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Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 69
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 70
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 71
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 72
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 73
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 74
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
×
Page 75
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 76
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 77
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 78
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 79
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 80
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 81
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 82
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 83
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 84
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 85
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 86
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 87
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 88
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 89
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 90
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 91
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 92
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 93
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 94
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 95
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 96
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 97
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 98
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 99
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 100
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 101
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 102
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 103
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 104
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 105
Suggested Citation:"Ecological Systems and Dynamics." National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere - Biosphere Program. Washington, DC: The National Academies Press. doi: 10.17226/1393.
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Page 106

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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

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.

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 3°C ~ 1.5°C (Jaeger, 1988~. Global temperatures will change rapidly, the rate of increase ranging between 0.1°C and 0.8°C 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 5°C, 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

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

~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.

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.

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 ~ ~ . ~

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

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

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 Current—the 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

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.

80 Three additional types of land use information are needed. First, reconstructions of past land use change are needed for use In con- junction with reconstructions of ecological systems. The overriding need with respect to global change is for data on a regional scale, extending back to about 1850 to cover the period of intensive human transformation of the earth's surface. Longer reconstructions on a regional and even on a continental scale are possible using the pollen record (Derby, 1956~. The timing of large-scare changes, such as the deforestation of Europe and the development of agriculture in China and other regions, should be compared with ice core records of changes in atmospheric composition. Second, land use data win need to become a routine component of contemporary monitoring efforts. Remote sensing can be utilized to provide a record of rates of change, especially in the tropics, where deforestation is proceeding rapidly. Third, future scenarios of land use change are needed to predict changes in biotic systems and interactions within the global system. This predictive process will require the combined initiative of biologists, demographers, development economists, and agricultural experts. Changes in Carbon Dioxide The primary effects of increased carbon dioxide concentration are increased photosynthetic rates and decreased stomata! conduc- tance, which reduces water Toss and causes changes in plant phe- nology (CarIson and Bazzaz, 1980~. Most studies with agricultural crops and species from natural communities indicate that produc- tivity and yield increase with elevated CO2 levels (Strain and Cure, 1985~. Plants with the C3 metabolic pathway generally show more enhancement of photosynthesis and growth in response to elevated CO2 than plants with the C4 pathway. This response has a clearly understood physiological basis. There are also differences in response to elevated CO2 among species and among genotypes within species, but the bases for these differences have not been fully investigated (Bazza~ et al., 1985~. Studies of whole communities, however, do not always show enhanced productivity. A community of short-statured annuals showed no response (Williams et al., 1988), and short-term measurements of some deciduous tree species showed only a small enhancement in growth (Williams et al., 1986~. The response to elevated CO2 productivity of natural vegetation is therefore still

81 somewhat unpredictable, but nevertheless essential for understand- ing the relationships between primary production and changes in the global environment. High CO2 concentrations may change the pattern of carbon avocation to plant parts and activities, but the available data are inadequate for describing general patterns (Bazzaz et al., 1985~. There may also be shifts in the kinds of chemical defenses in plant tissues and changes in rates of plant tissue consumption by herbivores (LincoIn et al., 1984~. Decomposition rates and nutrient cycling may change in response to altered plant C/N and Jignin/N ratios, as wed as changes in starch content, leading to feedback via decomposition restricting the positive effects of CO2 enhancement (MeliHo et al., 1982). Experimental studies in which whole plant communities have been subjected to elevated CO2 show that competitive hierarchies change. The resulting community structure is influenced by other environmental factors such as moisture, light, nutrients, an(1 temper- ature (Bazzaz et al., 1985~. It is important to note that the response at the community level may not be directly predictable from the re- sponse of individual species to elevated CO2 or to other environmen- tal factors because of changes in species composition and interactions with heterotrophic organisms. ()ur knowledge of past responses of natural systems to changed CO2 concentrations is also limited; the fossil record should be inspected for changes at the end of the last glacial period that can be related to changing CO2 concentration in the atmosphere. The response to enhanced CO2 concentrations at the level of ecosystems has been the subject of only a few experiments (Drake and Read, 1981; Oechel and Strain, 1985; Tissue and Oechel, 1987) en cl is largely unknown. Of concern, particularly with respect to the global carbon cycle, are the rates of CO2 storage and release of various ecosystems as affected by increased CO2 concentrations (Billings, 1987), and the relationship of these variables to nutrient circulation. Feedback to the hydrologic cycle may be influenced by changed water use efficiency. Other Changes in Atmospheric Chemistry and Pollution Other changes in atmospheric chemistry have potentially large, landscape-level effects on ecological systems, primarily through ef- fects on ecosystem components, especially those with slow turnover

82 rates such as soil organic matter en c] Tong-lived organisms such as trees and fish. The regional scale of pollution and the transport of pollutants across political boundaries make these issues of global concern. Synergistic effects of various pollutants are wed known from simple laboratory experiments (Mansfield et al., 1987), and these ef- fects will certainly be found in the study of ecosystems. Ecosystem changes attributed to acid deposition alone, for example, may be the result of interacting factors. Forest decline may result from acid depo- sition, associated pollutants, ozone, disease/pest outbreaks, changing forestry practices, and/or interactions among any or Al of these fac- tors (Hutchinson and Meema, 1987~. In lakes, changes inducible by acid rain (Harvey, 1982) are also inducible by certain metal pollu- tants (National Research Council, 1972) and fishery management practices. Experiments are needed to determine physiological toler- ances to multiple stresses. Tolerances to different stress factors are not linked and therefore are inherited independently; hence it seems unlikely that organisms will spontaneously evolve tolerances to mul- tiple simultaneous or sequential stresses. Furthermore, purposeful breeding for stress-tolerant organisms will also be difficult. Knowledge of which pollutants will increase in the future, and by how much, is needed to define the areas requiring attention in the IGBP. This information will be derived from expected rates of industrial and technological development. It is clear, however, that a focus on suites of associated pollutants and multifactorial responses at individual, population, and ecosystem levels is needed to unravel complex causes of damage to ecosystems. Because such experiments cannot be performed for each potentially important factor or inter- action, the proper approach wiD be to carefully integrate laboratory and field experiments with simulation models, and with small-scale studies aimed at particular organisms or interactions. Innovative in- dices of responses to pollutants are needed for both survey work and retrospective studies. These results win be best derived from whole- ecosystem experiments. The effects of pollutants on heterotrophs, pathogens, or symbionts such as nitrogen-fixing bacteria must be examined in addition to the effects on plants. The effect of pollutants on marine ecosystems is difficult to study, because suitable control sites are hard to identify, making the sep- aration of even major pollutant effects from "natural" variability problematic. Another problem is that impacts can occur via indirect pathways. For example, an oil spill in the Baltic Sea caused a signifi- cant (lecrease in hatching of herring eggs. However, the result was not

83 because of a direct toxic effect on the eggs themselves; rather, the of] decreased populations of gammarid amphipods, causing decreased amphipod grazing on fungi and consequently increased fungal in- fection of eggs (NelIbring et al., 1980~. Controlled ecosystem-scale experiments allow a holistic assessment of the responses of ecosys- tems to known exposure to pollutants, and are necessary to detect these indirect pathways. Changes in Ultraviolet Radiation Changes in atmospheric constituents, especially stratospheric ozone, wiD affect the intensity of ultraviolet radiation reaching the terrestrial and marine portion of the biosphere. Previous investi- gations in arctic/alpine regions indicate that changes in ultraviolet irradiance wiD cause changes both in productivity and in distribu- tions of specific vegetation types. More subtle effects on ecosystem processes like decomposition are less completely understood and yet have great importance in biosphere-atmosphere interactions. The IGBP needs, first, to test the responses of characteristic vegetation types to a range of potential ultraviolet levels, focusing specifically on more subtle responses such as rates of decomposition of organic matter, susceptibility to plant diseases, species interactions, and genetic change. Intensified ultraviolet radiation wiD increase mutation rates, but it is not clear what effect this will have on population structure and viability. Theory is yet to be developed that will generate testable hypotheses regarding the effects of fluctuating mutation rates on susceptible species. Second, the IGBP needs to conduct experiments and develop models to predict the productivity and distribution changes of terrestrial and marine biota in response to increased ultraviolet radiation. Methodologies for extrapolation from experiments and measured sites to global scales require more development. Sea Level Change The effects of the expected rate of sea level rise in the next century, nearly 1 cm/yr, may be somewhat analogous to the effects of sea level rise that occurred at the end of the last ice age, 80 m over an interval of 14,000 years (Bloom, 1988~. Although it is clear that major dislocations in estuarine, marsh, and nearshore ecosystems occurred during deglaciation, our knowledge of ecological response is

84 inadequate to allow firm predictions. Available information suggests, however, that the predicted rates of sea level rise are near the upper bound of possible rates of intertidal marsh growth (Boamann et al., 1984~. Consequently, sea level rise in the next century is likely to drown many if not ah salt marsh systems except in areas where the land is rising and reducing the rate of relative sea level change. Tidal marshes are important habitats because they are highly productive. They buffer nutrient availability in neighboring estuar ies and provide critical habitat for migrating birds. Drowning of salt marshes will change the hydrology of estuarine systems. Rearrange- ment of channels can erode previously deposited sediments, exposing buried pollutants such as dioxin and mercury. A rise of water level in estuaries will have repercussions on fish habitat far upstream, which may be difficult to predict, because the distance a saltwater wedge will move upstream clepen(ls not only on sea level, but also on dis- charge rates in the river, which wiB be affected by climate. Increased precipitation could change discharge and result in increased erosion en c! sediment transport. Coupled with rising sea level, the result would be an increase in siltation of river channels leading to greater flood potential, and higher turbidity in estuarine zones, especially when higher sea level causes rapid coastal erosion. Higher precipitation would also cause a rise in the water table and its propagation shoreward. This will be countered by the sea level increase. Thus it seems likely that the fresh and salt transition zone wiB move, although the direction is not predictable. Because this front is of great ecological significance, it wiD be necessary to examine the probable magnitude of the effect using numerical models of coastal zone aquifers. A question of vital interest, especially for low-lying islands, is whether coral reefs can grow at rates comparable to the projected sea level rise, particularly when the effect is combined with increased water temperature (Mathews, 1984~. Rocky shores in temperate latitudes support very productive in- tertidal and subtidal communities, which in turn support the detrital- based food webs of nearshore fish and shellfish communities, as wed as adjacent sandy beach fisheries. In some countries, seaweeds and invertebrates are also used directly and extensively by humans (San- telices et al., 1984; Tseng, 1981, 1984~. In the intertidal, as in many other ecological situations, the vari- ance of the physical parameters, especially air temperature, is more

85 important than the mean. Quite often it is the catastrophic event— the coincidence of daytime Tow tides with unusually hot weather, for example that determines the distribution patterns of longer-lived sessile organisms (Glynn, 1968; Hughes et al., 1987; I`essios et al., 1984; Loya, 1976a; Wethey, 1985~. Therefore the IGBP research program must include an experimental design that recognizes and accommodates these episodic events. WHAT P RO C ES S ES REQ UIRE FURTHER UNDERSTANDING IN ORDER TO MODEL BIOTIC RESPONSES TO GLOBAL CHANGE? A Tong-term goal for the {GBP is to develop interactive mocI- els coupling the exchange of matter, such as CO2, water, and trace gases, as well as exchange of energy and momentum, between ecosys- tems and the atmosphere. Currently, ecosystem and atmospheric models are for the most part separate, with ecosystem models us- ing externally specified climatic drivers, and climate models treating ecosystems as static boundary conditions. Modeling ecosystem response can take one of two forms, a process-functional approach or a population-community approach. Process-functional models simulate fixation, allocation, and decom- position of carbon, cycles of nitrogen, phosphorus, sulfur, and other elements that exchange across system boundaries. Population-com- munity models represent birth, growth, death, and movement of organisms or groups of organisms, and can simulate changes in abun- dances of species within and between ecosystems (e.g., Botkin et al., 1972~. The selection of one of these approaches over the other de- pends on the question being asked. Many research teams have chosen the process-functional approach because of the importance of biogeo- chemical cycles in short-term (1 to 100 years) ecosystem response to global environmental change. This is in part because rates of element cycling may change more rapidly than species composition (days to years versus years to centuries). However, as populations and com- munities change, so do controls over biogeochemical cycling such as plant tissue element ratios and detrital organic chemistry (Pastor and Post, 1986~. In a reciprocal fashion, a change in nutrients en cl other resources is a driver of population and community processes. Thus general models linking biogeochemical cycles to population and com- munity processes will be required to model ecosystem consequences

86 of global change (Pastor and Post, 1988~. Several models have suc- cessfully blended the two approaches by tracking changes in those properties of individual organisms or cohorts that are important at the ecosystem level, such as carbon or nitrogen content and decay rates of annual cohorts of litter (Figure 2~. Paleoecological data provide a key to unraveling the rates of transition between vegetation types resulting from climatic change (Jacobson et al., 1987~. Current models must address the issue of how much climate-induced process-level change (photosynthesis, decomposition, nutrient cycling) occurs within ecosystems before community turnover and biome-type change occur, and, conversely, to what extent species changes drive process-level changes in these systems. Records from the past may elucidate the time frame and the driving functions for these changes (e.g., Chen, 1986; Grimm, 1984~. The paleorecord provides data sets, especially isotopic records, that should be used more extensively in global change research to test process-functional model predictions. The testing of Tong-term ecological/process models is a chaDeng~ng area that involves inferen- tial tests from a wide and rich mixture of paleoecological, historical, and experimental data. Questions of scale must be resolved to link ecosystem models to models of atmospheric change. Physiological models simulate varia- tions in carbon dioxide exchange or transpiration on time scales of minutes to months, but typically have low resolution of spatial dif- ferences. Atmospheric models require scales of minutes to days, but with much broader spatial representation. Models linking ecosys- tem and atmospheric change must simulate spatial variations across large areas (regional to global), but have fine temporal resolution for exchange processes. New mode! structures will be required for this linkage. A number of ecological research teams are developing "generic ecosystem models" to investigate and contrast the likely responses of different terrestrial ecosystems to changes in the global environ- ment over the next century. These models incorporate the important components, processes, and linkages present in all terrestrial ecosys- tems so that the intercomparisons can be made from a common perspective with the same units. Unfortunately, they are limited to present-day ecosystems and are therefore of limited usefulness for predicting responses to future global change. Linkage of generic

87 CLAYS 3 200 cn z o OTHER OTHER (BASSWOOD) MAPLE ASPEN \ i_ SPRUCE 1 750 PRESENT CLIMATE 1 950 2050? WARMING ~ GREENHOUSE CLIMATE? SANDS O 200 CC IIJ z o z 9 100 LL LU m - b //~ ~~ \ MAPLE 01 1 750 BIRCH / ASPEN ACE \ \ OAK 1 950 2050? PRESENT CLIMATE ~ WARMING ~ GREENHOUSE CLIMATE? FIGURE 2 Predictions of biomass and species composition of Minnesota forests under climatic conditions predicted with CO2 doubling. The predictions are based on a population-based model that simulates forest growth, combined with a process- functional model that simulates soil moisture and nutrient cycling. Climatic inputs were the same for the two runs, but (a) simulates forest growth on fine-textured soils, and (b) simulates growth on coarse-textured soils. Note that biomass increases under greenhouse climate in (a), but falls to low levels in (b). Redrawn with permission from Pastor and Post (1988~.

88 models to population-community models will be critical for Tong- term future predictions, or for regions where rapid rates of change occur. The resulting models should permit simulation of biospheric processes coupled to atmospheric and biogeochemical dynamics. WHAT ARE THE TEMPORAL AND SPATIAL DYNAMICS IN RESPONSE OF ECOLOGICAL SYSTEMS TO GLOBAL CHANGE? Time lags in the ecological response to global change result not only from differences in longevity or life cycles among organisms, but also from nonlinear processes. For example, Tow availability of nitrogen, the most frequently limiting nutrient in terrestrial ecosys- tems (Pastor et al., 1984), may prevent ecosystems from changing in response to changing climate until after temperature thresholds are exceeded for dominant species. As previously dominant species are replaced by others, consequent changes in carbon and nitrogen cycles can happen rapidly, amplifying ecosystem response as illustrated in Figure 2 (Pastor and Post, 1988~. Time lags pose a challenge for the identification of cause-effect relationships, because due to delayed feedbacks, effects can be sep- arated in time from causal events. Threshold effects from a slowly increasing variable (gradually increasing levels of ultraviolet radia- tion, for example) may prove difficult to distinguish from delayed responses to a single causal event. Experiments and models are needed to describe the time course of the complex series of responses that can occur in intact ecosystems. PaTeorecords wiD be useful, es- pecially where time lags are on the order of centuries. For example, pollen and paleolimnological records indicate rapid soil destruction in northwestern England at the time of the Younger Dryas cooling 10,800 years ago, but slower soil buildup following the rapid warming at the end of this event (Pennington, 1986~. Species differ in response times to environmental change, en cl different environmental variables lead to different lags among various species (Davis, 1984~. Cumulative small changes (gradually decreas- ing rainfall) will have a different effect from changes in the frequency of (discontinuous events (e.g., droughts). Indirect effects, such as enhanced growth due to demise of competing species, or due to in- creased nutrient supply resulting from changed microbial communi- ties, complicate pre~lictions of lag times. In general, rapid responses,

89 within a year, are seen in biotic factors such as microbial commu- nities, plankton (lensity, reproductive success of plants and animals, transpiration, photosynthesis and respiration rates, pathogen out- breaks, animal behavior, and densities of annual plants and short- lived animals. In the open ocean, biotic responses to climate are influenced by ocean currents and mixing, which track the seasonal cycle with a lag of only a few weeks. A lag time of several years or decades can be corrected in other processes, such as annual plant community structure, changed plankton density or net primary pro- duction caused by changes in consumers (e.g., insect epidemics and consumption of plankton by fish), and density and community struc- ture of perennial herbs, shrubs, trees, and some animals. Moreover, changes in the genetic structure of populations depend on generation time. Whether redundancy within an ecosystem can delay responses of the system to environmental changes requires investigation, as this may be an important and perhaps unexpected influence of biodiver- sity on ecosystem processes. HOW WILL TRANSFERS OF MATERIALS ACROSS ECOSYSTEM BOUNDARIES BE AFFECTED BY GLOBAL CHANGE? The impact of climate change on the frequency of drought and other hydrologic events affects not only ecosystem-level processes but also related landform configuration such as the formation of deltas and changes in stream channels. Soil moisture influences veg- etation physiognomy, which in turn influences albedo and surface roughness (Dickinson, 1986~. Sediment loadings, types of particulate and soluble inputs, and flow rates can affect downstream terrestrial ecosystems and turnover rates, flow, and current dynamics in wa- ter bodies. Hydrologic dynamics dictate flooding, and thus affect wetlands formation, and gas emissions and nutrient exchanges under aerobic and anaerobic conditions. Aeolian transport carries fertilizer and dust to adjacent or more distant systems. There is evidence that arid lands "feed" each other; i.e., they transfer materials in part because of similar biotic compo- nents and because of wind-driven particulate pathways. Long-range transport is important for global change, especially because this path- way may be crucial for quantifying the cycles of phosphorus and trace

90 elements. Stable isotope ratios provide a promising method for de- tecting origins of wind-borne particulates. Appropriate atmospheric models are now needed to simulate aeolian pathways. In a related set of ecological processes, changes in climatic pat- terns will affect the distribution and/or intensity of fire, which is a major source of release of elements from terrestrial systems to the atmosphere. Thus models will need to incorporate these fire-driven processes into the description of aeolian transfers, especially as these transfers are related to changes in climate and land use. Transfers across the biosphere-atmosphere interface will also be affected by global change. For example, changes that lead to al- teration of wetting-drying cycles in seasonal wetlands, forest soils, and agricultural soils can alter the production and transport of bio- genic gases (Harries and Sebacher, 1982), significantly influencing atmospheric composition and global biogeochemical cycles. More- over, shifts between reducing and oxidizing environments can lead to changes in the relative quantities of trace gases such as CH4, CO2, N20, and NO released from the systems (Mooney et al., 1987~. Likewise, major changes in the moisture characteristics of ecosys- tems change decomposition environments, e.g., from decomposition in anaerobic environments with subsequent release of CH4 to het- erotrophic decomposition in aerobic environments, releasing CO2. This example of a change in the decomposition pathway could be particularly important in northern bogs and tundra and could have significant impacts on atmospheric composition. Global distribution of fluxes and their interactions should be studied using a combination of chamber estimates, aircraft in situ and remote sensing (e.g., LTDAR) measurements of gases, and eddy correlation techniques for estimating flux. These data should be collected and analyzed across gradients of moisture, temperature, or fertility, and used to develop and test models that describe the interacting nutrient cycles and fluxes. Data sets collected at multiple scales will facilitate development of models representing large areas. In the marine environment, climate change and an increase in wind erosion are likely to increase the delivery of fine particles by aeolian transport from the continents to the surface waters of the ocean, where productivity could be affected (Martin and Fitzwater, 1987~. Depending on their composition, such particles could have either biostimulatory or biotox~c effects on oceanic productivity and other marine processes. Another concern in the transport from land

91 to oceanic surfaces is the increase in nitrate, heavy metals, and aro- matic hydrocarbons in precipitation as atmospheric pollutant Toads increase from industrial sources. Oceanic waters are currently a major source of dimethyIsulfide (DMS) to the atmosphere. The oxidized products of this DMS add to aciclic rain over continents downwind of oceanic sources, may increase cloudiness through nucleation on SO4-- aerosols in the troposphere (CharIson et al., 1987), and may increase albedo in the stratosphere from SO4-- aerosols (Ryaboshapko, 1983~. Coccolithophorids are apparently a major source of DMS, and plankton processes clearly respond to mesoscaTe events and physical processes of a changing environment. Blooms and bloom conditions for these organisms can be observed through remote sensing of ocean color, sea surface tem- perature, and winch stress to document natural cycles of change in physical forcing and its biological response. The coccolithophorids are a special case, because in addition to chlorophyll the white light reflected by the calcite coccolith plates provides a unique signal that can be observed by satellite sensors (Holligan et al., 1983~. Thus the potential exists to better unclerstand the physical, chemical, and biological conditions associated with the onset and collapse of coc- colithophorid blooms, and to make regional estimates of the ocean biogenic source for DMS. Long time-series observations in oceanic regions are needed to document natural variability in physical forcing functions that con- tribute to seasonal and intersexual variability in rates of primary production and the flow of this material through the marine food web. This information is necessary to formulate an(1 refine hypothe- ses about how changes in the physical climate will affect plankton community composition and dynamics, and how these changes wiD feed back to climate via the residence time of photosynthetically fixed carbon in the sea. Two extremes with regard to carbon storage times can be envisioned. One might be the transfer of carbon from very small phytoplankton to protozoans and bacteria with a large fraction of the phytoplankton carbon respired to CO2 in a relatively short period of hours to days. Another would be the (lirect and rapid sinking of large phytoplankton cells, such as diatoms, to the deep ocean, with subsequent storage times for carbon on the order of decades and longer. Coastal wetlands (salt marshes, mangrove swamps) are sources of several atmospheric sulfur gases (DMS, H2S, COS, Cob. As sea level rises, the ability of wetland plants to oxidize their rhizospheres

92 could (lecrease owing to longer periods of inundation, increasing the flux of sulfur gases to the atmosphere. This hypothesis could be tested by examining the consequences of increased tidal flooding on wetlands currently experiencing subsidence, such as those in coastal T · . toulslana. Freshwater wetlands, both natural (peat bogs, marshes, swamps) and managed (rice paddies), are major sources of atmospheric CH4 (Harries et al., 1985~. CH4 flux is the result of two competing processes- CH4 formation and CH4 oxidation. Of the two, CH4 ox~ciation is much more temperature sensitive, increasing rapidly as temperature rises. Consequently, CH4 fluxes are greater when the temperature is Tower. Increases in wetiancT areas in high latitudes due to increased precipitation could greatly increase the atmospheric CH4 flux. An important component of the TGBP should be the study of estuaries and coastal regions that already experience high inputs of nutrients and other substances from terrestrial ecosystems. The resulting eutrophication is a growing problem of worI(lwide propor- tions. Inputs of both nutrients and pollutants to estuaries and coastal seas could change as climate affects rates of erosion and leakage of substances from terrestrial ecosystems. Primary production in many estuaries and coastal regions is limited by nitrogen (McCarthy, 1980), so potential nitrate outputs from terrestrial ecosystems receiving greater rainfall offer the possibility for significant enhancement of productivity. DOCUMENTING GLOBAL CHANGE IN ECOLOGICAL SYSTEMS Documentation of past changes in ecological systems is necessary to demonstrate connections and cause-effect relationships within the global system. The record of the past, discussed in detail in the back- ground paper on "Earth System History and Modeling," also pro- vides a measure of natural variability before human-caused changes in the global system began. Observations needed to document future changes are equally important, and are discussed below. Particular types of systems should be chosen for observation and study on the basis of their importance or their sensitivity and re- sponsiveness. Ecosystems are important if either (1) they are large contributors to critical global cycles or (2) they are important as resources for human society. For example, oceanic waters should

93 be observed and studied because of their potential as sinks of atmo- spheric carbon dioxide and as sources of atmospheric DMS. Estuaries and nearshore ecosystems should be observed and studied because of their role as providers of fish, shellfish, habitat for wildlife, and recre- ational opportunities. Tropical forests are important reservoirs of biodiversity and play an important role in the global carbon budget, boreal wetlands are important sources of methane, and temperate grassland and forest regions are important for human foot! produc- tion. Sensitive systems are easily changed and thus can be studied as early warning signals of global change. For instance, coral reefs in nearshore, tropical regions are sensitive to slight rises in temperature and relatively Tow levels of pollution (Hughes et al., 1987; Lessios et al., l9S4; Loya, 1974, 1976b; Roberts, 1987), and temperate grass- lands are sensitive to small changes in precipitation (Risser, 1985~. Process and population dynamics studies are necessary to interpret changes in indicator species, and to separate natural variations from effects of pollution, temperature rise, or other parameters. Sensitiv- ity has been determined primarily in reference to a particular impact, but the challenge to the IGBP is to consider the multiple causes- climate change, chemical loadings, direct human impacts that can synergistically affect ecological systems. Many transitional areas are either important glob ally or ex- tremely sensitive to global change, e.g., the tundra-taiga transition (to climate change), the desert-grassTand transition (which is un- dergoing (lirect human impact as well as being sensitive to climatic changes); and estuaries (areas of economic importance at the fo- cal point of pollutant stress). Other examples are systems that have rapid turnover of nutrients, that have individuals or populations that may be characterized as sensitive to change, or that may provide an index of the effects of change. Indices to measure and monitor in these systems include productivity, distribution and growth rate of species with economic importance, changes in species that play an important role in the community or ecosystem, and in other key in- dicator species, distribution of structural types/forms of ecosystems, carbon pools and dissolved and particulate carbon Tosses, nutrient Tosses, and carbon:element ratios in foliage canopies. An important criterion for choosing study systems is the po- tential for recovery and calibration of a paTeoecological record of responses to past changes. Paleorecords of all types that indicate the

94 nature of past change in response to climate, human impact, and so on, provide valuable indices of sensitivity. At each site chosen for Tong-term monitoring, responses to forcing factors should be studied through both empirical observations and experiments. The stations should be used to establish ground truth for satellite observation. The following aspects at the sites should be documented and understood: 1. Responses of individual species, especially key species that play major functional roles within communities, and distribution and abundance patterns of functional groups of species selected to characterize the response of particular systems should be studied. 2. Species interactions, including responses to pathogens and insect outbreaks should be investigated. 3. Ecosystem-level responses especially those that feed back to the global system, such as biogenic gases, albedo, and moisture exchange and nutrient transfers among and between terrestrial and aquatic systems—should be studied. 4. "Indicator" species, gradients in physical coalitions and/or biotic composition, or other parameters must be identified that can be observed easily (e.g., by remote sensing) and that respond rapidly, providing early warning of changes in the condition of ecosystems. The paleorecord may be utilized to identify aspects of the system that are sensitive to climate. 5. Changes in response dynamics should be studied in these particular systems to obtain better predictive capability for transient effects. 6. Spatial dynamics and sensitivities should be understood for the locality under study, including interactions with adjacent sys- tems. Because each system varies in its dynamics, variables to be stud- iec3 will be different in each system. For example, in some systems the variable of interest is the influence of physical factors on net primary production, and in others, it might be sedimentation and gas exchange, how the food web partitions primary pro(luction, or how predators high on the food chain feed back on production. A Tong-term monitoring program is essential, to be combined wherever possible with a retrospective fossil record that reveals the trajectory of change before the onset of global change. In addition to studies at individual sites, changes in the structure and function of biotic systems must be monitored on a global scale

95 to document future changes. Methods for monitoring terrestrial sys- tems include calibrated multiband spectral absorptance of the earth's cover through time to provide information on such changes as biotic cover classes (Malila, 1980), net primary productivity (Tucker and Sellers, 1986), chiorophyU as an indicator of water quality in freshwa- ter systems (Lathrop and Lillesand, 1986), and surface temperature (Mortimer, 1988~. Still in the experimental stage but potentially use- ful for monitoring are changes in radar signal to provide information on the three-dimensional structure of vegetation indicating its suc- cessional status (Paris and Kwong, 1988) and changes in microwave signal analysis to derive surface properties such as flooded areas and soil moisture (Ormsby et al., 1985; Wang et al., 1986~. Changes in human land use must be carefully monitored, especially in the tropics (MeliHo et al., 1985~. Two strategies are being adopted by the Global Ocean Flux Study to make long time-series observations in oceanic regions that provide important steps in this direction. The first involves the use of satellite observations for ocean color that serve as a proxy for near-surface phytoplankton biomass. This provides an ocean-scare view of the pattern of plankton distribution, and how this changes in time. The second involves in situ observations at sites selected for their regional significance (National Research Council, 1984~. This latter approach win provide a framework of water column observations of physical, chemical, and biological processes in ad- dition to rate measurements of certain processes such as primary production and the sinking flux of particulate material. The fre- quency of sampling and duration of these studies wiD be determined in the context of the processes that need to be studied at a particular site to improve our regional understanding of the coupling of ocean biogeochemical processes and the physical climate system. Ongoing efforts to obtain global estimates of populations (e.g., large mammals, birds, and pathogens) to monitor biological reserves, water and air quality, an(1 resources such as crops, forests, and fish, will provide valuable information for the IGBP. Of particular im- portance is increased attention to the assembly and analysis of ex- isting data on large-scare temporal and spatial variation. Promising existing data sources include TocaTity-specific data associated with museum specimens; the North American Breeding Bird Survey and Christmas Bird Counts; tree rings; data on censuses and distributions of the well-studied European biota; records of crop harvests taken

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

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

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 .,

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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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

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