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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
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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
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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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THE PANEL S ADVICE TO THE USGCRP 29 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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30 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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THE PANEL S ADVICE TO THE USGCRP 31 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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32 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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THE PANEL S ADVICE TO THE USGCRP 33 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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34 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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THE PANEL S ADVICE TO THE USGCRP 35 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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36 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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THE PANEL S ADVICE TO THE USGCRP 37 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: