor
explaining variability in present-day communities is increasingly recognized.
In the eastern United States and Upper Midwest, where reforestation has
dominated land use for the past one to two centuries, modern forests have not
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returned to their presettlement composition, and land-use history explains
much variation in current community composition (Foster 1992; Foster et al.
1992; MIadenoff and Whitel994~. Animals that thrive in early successional
habitats also increased in abundance when cleared areas predominated and are
now declining. For example, as forest cover across the New England states
increased to 75-90% during the past century (IrIand 1982), the New England
cottontail (Sylvilagus transitionalis) has declined substantially throughout its
range (Chapman and Stauffer ~ 981~. The pattern of decline is correlated with
losses of old fields and young forest habitats (Litvaitis 1993~. Recent work
in the southern Appalachians revealed a persistent effect of historical land use
on skeam fauna (Harding et al. 1998~. Studies of forest-harvesting patterns
have also demonstrated that landscape patterns created by land use may
persist for a long time, even after the regime changes or all harvesting ceases
(Wallin et al. ~ 994~. Thus, increased understanding of the legacy of past land
use and the trajectory of landscapes and their biotic communities when land
use changes is important, and research is needed.
Ecosystem Functioning
Land use and management often have a direct influence on nutrient and
water cycling and soil quality. For example, agricultural practices often add
nutrients in fertilizer and water by means of irrigation. However, there are
sometimes indirect effects on ecosystem cycling. The presence of large
expanses of concrete and asphalt or compaction due to livestock can change
patterns of nutrient and water runoff. Patterns of soil erosion or sedimentation
are often altered with intensified land management. Changes in water flows
and temperature in aquatic ecosystems often occur as a result of land-use
changes. Thus, elements of the landscape may serve as sources, sinks, or
transformers for soils, nutrients, sediment, and pollution loads.
Agriculture, forest harvest, and urban areas contribute to degradation of
freshwaters by increasing inputs of sift, nutrients, and pollutants (Carpenter
et al. 1996~. The nutrients most often considered in studies of land-water
interactions are nitrogen and phosphorus. Economic and health concerns
about excess nitrogen inputs into aquatic ecosystems are growing throughout
the world (e.g., Cole et al. 1993, Vitousek et al. 1997a). In rivers, nitrogen
biogeochemisky is sensitive to land-use patterns, the structure of the riparian
zone, end river flow-regimes. Accumulation of excess phosphorus in lake and
stream systems has long been recognized as a cause of eutrophication.
Riparian vegetation zones, including wetlands and floodplain forests, are
conspicuous elements of many landscapes and important mediators of land-
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GLOBAL CHANGE ECOSYSTEMS RESEARCH
water interactions (Neiman and DeCamps ~ 997~. Riparian vegetation can take
up large amounts of water, sediment, and nutrients from surface water and
groundwater draining agricultural areas within a catchment, substantially
reducing the discharges of nutrients to aquatic ecosystems. Thus, freshwaters
are especially sensitive to changes in these adjacent lands (Correll et al. ~ 992;
Osborne end Kovacic 1993; Correll 1997; Lowrance etal. 1997~. Worldwide,
wetlands, floodplains, and riparian vegetation zones have often been altered
by agricultural and urban development. For example, woody riparian vegeta-
tion once covered an estimated 30-40 million ha in the contiguous United
States (Swift 1984~; at least two-thirds of that area has been converted to
nonforest land uses, and only 10-14 million ha remained in the early 1970s.
it is important for scientists to better understand the interactions between land
(soil) and water, the scales over which they are manifest, and how future land-
cover changes are likely to affect riparian zones.
Many land uses are specifically designed to enhance productivity (e.g.,
agriculture and managed forests). Other land uses, such as suburbanization,
reduce the biological productivity of a site. Alteration in the manageUproduc-
tivity of a system often changes the vegetation structure, which influences
faunal diversity dependent on those structures. For example, timber harvest-
ing in oak forests jeopardizes the endangered cerulean warbler (Dendroica
cerulea), which nests only in oak canopy.
Key Questions
1. What are the current global patterns and rates of change in land use
and land cover? What explains the differences among regions? How might
the answers to these questions be used to predict future patterns of land use
and land cover? What are the critical socioeconomic and cultural driving
variables that are needed?
2. How do changes in climate, land use, and disturbance regimes
interact to produce dynamic landscapes? Are there thresholds in land-cover
patterns (e.g., connectivity of habitats or sizes of patches) beyond which large
changes in biodiversity and ecosystem functioning are likely to occur? Under
what combinations of climate, land use, and disturbance are these thresholds
likely to be crossed?
3. Are the legacies of prior land-use activities and spatial patterns
similar among different world biomes? To what extent are today's communi-
ties explained by historical land use?
4. How well do we understand the interactions between human activities
and ecosystem services? In other words, how well do we understand the
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factors that influence the human activities that most strongly affect ecosystem
services, and how well do we understand humans' responses to changes in
ecosystem services?
CHANGED BIOTIC MIX
By changed biotic mix, we mean changes in the kinds or proportions of
the species constituting a community. Any addition or deletion of a species
automatically constitutes a changed biotic mix, but so do disproportionate
increases or decreases in existing species.
Species Diversity
Conserving biological diversity, especially species diversity, has re-
ceived much attention. The ecological significance of species diversity is
discussed in the next section. Here, we point out that protecting endangered
species and biological diversity in general has been important to many people
for a long time. Although it has been difficult to establish an economic
argument for protecting species diversity, there are strong ethical, moral,
cultural, and aesthetic reasons for doing so (Sagoff 1996~. Many societies
reflect those values in their laws, as for example the U.S. Endangered Species
Act of ~ 973 (see NRC ~ 995), which declares it to be the policy of Congress
to conserve endangered and threatened species.
Nonindigenous invasive species ~S) are radically changing the biotic
mix in many communities (Bright 1998, Cohen and CarIton 1998), with a
variety of potential impacts on biological diversity and ecosystem functioning.
When they become established, such invaders can eliminate particular native
species, either globally or locally, through a variety of means. For example,
the Nile perch (Lates nilotica) in Lake Victoria has driven more than 100
species of endemic cichlid fishes to extinction (Goldschmidt 1996~. The
brown tree snake (Boiga irregularis) has eliminated almost all the forest birds
in Guam (Williamson ~ 996; NRC ~ 997~. Goats have extinguished more than
50 plant species just on the island of St. Helena (Groombridge ~ 992~. NIS can
compete with native species (e.g., rangeland plants in the western United
States [Mack 19893; house gecko (HemidFactylus frenatus) on islands tPetren
and Case 19961) or attack them (e.g., the brown tree snake ARC 19971;
African ice plant fMesembryanthemum crystallinum] in Califomia, whose
salty leaves make the surrounding soil uninhabitable for other plants nearby
tVivrette and Muller 19771; fire ants tSo1tenopsis invicta] fTschinkel 199311.
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GLOBAL CHANGE ECOSYSTEMS RESEARCH
NIS also can cause diseases in native species (e.g., whirling disease tthe
metazoan parasite Myxobolus cerebralis] in trout tRobbins 1996], avian pox
virus and malaria tPlasmodium species] in Hawaiian birds Ivan Riper et al.
~ 9863~. NIS can even hybridize with native species, causing a kind of genetic
extinction (e.g., mallards fAnas platyrhynchus] with the Hawaiian duck EAnas
wyvilliana] and New Zealand gray duck fAnas superciliosa] tRhymer and
SimberIoff 1996], introduced rainbow trout fOncorhynchus mykiss] with
native cutthroat Rout LO. ciarki] in the Rocky Mountains tVariey and
Schullery 19983~.
The global sum of these impacts of NIS is staggering, even if many of
them are individually small. It is almost impossible to estimate the total
annual cost of damage caused by NIS and of attempts to control them, but
Pimentel et al. (1999) recently reviewed the literature. Examples of cost
estimates in the United States include $97 billion for 79 species from 1906
through 1991 (OTA 1993~; $14.5 million per year in Florida alone to control
the aquatic plant hydrilla (Hy~lrilla verticillata) (Center et al. 1997~; $44
million per year in economic damage to commercial shellfish caused by the
green crab Carcinus maenas (Lafferty and Kuris ~ 996~; and $200 million in
yearly damage to docks and ships by the shipworm Teredo navalis (Cohen
and CarIton ~ 995~. After habitat loss, nonindigenous species are the second
greatest cause of recent endangerment and extinction (NRC 1995, Bright
1998~. As noted previously, the Nile perch alone has globally eliminated at
least 100 species, while the New World rosy wolf snail (Euglandina rosea)
has caused the global extinction of at least 30 endemic snail species on Pacific
islands (Civeyrel and Simberioff 1996~. Just three introduced tree species in
south Florida form nearly monospecific stands over nearly 600,000 ha
(Schmitz et al. 1997), while invasive nonindigenous weeds in rangelands of
the West moderately or heavily infest more than 40 million ha and currently
spread by more than 2,000 ha per day (Westbrooks 1998~. This is a global
change of the first order.
Various types of harvest can locally or totally remove one or more
species from the landscape. For instance, commercially valued tree species
like mahogany are often locally eliminated by selective forestry. Agriculture
can also eliminate particular species or groups of them, while leaving others
relatively unscathed. For example, freshwater mussels in the United States
have proven extremely vulnerable to water pollution and sedimentation
largely generated by agriculture; perhaps 10% are already extinct, while
another 60% are threatened (Stein and Flack ~ 997~. Aquaculture similarly can
eliminate particularly sensitive species through pollution and also by habitat
conversion. For example, the effluent from shrimp-farming is toxic to some
species, and large areas of mangrove habitat have been converted to aqua-
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culture, with Toss oftypical mangrove forest inhabitants (lin-Eong ~ 995; NRC
l 999b).
A modified disturbance regime may be inimical to one species and
favorable to another. In southern U.S. pine forests, routine controlled burning
that was substituted for lightning-caused fires was generally performed in the
winter, whereas lightning in that region occurs primarily May through July.
This change led to the gradual replacement ofthe originally dominant ground-
cover plant in Tongleaf pine (fin us opalustrus) forests, wiregrass (Aristida
stricta), by other species, such as Bromus spp. (Clewell 1989; Seamon et al.
1989~.
Large changes are occurring in the genotype frequencies of many species
(e.g., Policansky 1993), although the scope and consequences of these
changes are far more poorly understood than changes at the species level.
Many ofthe invasive species mentioned above certainly change the genotypes
of the native species they affect, even if they do not cause their extirpation.
l:n many cases, supplementation of depleted wil~populations throughhatcher-
ies changes genotypes and ecotypes of the remaining wild populations (e.g.,
NRC 1996b; Waples 1999~.
Ecosystem Functioning
The ecological significance of species diversity has been of scientific
interest for many years (e.g., Hutchinson ~ 959~. How species diversity affects
ecosystem functioning continues to be of great scientific interest today (e.g.,
Mooney et al. 1995; Tilman 1996; Grime 1997~. While it is clear that if
enough species are lost, ecosystem functioning will be impaired, it is not clear
how many species are "enough," and in general how the kinds and numbers
of species in an ecosystem affect a variety of ecological processes. Because
human activities are affecting both the number and kinds of species in most
ecosystems, especially terrestrial and freshwater ones, this general scientific
question assumes heightened importance and urgency in any global change
research program.
The most far-reaching ecosystem-wide impacts are usually caused by
plant NIS that replace the previous dominants. In south Florida, Australian
Melaleuca quinquenervia (paperb ark tree), introduced cat l 900, increased to
200,000 ha, replacing sawgrass (CIadium spp.) and muhly prairies, as well as
some cypress (Taxodium spp.) swamps. It overgrows the native plants,
outcompeting them for light. Tn addition, Melaleuca is flammable; it reseeds
readily after fires, but the native plants do not. It uses large amounts of water
and has Towered the water table in many areas. It also changes water flow
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GLOBAL CHANGE ECOSYSTEMS RESEARCH
patterns by deposition of abscised leaves. All of these changes have led to
virtual monocultures of Melaleuca, as few native plants can tolerate the
environment Melaleuca has created. Animals that typically inhabit the native
plant communities then largely disappear (Schmitz et al. 1997~. Eurasian
cheatgrass (Bromus tectorum) dominates 7 million ha of rangeland in Idaho
and Utah alone, not by overgrowing native plants, but by facilitating fires and
by having a deep taproot, as opposed to the shallow root mass of the natives.
Once established by virtue of its impact on the fire regime and hydrology,
cheatgrass replaces native plants and populations of many animals subse-
quently plummet (Westbrooks 1998~. South American water hyacinth
(Crassipes eichhornia) at one time completely covered over 50,000 ha of
Florida waters. Today it blankets thousands of hectares of Lake Victoria. The
mats shade out submersed vegetation, which dies. Eventually the dying parts
of water hyacinth itself contribute to a growing oxygen deficit, and animal
populations decline. In all these cases, the MS has direct population impacts
on certain species (usually by competition), but many more indirect impacts
on numerous species generated through ecosystem effects. A similar situation
arises in Hawaii, where the Atlantic firebush (Myrica faya), a nitrogen-fixer,
has heavily invaded nitrogen-poor volcanic soils. Its subsequent nitrogen-
fixing activities have changed successional pathways to favor other
nonindigenous plant species that could not have thrived in the original voica-
nic soils (Vitousek 19861.
Nonindigenous plant species can hybridize with natives to form an
invasive pest. The North American corUgrass Spartina alterniflora, intro-
duced to England around 1829, was a harmless minor component of the
British flora, occasionally producing sterile hybrids with the native S. mari-
tima. Then, sometime before ~ 890, one such hybrid underwent a spontaneous
doubling of chromosomes to become fertile, becoming a major invasive weed
(Thompson 1991~.
A nonindigenous pathogen that removes a formerly dominant native
plant can also have ecosystem impacts. The chestnut blight, Cryphonectria
parasitica, virtually eliminated American chestnut (Castanea dentata) from
the forest canopy. The chestnut had been the dominant tree in much of the
Eastern U.S. (von Broembsen 1989~. This near extinction almost certainly
had numerous ripple effects on both ecosystem processes and particular
species associated with chestnut, but few were studied. It is known that at
least ten insect species host-specific to chestnut subsequently went extinct
(Opler 1979~. A pathogen that attacks dominant herbivores can similarly
greatly change the mix of species in native communities. For example, the
introduction into African cattle of the rinderpest virus led to the infection of
many native ungulate species, with mortality in some species reaching 90°/O
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and geographic distribution of certain species altered for a century (Dobson
1995~.
A nonindigenous animal can effect substantial
changes in ecosystem
functioning, for example, by either modifying the dominant plants (e.g., the
periwinkle on the coast ofthe Northeastern United States and adjacent Canada
tBertness ~ 9843) or by constituting a new habitat (e.g., zebra mussels on soft-
bottom areas in U.S. rivers and lakes tRicciardi et al. 19973~.
Change in a disturbance regime over large areas can affect entire ecosys-
tems. For example, suppression of fire in longleaf pine forests of the South-
east has led to the replacement of pine-dominated forests by hardwoods. The
precise ecosystem consequences have not been studied, but such processes as
nutrient cycling must be greatly changed. Replacement of storm-driven
floods along the Colorado River by floods regulated by dams built for hydro-
electric power led to the establishment of forests of Asian tamarisk (Tamarix
spp.) and native willows (Salix spp.) on formerly bare banks.
Key Questions
~ . What combination of nonindigenous species and potential recipient
community will lead to establishment and spread of the NIS? Only about
10% of species introduced to and surviving in a new range actually become
invasive (Williamson ~ 996~. Sometimes the same species can be invasive in
one region and rare and innocuous in another. There has been some limited
success in attempting to predict invasiveness simply from species traits (e.g.,
Rejmanek and Richardson ~ 996), but little experimentation to understand why
some habitats appear more susceptible to invasion than others.
2. Why do many NIS remain harmlessly noninvasive for many genera-
tions, then suddenly begin spreading rapidly (e.g., Brazilian pepper [Schinus
terebinthifolius] in Florida)? Is this due to a change in the species, a change
in the community, or an expected demographic lag? Conversely, why do
some NTS that are so invasive as to be considered scourges dramatically
decline, sometimes to near-rarity (e.g., Canadian pondweed fElodea canaden-
sis] in Great Britain and the giant African snail fAchatina fulica] on many
Pacific islands)?
3. What role does genetic change play in changed invasiveness? To
what degree are lessons learned from the evolution of virulence in pathogenic
disease vectors transferable to the understanding of genetic changes leading
to invasiveness or to increased invasiveness after an exotic becomes estab-
lished?
4. The substitution of nonnative genotypes, often uniform ones, in
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GLOBAL CHANGE ECOSYSTEMS RESEARCH
populations of forest trees, game and food fishes, and other organisms, may
have major consequences forbothbiodiversity and ecosystem functioning, but
these have barely been studied. Brook trout (Salvelinus fontinalis) have been
genetically homogenized by repeated introductions and use of hatchery stock,
while tree species like Pinus radiata have been planted in huge expanses.
Will such populations be as resilient to subsequent biotic and abiotic environ-
mental change as the genetically more heterogeneous ones they replaced?
Will they be able to evolve as quickly?
IMPLEMENTATION
The research questions at the core of each element of CHIEF are so
important for the future of ecosystem science, global change research, and the
sustainability of the biosphere that each warrants a substantial and sustained
investment. Some ofthe questions are ripe for rapid progress in the next few
years. Others, no less important, will yield more slowly, and the major
breakthroughs may be a decade or even more in the future.
Progress in answering each of the major questions posed in this report
will almost certainly require information and insights from and even beyond
the full sweep of global change research. Ultimately, the answers to each
question will need to be integrated with all of the others. Indeed, it is likely
that the major factors that control processes within each element in CHIEF are
mechanisms that unfold in other elements as well. For example, invasive
species may be among the strongest influences on ecosystem functioning;
similarly, changes in land use might be the strongest influences on changes in
the major cycles. As the elements of CHIEF, plus other components of the
global change research agenda, converge into a single suite of interlocking
processes, the agenda will reach its richest phase.
Mounting a coordinated and sustained commitment to advancing the
CHIEF agenda should be a priority for the USGCRP. Although significant
new funding will be required, and the distribution of resources among pro-
gram elements will likely need to be adjusted, the initiative should not require
major changes in the current administrative structure of the USGCRP. The
structural changes needed to implement this research initiative involve pri-
marily a shift in focus that is more inclusive than exclusive, and the incorpora-
tion of current federal research programs that would benefit from the initia-
tive. A CHIEF research initiative would do for ecological research what the
USGCRP has done for atmospheric research: it would stimulate and better
coordinate, at the highest administrative level, agency research programs in
nutrient cycles, habitat changes, invasive species, and ecosystem functioning.
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Recent progress has been so rapid, and the need for integration is so
great, that the identity of key questions and the boundaries between disciplines
need to be flexible at a level that has never been required in the past. The
world is changing too rapidly for science to address the challenges of global
change with traditional, incremental approaches.
Representative terms from entire chapter:
ecosystem functioning
