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4 Materials and Energy Organisms depend on the input of energy (in the form of sunlight or high-energy molecules), water, and mineral nutrients for metabolism and growth. Human societies depend on harvesting organisms or parts of them, so many of our perturbations of ecosystems are intended to maintain or increase production of organisms. Methods of increasing production of organisms useful to people include domestication, increasing the propor- tion of production that comes from species of economic value, removing predators, reducing competition, and increasing supplies of the resources that support production (e.g., by fertilization, irrigation, and modification of microclimates). Some valued ecosystem traits, such as processing of wastes and generation of aesthetically attractive landscapes, depend on the total production of species in the system, but often only a portion of total ecosystem production is of immediate value. Environmental problem- solving is often aimed at increasing allocation of production to particular forms, such as wood production in forests or seed production in agricul- ture. The overall determinants of biological production in terrestrial ecosys- tems are generally well known. The rate of photosynthesis depends on solar radiation, the availability of water in the soil, abundance of mineral nutrients, and temperature. All these are commonly manipulated to in- crease productivity of agroecosystems. Productivity is tied to fluxes of energy and matter. Sometimes it is measured in units of mass per unit time, as when one assesses the growth of vegetation or livestock. Sometimes it is measured in units of energy 61
62 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS per unit time, especially when the objective is to estimate the efficiency with which one form of biomass (say, a plant) is converted to another (an animal). Viewed as energy transformation, productivity conforms to the laws of thermodynamics; viewed as material flux, it exhibits conservation of mass. Hence, the measurement of nutrients and nutrient fluxes is a logical companion to productivity studies. Production, as opposed to productivity, is the material produced and hence is measured in units of biomass. It can be differentiated into gross production and net production. Not all gross production (PG) is available for harvest, because materials and energy are expended metabolically by living organisms through respiration (R) to maintain themselves. Net pro- duction (PN) is the portion of gross production that exceeds maintenance (PG = PN + R). In most ecosystems, NP corresponds to the amount of material that can be harvested or processed by detritivores. Measuring the production of entire ecosystems is difficult, and only incomplete data are available on even the most intensively studied eco- systems. Nonetheless, enough is known to enable us to see general patterns in the allocation of the products of photosynthesis to compartments in the systems. For example, interesting patterns are revealed by examining ratios of energy stored in long-lived tissues to energy stored in short-lived tissues of plants. If the ratio is high, much energy is tied up in tissues that remain intact for most of the life of the plant, such as trunks, branches, and large roots of trees and shrubs and rhizomes of some perennial herbs and grasses. The patterns revealed by such a comparison are as follows (Jordan, 19711: · The ratio of production of wood (long-lived) to production of litter (short-lived) in forests generally increases as solar energy available during the growing season decreases that suggests the increasing importance of energy storage in low-energy environments. · The ratio of wood production to litter production decreases as pre- cipitation decreases that suggests that increases in size are less valuable than the production of photosynthetically active tissue in dry environments. An important implication of these patterns is that a higher proportion of energy tied up in long-lived tissues means that less energy is available for use by other organisms in the community, because long-lived tissues are not heavily used as food. Consideration of ratios directs attention to trade-offs between different patterns of allocation. A plant, for example, can allocate a quantity of energy to wood, leaves, flowers, pollen, nectar, fruits, or defensive chem- icals. An increase in the allocation to one comes at the expense of allocation to at least one of the others. Domestication involves major changes in allocation patterns to increase the energy devoted to products of value to
MATERIALS AND ENERGY 63 people (Snaydon, 1984). The trade-off often expresses itself as a reduction of resistance to harsh climatic conditions, predators, parasites, and path- ogens. Protection from these agents must be provided by people e.g., through weeding, application of pesticides, and killing of predators. PERTURBATIONS AND PRODUCTIVITY Natural perturbations such as floods, droughts, windstorms, and f~re- can cause extensive and profound changes in an environment, sometimes even the elimination of living organisms. Perturbations caused by people can be just as extensive geographically, and their cumulative effects can be profound, e.g., conversion of forest to croplands or cropland to resi- dential areas. Productivity often increases as a result of perturbation, but decreases as perturbation becomes more severe (Odum et al., 19791. Sulfur dioxide and nitrous oxide at low concentrations in the air over a cornfield can increase production, because of the fertilizing effect of the sulfur and nitrogen. But, as their concentrations increase, productivity of the corn- field decreases, eventually to far below the preperturbation level. Common physical perturbations are those imposed deliberately in land management, e.g., logging or selective harvesting of forests (Chapters 19 and 23), channeling or damming to alter water flow (Chapter 21), and mining (Chapter 181. The disturbance might be simply the harvesting of the species of interest (Chapter 121. Development projects can also in- advertently affect species (Chapter 161. Effects of perturbations caused by chemicals can be more subtle than those caused by physical alterations. Pollutants that have been taken up by photosynthesizers can be passed on to grazers or detritivores, stored in those organisms, and concentrated (as in the case of some radionuclides and fat-soluble substances). When the grazers are eaten by predators or scavengers, the pollutant burden is passed along. Other predators or scav- engers might eat those and thus continue the process of concentration. Eventually, pollutant concentration can be high enough to cause significant biological effects (Chapter 221. CHEMICAL PATHWAYS AND BIOLOGICAL CONCENTRATION Biological concentration of organic chemicals, especially hydrophobic, lipid~soluble ones, can be extreme. For example, polychlorinated biphen- yls (PCBs) can accumulate to more than 100,000 times the concentration in water. Eating 0.5 kg of Lake Erie fish can cause as much PCB intake as drinking 1.5 x 106 L of Lake Erie water. Such pollutants cycle both
64 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS through the inorganic parts of the environment and through the biota, and their presence can affect primary and secondary productivity directly or indirectly. The DDT case study (Chapter 24) shows how a pollutant can affect populations by altering reproductive performance. The pollutants that follow a decomposition pathway might appear, in some form, in the sediments of a water body or in the soils. Some metals released into the environment are transformed by bacterial action and become either more or less toxic to higher organisms. For example, rel- atively insoluble inorganic mercury, which is not highly toxic, can be methylated and become biologically mobile and highly toxic. Effects of chemical perturbations become evident when death or illness results from excessive concentrations of toxic materials. Chemical per- turbations can also affect ecosystem processes and patterns. For example, increased input of phosphorus often results in eutrophication of lakes, with substantial increases in primary productivity and changes in species com- position (Chapter 201. NUTRIENT FLUXES The major elements of organisms e.g., carbon, nitrogen, oxygen, sulfur, and phosphorus are involved in massive global cycles that involve both living and nonliving components of the biosphere. Except for phos- phorus, these elements can be present as gases, and the magnitudes of their movement are being substantially modified by human activity. For example, the anthropogenic fluxes of sulfur today are approximately equal to those of the natural sulfur cycle (Andreae and Raemdonck, 1983; Franey et al., 19831. Changes in global biogeochemical cycles affect environ- mental problem-solving at specific sites and are beyond the control of managers. But they are the most important of the cumulative effects of individual projects, because the perturbations are largely the sum of the inputs of specific projects. One of the most challenging problems in main- taining high environmental quality is to find ways of reducing undesirable effects of individual projects on biogeochemical cycles. This requires dealing with the cumulative impacts of many projects. Patterns and rates of nutrient fluxes have traditionally been studied along with biological productivity, especially in agricultural systems where re- lations between nutrients and productivity were first demonstrated. Con- struction of "nutrient budgets" for ecosystems by tabulating the incomes and outputs of various elements is an application of the first law of ther- modynamics, i.e., net energy changes in a system are determined by the inputs and outputs, whatever the internal pathways or mechanisms. Nu- trient conditions in both Lake Washington (Chapter 20) and Southern
MATERIALS AND ENERGY 65 Indian Lake (Chapter 21) were predicted essentially in this way. Attaching biological relevance to nutrient values, however, requires an understanding of physiological processes. The importance of phosphorus as a funda- mental and controllable limiting nutrient was central to arguments about Lake Washington, for instance. Among the elements that constitute major proportions of cellular mass (carbon, nitrogen, oxygen, and phosphorus), phosphorus is unique in not having a gaseous atmospheric phase. For this reason, plant production in lakes is often limited by rates of supply of phosphorus and can be manipulated over long periods. Because the po- tential for plant production rises and falls with the availability of phos- phorus in the water, growth decreases if phosphorus is removed from the water and its input is lessened. (The rate of decrease depends on the size and shape of the lake and the flushing rate of water.) Although early work on phosphorus in lake waters discounted its importance as a regulating nutrient (Juday and Birge, 1931; Juday et al., 1928), experience with eutrophic systems showed its pivotal role by the middle of the twentieth century (Hasler, 19471. Domestic sewage is especially rich in phosphorus from animal wastes and the polyphosphate complexes that are commonly used as builders or surfactants in laundry detergents. Relationships between production and light are sometimes more im- portant than those between production and nutrients. For example, South- ern Indian Lake remained thoroughly mixed after impoundment, and suspended solids from an eroding shoreline increased abiotic turbidity and diminished light penetration (Chapter 21~. Lake Washington was moving to a somewhat similar situation of low water transparency during 1963 and 1964, the peak years of enrichment (Chapter 204. In Southern Indian Lake, the element that received most attention after impoundment was not the one that had been targeted by the impact as- sessments. Impact studies had considered phosphorus because it is often a limiting nutrient in freshwater lakes, but mercury emerged as more important. The environmental problem became chemically much less like that of Lake Washington than like the problem with DDT (Chapter 241. In Southern Indian Lake, the deleterious material was not manufactured and added artificially to the system, but was released as a consequence of the manipulation. INTERACTIONS AMONG PRODUCTIVITY, BIOMASS, AND NUTRIENTS Development of our understanding of productivity, biomass, and their relations to nutrients was spurred in part by practical concerns. During the 1960s, a debate arose about the causes of lake eutrophication and the
66 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS most potent management options (Edmondson, 1974; Likens, 1972; Na- tional Research Council, 1969). That phosphorus and phosphorus loading were basic for controlling productivity in eutrophic basins was shown by convincing logic and careful analyses of cases in which loading was altered by design (e.g., Edmondson, 1969, 1972; Schindler, 19741. Current models are based on findings that are embodied in quantitative relations among hydrological conditions, basin structure, and nutrient and chlorophyll concentrations in lakes (Chapra and Reckhow, 1983; Dillon and Rigler, 1974; Vollenweider, 1969, 19764. These models help to relate physical and chemical circumstances to productivity and trophic state. They are relevant both to applied issues and to studies of the constraints on energy transfer among trophic levels and of the magnitude of the production base. A key feature of the models, however, is not so much what they explain, but what they do not. They cannot predict algal bio- mass, chlorophyll, or related entities even to within an order of magnitude when a set of heterogeneous basins are compared. Substantial residual variability in lake productivity cannot be explained by nutrient loading (Carpenter and Kitchell, 1984; Harris, 19801. For single lake basins or lake districts, the fits are usually far better, particularly when variation in externally derived nutrient loading is large (Edmondson and Lehman, 1981; Schindler et al., 1978), but even in such basins, changes in chlo- rophyll and water transparency can be extreme, despite relatively constant nutrient loads (Edmondson and Litt, 19824. INDEXES OF ECOSYSTEM FUNCTIONING Productivity is used as an index of perturbation in a number of ways. In aquatic ecosystems, following the ideas of Eppley and Peterson (1979) and Harrison (1980), primary production can be considered as a sum of production based on "new" nutrients and production based on "regen- erated" nutrients. The "new" portion of production is the primary carbon fixation that is supported by externally supplied nutrients, e.g., from upwelling, chemical weathering, and stream or overland runoff. The "re- generated" portion of production is the availability of nutrients in situ from excretion and decomposition of cells. Production that is based on regeneration cannot be harvested without compromising the production base itself. Measuring "new" and "regenerated" nutrients is not easy, but the data needed are not much different from those needed to construct quantitative nutrient loading models (Vollenweider, 1969, 1975, 19761. The techniques for doing so have become very sophisticated (Reckhow, 1979). The cycling index (Finn, 1976, 1978) is a useful measure of the effects
MATERIALS AND ENERGY 67 of perturbations on nutrient fluxes in ecosystems, especially terrestrial ones. The cycling index is the ratio of the amount of nutrients recycled in an ecosystem per unit time to the amount of nutrients moving through the system. For most undisturbed terrestrial ecosystems, the index is be- tween 0.6 and 0.8. The 20-40% of the nutrients lost must be made up either from the atmosphere or from weathering of bedrock, if the ecosystem is to remain in a steady state. In agricultural or other disturbed systems, the index is often much lower, and productivity is maintained by fertil- ization. CONCLUSIONS Our efforts to extract materials from ecosystems, to use them as waste processors, and to manage them for productivity or maintenance of species richness often conflict. Managing the conflict requires understanding of the determinants of biological productivity and of how those determinants are influenced by human-induced perturbations. The most difficult changes to predict are those involving dynamics of important nutrients, and most surprises result from incorrect prediction of the effects of projects on the behavior of nutrient elements. The same problem is central in understand- ing the influence of human activities on global air circulation patterns and climate issues addressed only peripherally here. Changes in behavior of nutrients seldom attract attention, because they are often not directly per- ceivable by the unaided senses, although they do become obvious, for example, when they impair visibility in scenic areas. Other conflicts arise over decisions to divert energy flow patterns in ecosystems toward species of indirect value to people and away from species with no current value. Indeed, much of the difficulty in preserving species richness is related not to overexploitation of species, but to con- version of the ecosystems to uses that are incompatible with preservation of the community of species originally present.