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Ecological Systems and Dynamics COORDINATOR: MARGARET B. DAVIS Ecological systems play a complex role in global change, as previ- ous documents have emphasized (Bolin et al., 1986; ICSU, 1986a,b; National Research Council, 1986a). The following questions illus- trate the information required to improve understanding of the role of ecological systems in global change: What are the most significant global variables affecting the dynamics of ecological systems, and how can biotic responses to global change be predicted? O What ecological processes and mechanisms require further understanding, and what data sets are essential to model biotic responses to global change? What are the temporal en cl spatial dynamics in the responses of ecological systems to global change? How can they be docu- mented? How will transfers of materials across ecosystem boundaries be affected by global change? This paper is the result of discussions at two workshops on ecological systems and dynamics, one focusing on terrestrial systems and one on marine systems. The contributions of those participants listed in the appendix to this paper are gratefully acknowledged. 69

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70 ~ How clo past responses of ecological systems, recorded by the fossil record, aid in predicting future response to global change? ~ What are the characteristics and generalities of the feedback processes between ecological systems and the global system? The first five questions are discussecl below. Feedbacks to the global system are referred to throughout this paper, and are ad- dressed specifically in the companion paper on "Climatic and Hydro- :togic Systems." The final section discusses research priorities for the ecological component of a research program on global change. Responses of ecological systems to global change are complex because of the inherent intricacies of ecological systems and their interactions with the physical system, and because those processes depend on the influences of history en c] scale. Furthermore, ecological systems encompass a vast array of temporal variability, with response times varying in different parts of a single, interacting system. In addition, multiple stresses inevitably affect virtually all biotic sys- tems. As simple examples, Great Lakes fisheries are simultaneously influenced by the multiple impacts of eutrophication, toxic pollution, and the introduction and removal of species (Evans et al., 1988~. For- est dieback occurs under changing conditions of acid precipitation, heavy metals, ozone, drought, complex forest demographics, and as- sociated nutrient cycles (Klein and Perkins, 1987~. The marine biotic community is also complex and highly variable, frequently clemon- strating situations where the effects are removed in time and space from the events that caused them (Rothschild, 1986, 1988~. These examples caution against efforts to predict biotic responses, for ex- ample, by simply correlating biological phenomena with the spatial distribution of climate or by using single-factor causation theories. Only limited success would be likely with these techniques because biotic responses are often nonlinear and involve feedbacks at many temporal and spatial scales (National Research Council, 1986b). Because of these particular characteristics of ecological systems, studies undertaken in the IGBP must include experimental ap- proaches that address multiple stresses at several levels of organiza- tion, including whole ecosystems. The following discussion (lescribes the challenge of extrapolating local observations and experiments to larger scales, in order to connect the biotic and abiotic components and to understand feedbacks to the global system.

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71 WHAT VARIABLES DRIVE CHANGES IN BIOLOGICAL SYSTEMS? Changes in Climate A doubling of the concentrations of radiative gases in the atmo- sphere is expected to lead to a rise in global temperatures of 3C ~ 1.5C (Jaeger, 1988~. Global temperatures will change rapidly, the rate of increase ranging between 0.1C and 0.8C per decade (see Figure 2 of the companion paper on "Climatic and Hydrologic Sys- tems"~. The spatial and temporal patterns of temperature increases will be heterogeneous and are expected to be greatest in the northern mic3-continent region of North America and Eurasia (R.E. Dickinson, personal communication). Although general circulation moclels do not agree on the magnitude of change, ah models predict a change in precipitation in the m-continent. An important observation is that not only will the predicted temperatures be higher than any expe- rienced during the last several million years but the rates of change are more than an order of magnitude faster than any recorded in Quaternary history (M. Davis, personal communication, University of Minnesota). Ecological systems would therefore be required to respond to quite different temperature conditions from those of the past millions of years, which raises questions about their potential adaptive response. The enormous significance of the rate of temperature change can be appreciated when spatial displacement is considered. A temper- ature rise of 5C, for example, would imply northward displacement of isotherms in North America by 500 km. This change could occur within 100 years (Jaeger, 1988~. Range extensions or movement of forest trees during the Holocene, recorded by fossil pollen, was only 25 to 40 km per century (Davis, 1981; Huntley and Birks, 1983), with the fastest range adjustment by spruce into northwestern Canada at 200 km per century (Ritchie and MacDonald, 1986~. Given these rates of plant dispersal, vegetation would not be able to change its geographical distribution as fast as the changes in suitable habitat. As a result, there would be lags decades in length in the adjustment of ecological systems to rapidly changing climatic conditions. These lags in the match between climate and vegetation will become appar- ent in the mid-continent Tong before doubling of carbon dioxi(le has occurred. This phenomenon is described in Figure 1, which shows an example of how the geographical distribution of suitable climate

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72 / / 0 400km , .... \, ~ ~ \ \, ~ ~ \ \.4 ~ ~ a . . FIGURE 1 Present and future range for a common forest tree, eastern hemlock (Tsuga canadensi~J, under climate scenarios predicted by (a) Hansen et al. (1983) and (b) Manabe and Wetherald (1987~. Horizontal shading is the present range, and vertical shading the potential range with CO2 doubling. Cross-hat~hed area of and the actual geographical distribution of a tree species may fait to coincide 100 years from now (Zabinski and Davis, unpublished data). Ecophysiological responses to stress and the ability of plants and animals to reproduce and establish themselves ultimately determine the geographical range limits of individual species. Frequently, for example, in Figure 1, critical threshold values are now deduced from climatic correlations with geographical distributions, but more quan- titative experimental data on responses are needed, as wed as addi- tional empirical observations, e.g., from dendrochronology (Garfinkel

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~3 1~ , ~ .' 0 400km I 1 ~ 1 I ~ ~ ~ ~ 1 -__., >A ~( \\ ~ an\ I . . )^ b ovens is where the tree we Only to be Id 1~ yea boa now. React coupes ago persist to the south in po^ts of gargle e~lr~nt. S1~c~t adduce to the Tab is ~1~ ~ Eta ~ the pat were ^out 25 ~/100 yr, ad the most regild ate ~~n Boa the Sail aced tar spruce) is only 200 -/100 a. As a conse-~ce, ~^ of the potently rage w10 reran Coupled. and Blubbers lOSO). Dlrect physi~o~c~ observ~t10ns are needed r plants and animus near tbelr range bmUs. In ~ddldon to the ecophyslologlc~ processes of the org~nlsms, reproduchon dyn~mlcs wlU ^ct rates of in Tin popul~tlon cb~nge and the rate of popul~tlon dl~slon to new geogr~phlc locations (Brub~ker, 1986). ~lgr~tlon and the away to c~onlze new b~blt~s ~1 ^ct the Salty ~ species popul~tlons to track dlm~tlc cb~nge.

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74 Existing data on exotic species may prove particularly useful in pre- dicting species' responses to environmental change. For example, the population dynamics of successful invaders (Harper, 1977) may provide information needed to predict which species will spread and expand in response to changed future environments. Such an anal- ysis would define the genetic characteristics of a species that allow it to expand or that contribute to its extinction in response to en- vironmental change. It would also provide data to indicate how the genetic structure of future populations would be affected. Responses to past catastrophes, such as major extinctions of organisms, and documentation of which species survived and which became extinct, will be valuable to these studies. Also, evaluation of the habitats susceptible to biological invasions win permit a complementary ap- proach for identifying likely spatial responses of biotic components to changing global climate. Quaternary paleorecords document rates of range expansion and also show that species do not necessarily move as a group but have responded to change individualistically (Davis, 1981~. As a result, ecosystems that are of limited spatial extent today may have been much more expansive in the past. An example is the oak savanna, which today forms the narrow ecotone between prairie and forest in North America, but which covered an area hundreds of kilome- ters wide during the mid-Holocene (McAndrews, 1967~. Species that are rare or geographically localized today, such as bristIecone pine (Pines aristata), were abundant in the past, while ponderosa pine (P. ponderosa), the dominant tree over large regions of the Rocky Mountains today, was very rare during the last glacial period (Spauld- ing et al., 1983~. Spruce (Picea), which now characterizes the vast boreal biome, was sparse throughout North America in the early Holocene (Webb, in press). These examples show that one cannot assume that existing blames will remain intact under future changes Of global climate. The fossil record shows clearly that communities may be disassembled and species reassembler! in new combinations in response to new climatic conditions (Davis, 1981; Graham, 1986~. The resulting new combinations of vegetation, climate, and soils can result in altered spatial patterns of such fundamental processes as net primary production (Pastor and Post, 1988~. More subtle, but still important, processes such as evolved host-pathogen relationships may also be disrupted by the stress of new conditions, resulting in increased frequency of epidemics (Leonard and Fry, 1986~. Global change will have a major impact on biological cliversity.

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75 Although absolute, and even relative, rates of species extinction and formation are not known precisely, changes in species and species number have presumably been a continuing process. There are now, however, conditions that have not been present over the past thou- sands of years. Human activity now adds substantially to changes in habitat on a global basis and, as a consequence, increases the loss of species (Lovejoy, 1980~. This factor will be exacerbated by future cTi- matic changes, which wiD alter the scale and patchiness of landscape units (e.g., forested versus unforested areas), both through direct effects en cl indirectly by changing patterns of land use by the hu- man population. To analyze the combined impact of these changes, information is needed on (1) how landscape pattern affects species survival and extinction, (2) the effect of patchiness on dispersal to new habitats, (3) the effect of species interactions on survival and extinction, and (4) direct effects of human activities on abundance and distribution of organisms. Certainly, fragmentation will result in fewer pathways for species migration toward favorable habitats. In this regard, small ecological reserves may be especially vuinera- ble to the effects of climate change (Peters and Darling, 1985~. A likely result will be the extinction of species that such reserves were established to preserve. A fundamental question in the context of global change is whether changes in species composition and diversity will significantly af- fect ecosystem function. There are examples where ecosystem func- tion appears to be relatively independent of species composition (Schindler, 1988~. There are also examples where changes in species have had remarkable alterations in ecosystem processes, for exam- ple, the alterations produced by key predators such as the starfish Pisaster in the rocky intertidal (Paine, 1966) or the wolf on Isle Royale (Mech, 1966~. A single nitrogen-fixing species can have a large effect on succession (Crocker and Major, 1955; Vitousek et al., 1987~. Thus, quite aside from the question of biodiversity per se, the question of species replacement in ecosystem function must be addressed in a much wider array of ecosystem types. Biological and atmospheric properties are coupled through sev- eral funilamental cycles, particularly the carbon and hydrological cycles. Predicting feedbacks to the climate system involves pre- (licting how communities and key species in those communities will respond to changed climate. A major question is whether responses can be generalized from existing ecosystems. Because climate change will be large, species turnover wiD often occur, changing ecosystem ~ ~ . ~

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76 properties in many (although not all) cases. Thus the influence of species composition on ecosystem properties becomes a critical issue. Climate change wiB also have direct effects on ecosystem pro- cesses and gas exchange with the atmosphere. General principles linking environmental factors (temperature, light, nutrients) to whole plant carbon fixation and avocation above and below ground and to photosynthetic versus nonphotosynthetic tissue are just now emerg- ing (Bazzaz et al., 1987~. Effects of climate variables on plants and soils can be determined using field and laboratory approaches. How- ever, complex interactions of soils, plants, and climate occur. Recent simulation of climate change in semiarid grasslands revealed a strong, transient (ca. 50 to 100 years) increase in net primary production, despite drier conditions in the model (D. Schimel, personal commu- nication). This was because higher temperatures led to higher rates of microbial mineralization of soil nutrients. The increase in nu- trient availability in the simulations compensated for the reduction in moisture until soil reserves of nutrients were depleted, at which time production crashed. The intensity and timing of the transient varied regionally, depending upon levels of primary production and initial soil organic matter. Similar interactions are simulated for forests (Pastor and Post, 1988~. For global applications, results from such stuclies must be generalized from the systems in which detailed studies are available. This research indicates that measurements of variables that control ecosystem response to change must be made over large areas for input to global models. For aggregation of biological data from regional to global scales, remote sensing of vegetation structure and composition, land form characteristics, and certain biophysical characteristics will be neces- sary. Because of the potential for repeated coverage of the globe, satellite observations should provide useful data at many scales. Re- mote sensing provides information on states of specific variables, but it does not yet provide actual measurements of a process or flux. For these measurements, new technology is becoming available. Airborne and ground-based laser-based systems, such as I,IDAR (light detection and ranging) technology, tunable diode lasers, and Fourier-transformed infrared spectrometry, allow measurement of at- mospheric gas concentrations at varying scales (Harries et al., 1988; Matson and Harriss, in press). Changes in gas concentrations can be coupled with aerodynamic physical flux measurements to estimate exchange of gases (such as CO2, CH4, N20, and NO) between ter- restrial ecosystems and the atmosphere. These approaches are just

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77 beginning to be applied generally to ecological questions, but initial tests have proven very encouraging (Gosz et al., 1988~. In the marine environment, the record of the past leaves little doubt that global warming will result in different distributions of planktonic organisms than those of to(lay (CLIMAP Project, 1976~. If in the simplest case the ocean warming were to be positively corre- lated with latitude, one would expect that the expansion of habitat in a poleward direction, which has occurred during the Holocene, would continue. But when one recognizes that the mean global warming projected for the next several decades is comparable to that experi- enced in the last 20,000 years, questions immediately arise regarding the potential of the biota to accommodate to these rates of change. Also, our knowledge of plankton distributions in the past is based on data for the few taxa, such as the foram~nifera, that have easily preserved hard parts. Because the ecological role of these protozoans is not wed known, it is difficult to predict the degree to which changes in their species composition indicates a change in the plankton com- munity in general. Increased warming and precipitation will decrease the density of surface waters, especially at high latitudes. If the warming and fresh- ening of the surface water outpace the processes of convection and isopycnal mixing, vertical diffusion between the surface and the main thermocTine could be much slower. This isolation of the main ther- mocline could severely impede the vertical transport of remineraTized nutrients, severely diminishing the huge spring bloom characteristic of the North Atlantic. At high arctic latitudes, the reverse could oc- cur. At present, very strong stratification is maintained because the freezing, high-salinity waters produced on the continental shelves during pack ice formation are incorporated into the thermocI;ne. Thus the warm inflowing "Atlantic water" cannot upwell and is com- pletely isolated from the surface layer. Warming and freshening will diminish ice formation and could lead to an ice-free Arctic. Ventila- tion of the main thermocTine of the Atlantic from the Arctic via the Greenland-Scotland overflows could occur, with the result of much higher rates of primary production and nutrient cycling. The regional effects of global warming on the plankton habi- tat in near-surface waters are unpredictable at present. Strength of wind fields and their orientation will vary. Along-shore winds con- tribute to the upwelling process in many coastal waters and across the equatorial Pacific. The direction, intensity, duration, and frequency of these wind events determine the extent and timing of upwelling

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78 events. Because this process, which is typically highly seasonal, is very important in stimulating the primary production processes that lie at the base of the food webs for many commercially exploited species, it can be anticipated that global climate change will have significant economic consequences, especially for the fish-harvesting nations. Changes in the intensity and frequency of stratification and de- stratification processes will have differential effects on plankton, de- pending on their physiology and anatomy. Diatoms, for example, are phytoplankton that typically dominate in cold nutrient-rich waters, such as those that have recently upweHed. Because of their high sink- ing rates, diatoms are also major contributors to the flux of carbon to the deep ocean (Smayda, 1970~. A turbulent mixed layer also seems to be a requirement for diatom success. Diatoms are the preferred food of many organisms in the food webs of commercially exploited fishes, and when replaced by other types of phytoplankton the di- noflagelIates, for example, in the case of the Peru Currentthe yield of the fish of greatest economic interest is reduced dramatically (Bar- ber and Chavez, 1983~. There is some evidence that at least some economically impor- tant seaweeds may be quite sensitive to increases in water temper- ature. During the 1982-1983 El Nino event in Chile, the northern populations of the alga Durvillea disappeared and have not yet re- colonizec3 (Tomicic, 1985~. The kelp, Laminaria japonica, is grown extensively in the warm waters of China because one phase in the life cycle that is particularly temperature sensitive can be cultured, after which young sporophytes are outplanted on rafts, where they grow to harvestable size (Tseng, 1981~. Because the sporophytes are probably near the limit of the temperatures in which they can survive, an increase in water temperature of only a few degrees could eliminate the entire industry. Populations of valuable fish and shellfish undergo fluctuations in abundance of orders of magnitude on time scales ranging from one to a hundred years. Examples include the collapse of the Peruvian anchoveta and the California sardine population (Murphy, 1977), the decline and then increase of the Japanese sardine (Kondo, 1980), substantial declines of shellfish such as the oyster of Chesapeake Bay (Kennedy and Breisch, 1983), and changes in distribution, such as contraction of Atlantic salmon distribution along the coast of North America in the present century (B. Rothschild, personal communi- cation), or the areal expansion of triggerfish to much of the coast of

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79 Africa during the past decade (Guliand and Garcia, 1984~. It appears that a number of these population changes result from changes in the distribution of temperature. A more fundamental unclerstanding of ecological processes is needed to understand the consequences of climate change, particularly the interactions with the dynamics of associated populations, and especially plankton. Changes in Human [and Use For the last century, and presumably for the next, land use has been more important than climate change in forcing changes in eco- logical systems and dynamics. In addition to examples such as the effect of deforestation on biodiversity in the tropics, land use changes affect a large number of ecological and physical ecosystem properties that control interactions with the atmosphere and hydrosphere. Land use affects storage of carbon, nitrogen, and phosphorus in the soil, as well as element storage in the biota. For example, slash-and-burn agriculture releases nutrients from the biomass to the soil, with con- comitant releases of gases to the atmosphere (Mooney et al., 1987~. Mechanical disturbance of soil, by plowing, results in organic matter loss and alterations in soil structure and porosity, which in turn alter moisture regimes, microbial processes, and emissions of trace gases to the atmosphere. Removal of vegetative cover, as well, decreases net primary production and net ecosystem production, and fluxes of water to the atmosphere through evapotranspiration. Deforestation dramatically increases sediment and dust production, runoff, and solute concentrations, with consequences for biota in lakes, estuar- ies, and coastal zones (Bormann and Likens, 1979~. Finally, land conversion affects the diversity of ecosystem types both globally and regionally and, in particular, causes Toss of species (Lovejoy, 1980~. What aspects of land use need to be characterized in order to address potential changes in ecological systems? The primary need is for data in categories of cover types: natural vegetation (specified in terms of biomass and stature), arable land, grazing land, permanently flooded land, nonproductive land. Within the arable land category, data are needed on three additional factors to characterize land use: water use, fertilizer use, and biocide applications. For each expressed need for land use data, it is necessary to specify the scales and resolu- tion (time and space) and the level of accuracy needed for data to describe land use adequately to assess its relation to the changing global environment.

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96 from the agricultural literature; paleoecological data; and written historical records. A close coupling of process studies with the long-term observa- tions will be required. The proper spatial and temporal scales for the long-time series observations and process studies will be determined by the specific question being asked. However, as a general strategy observations should be made along important environmental gradi- ents, including gradients of physical turbulence, temperature, and pollution. Ongoing programs relevant to the above include the World Ocean Circulation Experiment, Global Ocean Flux Study, Global Ecosys- tem Dynamics, and the U.S. National Science Foundation's Long- term Ecological Research network. PRINCIPAL ISSUES AND PRIORITY RESEARCH CHALLENGES An important aspect of IGBP research is an understanding of the mechanisms of biotic response to global change. An understanding of functional processes is essential for predicting Tong-term and tran- sient responses to circumstances that do not now exist anywhere on the globe. Both marine and terrestrial systems are highly variable in space, and marine biota in particular show great variability in time. The required research effort can be organized around several ques- tions. How will biota respond to changes in forcing factors? What are the positive and negative feedbacks? And what are the effects on global processes if particular ecosystems or particular species are lost or drastically reduced in abundance? Three research approaches should be used to increase our ability to predict biotic responses to global chance and fe~`lhnck~ to the global system: _ ~ ~ O_ _ _ w ~ ^ ~ _ ~~ ~ ^ a_ ~ AL ~ ~ ~~ ~ I/ ~ tJ 1 ~ ~ 1. Laboratory and field experiments at the organism level, and compilation of existing data on population and community patterns in response to environmental variation and land use patterns on large spatial scales, are needed. Field and growth-chamber experi- ments must quantify the responses of whole plants to temperature, moisture, carbon dioxide, and other forcing factors. These data are needed to understand global change and also to parameterize whole-plant ecological models. Features of plants and animals that influence their dispersal and successful invasion of new environments are important in predicting the time course of changes in vegetation

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97 and in biodiversity in the face of global change. Fine-scaTe paJeo- records of vegetation and faunal change will also be utilizecI. The genetic architecture of species and the way in which central ver- sus peripheral populations wid be influenced by rapid environment e] change must be incorporated for realistic predictions. Invasions by alien species may provide a particularly valuable model for evaluat- ing the response of species to new conditions such as alien predators, competitors, and pathogens. Existing data bases on plant and ani- mal distributions and growth (e.g., tree rings) must be compiled for correlation with climate, land use, pollution, and other variables. 2. Experiments are needed on intact ecosystems, using large- scaTe manipulations and taking advantage of natural experiments. These large-scare experiments are necessary to expose intact ecosys- tems to changed temperature, water, nutrient levels, carbon dioxide, and pollution inputs, singly and in combination. Experimental ap- proaches in terrestrial, lacustrine, and intertidal systems win include the use of portable greenhouses that enclose organisms and substrate and permit the monitoring of plants and soils under changed temper- ature, moisture, carbon dioxide concentration, and so on. Wetlands and upland watersheds can be subjected to hydrological manipula- tions and altered precipitation chemistry, and species of animals or plants can be introduced or subtracted from lakes or enclosed areas of lanciscape. Because of the inherently Tong lag times in ecologi- cal systems, responses must be monitored for many years (1 to 50), depending upon the particular processes in question. Population re- sponses, system responses, and changed outputs to the atmosphere should be measured to parameterize models. The use of environ- mental gradients of temperature, salinity, nutrients, pollution, and human exploitation will be a powerful study approach. In the oceans, natural experiments should be exploited that simulate conditions of global change. For example, regions of sub- sidence can be used to simulate the effects of sea level changes on tidal marshes or intertidal communities. In shelf areas, fish harvests alter the abundances of species, providing opportunities to study community structure. In the open ocean, eddies enclose water that is subsequently moved across major oceanographic meteorological fronts, providing a natural climatic change experiment. Anomalous years that change the position of the arctic front in the North At- lantic provide a means for studying the relationship between the extent of warm water and the mixing of nutrients. An international agreement should be made to ensure that in anomalous years, there

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98 is capability for scientists to reach sites where changes in the physical environment are occurring. 3. In the Tong term, ecosystem models must be assembled that couple population-community models with process-functional models for simulation of the response of ecosystems to rapid, large changes in environmental factors. This combined approach wiD be used in the development of ecosystem models that are coupled with appropriate atmospheric models, to begin the process of developing sufficient un- derstanding of linkages and feedbacks with the global system. These models shouIc! have predictive capacity for ecosystem responses to forcing factors, alone and in combination, predicting the biota for entire regions and describing consequent fluxes to the atmosphere and oceans. Two critical problems arise in attempting to build a model that can be used to predict responses to global change. First, the vari- ability of biological materials and the heritability of responses to environmental variation are inadequately known. Present models as- sume that ah individuals are identical, but as more complex models are built, scaling up from single plants to landscapes and incorpo- rating a spatial dimension, patterns of genetic variation must be incorporated. Second, the use of typological landscape or ecosystem descriptions (prairie, savanna) is inappropriate since global change may result in the development of new landscape or ecosystem "types" or biomes. Element cycling and species composition of ecosystems are interdependent aspects of ecosystems linked by complex feedbacks. Changing species can influence element cycling via a number of mech- anisms; similarly changing amounts or ratios of nutrient elements wiD influence species and community composition. Because of this, long- term models of ecological response to global environmental change must represent both process-functional and population-community aspects of ecosystems. Certain species play critical roles in ecosys- tems; if change is so rapid that these populations are killed out- right, drastically changed or eliminated processes such as primary production, decomposition, and nutrient cycling wiD also respond. Prototypes of linked models exist to predict biomass and species com- position with changes in climatic condition (see Figure 2; Pastor and Post, 1988~. Substantial integration of observational, experimental, statistical, and modeling approaches will be required to generalize this type of representation for global application. Once combined models are develop e(l, they will then be coupled .,

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99 with the next generation of atmosphere-ocean circulation models. Such combined models should be capable, for example, of predicting the following on a regional scale: net primary production, water- holding capacity of soil, albedo, surface roughness, canopy height, and trace gas production. Steps in the development of these models are as follows: ~ Continue to develop whole-plant models of important plant forms (e.g., trees, grass, and shrubs) that incorporate carbon, nutri- ent and water exchange, and responses to the forcing factors, singly and in combination. Couple the whole-plant function mode! with population-com- munity and ecosystem models to simulate ecosystem processes as affected by population change. ~ Link the combined model with existing soil models, as has already been done for certain forest simulation models. ~ Couple the resulting model with atmospheric models at the landscape scale (tens to hundreds of kilometers). ~ Test the model over an appropriate range of systems, con- sidering the effects of climatic change, changes in carbon dioxide concentration, precipitation chemistry and other forms of pollution and so on. 1, Whole-ecosystem experiments will be relevant at all stages in the development of these models. The experiments will be necessary for the scaling-up of models from plants to ecosystems, and will also be used to validate models, to assess responses to multiple impacts, and to parameterize models. Similarly, the model development wiD suggest needed experiments and aid in their design. Validation of the incorporated processes can utilize the paTeorecord of vegetation and hydrology as it responded to climate in the past. Fossil data have been compiler! at a scale of resolution similar to general circulation models for Europe, eastern North America, and Japan, but adclitional records are needed, especially for Asia and for the tropics. In order to develop and vaTidat e models, research sites are needed to test their specific predictive capacity. Research sites shouIcI be cho- sen on the basis of sensitivity to global change, importance via feed- backs to the global system, importance regarding human resources, the existence of ongoing studies that provide baseline information and background understanding, and the potential for developing and calibrating paleorecor~ls that (remonstrate responses to environmen- tal changes in the past.

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100 In summary, the initial priority for the IGBP is to obtain addi- tional experimental data, so that new models can be developed to extrapolate ecological responses to environmental changes that have not been experienced in the past. Experiments to determine the response at organismal and community levels, as well as large-scare experiments directed toward! scaling up from leaf to plant to stand to watershed to blame to global levels, are needed. Imp act s from multiple stresses can be studied by means of fixed experiments on ecosystems. Natural conditions that expose entire systems to changes in environment that simulate some aspect of global change can also provide valuable information. Studies at specific sites are needled in order to develop the ability to predict changes in community and ecosystem structure, and to predict changes in fluxes of materials to the global system. Such sites need to be selected using a variety of criteria including sensitivity to global change, importance to the global system, relevance for society, and availability of background! information about the system. REFERENCES Barber, R. T., and F. P. Chavez. 1983. Biological consequences of El Nino. Science 222:1203-1210. Bazzaz, F. A., K. Garbutt, and W. E. Williams. 1985. Effect of increased atmospheric carbon dioxide concentration on plant communities. Pp. 155-170 in Direct Effects of Increasing Carbon Dioxide on Vegetation, B. R. Strain and J. D. Cure, eds. Springfield, Va.: National Technical Information Service. Bazzaz, F. A., N. Chiarello, P. D. Coley, and L. PiteLlca. 1987. The allocation of resources to reproduction and defense. Bioscience 37:58-67. Billings, W. D. 1987. Carbon balance of Alaskan tundra and taiga ecosystems: Past, present and future. Quaternary Science Reviews 6:165-177. Bliss, L. C. 1985. Alpine. Pp. 45-65 in Physiological Ecology of North American Plant Communities. B. F. Shabot and H. A. Mooney, eds. New York: Chapman & Hall. 351 pp. Bloom, A. L. 1988. The exponential shape of late glacial sea level change. In AMQUA Program and Abstracts, 1988. Pp. 10-11. Boamann, R. H., J. W. Day, Jr., and C. A. Miller. 1984. Mississippi deltaic wetlands survival: Sedimentation versus coastal submergence. Science 224:109~1095. Bolin, B., B. R. Doos, J. Jager, and R. Warrick (eds.~. 1986. SCOPE 29: Greenhouse Effect, Climatic Change, and Ecosystems. Chichester: John Wiley & Sons. Bormann, F. H., and G. E. Likens. 1979. Patterns Process in a Forested Ecosystem. New York: Springer-Verlag. 253 pp. Botkin, D. B., J. F. Janak, and J. R. Wallis. 1972. Rationale, limitations and assumptions of northeastern forest growth simulator. IBM Journal of Research and Development:101-116. Brubaker, L. B. 1986. Responses of tree populations to climatic change. Vegetatio 67:1 19-130.

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106 William C. Clark, Harvard University Robert E. Dickinson, National Center for Atmospheric Research Pamela Matson, NASA Ames Research Center Michael B. McElroy, Harvard University Jerry Melillo, Marine Biological I,aboratory Harold A. Mooney, Stanford University Gene Nainkoong, North Carolina State University Paul G. Risser, University of New Mexico David S. Schimel, Colorado State University Herman H. Shugart, University of Virginia Walter G. Whitford, New Mexico State University Working Group on Ecological Systems and Dynamics: Marine Systems February 18-19, 1988 Cambridge, Massachusetts Margaret B. Davis, University of Minnesota, chairman William C. Clark, Harvard University Robert E. Dickinson, National Center for Atmospheric Research John Edmoncis, Massachusetts Institute of Technology Robert Howarth, Cornell University John Imbrie, Brown University Jane Lubchenco, Oregon State University James J. McCarthy, Harvard University Michael B. McElroy, Harvard University Brian Rothschild, Chesapeake Biological Laboratory