Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
9 The Special Problem of Cumulative Effects THE NATURE OF THE PROBLEM The continuing degradation of ecological systems due to repeated per- turbations is a difficult environmental and scientific problem. Because current science and policy focus primarily on the environmental effects of single projects or actions, little progress has been made in dealing with cumulative effects. Recent awareness of such serious cumulative effects as acid rain, the rapid loss of tropical forests, and the threatened extinction of many species has brought increasing political and scientific attention to cumulative environmental effects. Many cumulative effects result from the movement of materials through the environment. An analysis of the propensity for natural processes to concentrate or disperse materials can help to determine where cumulative effects will be most severe. Several processes lead to the concentration of materials. The most important is movement of the medium itself, such as atmospheric mixing and stratification, water currents, and downward movement of water in soil. Other processes are relatively independent of the medium itself, as when chemical interactions cause flocculation and settling of suspended particles entering estuaries and when particles settle because of gravity. Living organisms are responsible for considerable concentration of materials, and their patterns of movement and dispersal can result in increased exposure to environmental contaminants. Some organisms migrate great distances, and many species aggregate for feeding or 93
94 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS breeding. Passage of material through food chains can lead to great concentration; top carnivores are exposed to much higher concentrations of contaminants than are plants at the bottom of the food chain (Chapter 241. Processes that lead to dispersal of materials are sometimes similar to those concentrating them. Water currents, wind, the migration of or- ganisms, and the dispersal of their propagules can all serve either to disperse or to concentrate materials. Dispersal can either increase or decrease the ecological effects of materials, depending on its rate in relation to the biological activity of the materials. Dispersal of highly toxic substances that remain active for a long time might simply increase the area of effects. An inadequate understanding of how such processes work in specific cases can lead to undesirable results. Recent discoveries of areas of toxic waste buildup in Puget Sound, Washington, indicated that the processes of dilution originally counted on to disperse toxic materials and reduce them to harmless concentrations were not func- tioning as expected. The balance of processes that disperse and con- centrate materials in different environments can result in the manifestation of effects over widely differing scales. The effects of environmental perturbations accumulate when the fre- quency of individual perturbations is so high that one comes before the system has recovered from the previous one or when the ecological effects of perturbations in adjacent areas overlap. Thus, cumulative effects result from spatial and temporal crowding of environmental perturbations. The combined effects of repeated perturbations can be more severe than and sometimes qualitatively different from the sum of the effects of individual events. Comparing the problem of cumulative effects with the "tyranny of small decisions" described by the economist Alfred Kahn, Odum (1982) pointed out that, when numerous small decisions on related environmental issues are made more or less independently, the combined consequences of the decisions are not addressed; therefore, no provision is made for analyzing the patterns of the perturbations or their effects over large areas or long periods. A step toward identifying the major issues involved in the assessment of cumulative impact was made in a recent international workshop held in Toronto, Ontario, under the joint sponsorship of the present committee and the Canadian Environmental Assessment Research Council. Some of this chapter draws on the proceedings of the Toronto workshop (Beanlands et al., in press).
THE SPECIAL PROBLEM OF CUMULATIVE EFFECTS KINDS OF CUMULATIVE EFFECTS 95 Several types of perturbation can produce cumulative effects. Many involve the addition of materials to the environment, for example, the addition of sewage effluent to Lake Washington from multiple sources over many years (Chapter 201. In that case, a major cumulative effect was the rapid deterioration of water quality with attendant changes in species composition. The materials added can also be toxic (Chapter 241. A second major type of cumulative effect involves the repeated removal of materials or organisms from the environment. In forestry, for example, both trees and minerals are removed (Geppert et al., 19841. The effect of harvest rate on population dynamics is seldom a linear function of pop- ulation density, and the response of a population to an incremental increase in harvesting rate can vary substantially with conditions. Increasing the intensity of harvesting can lead to population collapse if a critical threshold is passed, especially if the increase in harvesting is combined with natural environmental changes (Beverton, 1983; Murphy, 1977; Parrish and MacCall, 1978; Peterman, 19781. A third kind of cumulative effect can occur when management decisions result in environmental changes over large areas and long periods. In the New Brunswick forest (Chapter 19), forestry was based on single stands, instead of a large area of forest. The cumulative effect was a decrease in average yields with an increase in annual variability. Harvesting practices that seem appropriate from the standpoint of population dynamics often result in cumulative genetic effects, such as loss of genetic variability or inadvertent selection for undesired traits (Chapter l). A classic example of the cumulative genetic effects of repeated interventions is the evolution of pesticide resistance (Chapter 1~. A fourth, more complicated situation arises when stresses of different types combine to produce a single effect or suite of effects. For example, the construction of too many wells, developments, and drainage canals lowered the water table in the Florida Everglades substantially. Similarly, the construction of roads in national forests can trigger logging, recrea- tional activities, and poaching, which might combine with acid rain and other influences to threaten sensitive populations of forest species. Complex cumulative effects also occur when many individual areas in a region are repeatedly altered. The result can be dramatic changes in the mix, arrangement, and internal characteristics of the habitats of species. Large habitats can become fragmented into patches separated by areas that are inhospitable to many organisms. As habitat patches become smaller and more isolated, species that depend on them become less able to find them and to maintain populations in them (Chapter 171. The conservation
96 KINDS OF ECOLOGIC KNOWLEDGE ED THEIR PLACATIONS of species in the face of severe habitat fragmentation could be our greatest terrestrial environmental problem. Cumulative effects need not be adverse, but might be considered by many as improvements in the environment. Sometimes, repeated attempts to "improve" the environment have the greatest cumulative effects of all. The conversion of deserts to arable land by massive irrigation projects destroys desert habitats, but creates economic benefits and new habitats that favor species adapted to exploit irrigated farmland. Even the attempt to keep some environments in a constant state through repeated interven- tions can have unexpected consequences. Fire is often controlled to protect habitats, but some communities require periodic fires to maintain their typical species composition (Kozlowski and Ahlgren, 19741. Moreover, reducing the frequency of fires can increase the severity of fires that do occur by fostering the accumulation of flammable material. DEFINITION OF CUMULATIVE ENVIRONMENTAL EFFECTS · Time-crowded perturbations. Cumulative effects can occur because perturbations are so close in time that the effects of one are not dissipated before the next one occurs. An example is repeated harvesting of agri- cultural crops or forests that removes some nutrients faster than they are regenerated between harvests (Geppert et al., 1984; Krebs, 1985, Ch. 281. Similarly, the evolution of resistance to pesticides occurs because the susceptible genotypes are repeatedly reduced in number each time the pesticide is applied (Georghiou et al., 1983; May and Dobson, in press). · Space-crowded perturbations. Cumulative effects can occur when perturbations are so close in space that their effects overlap. An example is power plants close enough that the heat plumes of their cooling water overlap (e.g., Slawson and Marcy, 19761. · Synergisms. Different types of perturbations occurring in the same area can interact to produce qualitatively and quantitatively different re- sponses by the receiving ecological communities. For example, several pollutants might interact to produce toxic mixtures (e.g., National Re- search Council, 1982, 1983b; for examples of mixtures toxic to humans, see Reif, 19841. · Indirect effects. Cumulative effects can be produced after or away from the initial perturbation or by a complex pathway. For example, when the level of Southern Indian Lake in Manitoba was raised, the inundation of the lake shorelines resulted in the release of mercury into the lake (Bodaly et al., 1984) and increased the turbidity of the water (Hecky, 19841. Neither of these consequences was predicted by knowledgeable limnologists (Hecky et al., 19841.
THE SPECIAL PROBLEM OF CUMULATIVE EFFECTS 97 · Nibbling. Incremental and decremental effects are often involved in each of the above categories, but not always. However, this "nibbling" of environments is so important that it should be given its own category. The numerous examples include time and space crowding (the addition of power plants to a river one at a time or the introduction of several pollutant sources into a lake), as well as removal of habitat piece by piece, as in the degradation of Chesapeake Bay (Flemer et al., 19831. · Other types of impact have sometimes been equated with cumulative effects, such as threshold developments that stimulate additional activity in a region (e.g., new energy developments in northern Canada) or projects whose environmental effects are delayed (time lags) or are felt over large distances (space lags). These types of effects can be cumulative if their impacts overlap in time or space or are synergistic with those of other developments. DIFFICULTIES IN PREDICTING AND CONTROLLING CUMULATIVE EFFECTS A fundamental problem in predicting and controlling cumulative effects is the frequently large mismatch between the scales or jurisdictional bound- aries of management authority and the scales of the ecological phenomena involved or their effects. Affected environments such as river basins, airsheds, and estuaries usually cross local and state, and sometimes even national, boundaries. Conversely, an environmental insult that originates in several jurisdictions might exert its effects largely or wholly in one area. The history of the Delaware River Basin Commission project is a striking example of the complexity of managing such a situation scien- tifically and equitably (Ackerman et al., 19741. As difficult as management of cumulative effects can be for regional authorities, management by local authorities is even more so. Local au- thorities are ill equipped to deal with regional trends in environmental deterioration. They often lack the legislated responsibility, motivation, expertise, and resources to perform the necessary analyses on an appro- priate scale. When effects appear far from their sources, the local juris- dictions with the sources might not even become involved. A major impediment to the control of cumulative effects is the percep- tion that the effects are minor. How often are projects stopped because they appear to be contributing (however slightly) to a trend in environ- mental deterioration? What happens if scientists recognize a trend, but cannot detect or distinguish the effects of individual actions? The problem of multiple "insignificant" effects is at the core of our failure to deal with the continued nibbling of coastal and other habitats. It is extremely difficult
98 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS to predict the point of attrition that constitutes a critical threshold. Below what population size will a species become unable to recover (Chapter 14~? Beyond what degree of fragmentation will an ecosystem begin to lose species rapidly or change markedly in its functioning? At what con- centration will an environmental contaminant cause qualitative changes in community organization (Chapter 201? Predicting the effects of perturbations is particularly difficult when many sources of different types act in concert. Accurate information on the nature of each source is difficult to obtain, and their interactions can be complex. The current controversy over the sources of acid rain and its effects in particular areas is a case in point (National Research Council, 1983a, in press; U.S. Environmental Protection Agency, 19801. The "proof" required in this socially and politically charged case is prohibitively ex- pensive to acquire. Control of eutrophication in lakes has been the greatest success in solving a multiple-source problem, possibly because of the limited areas involved. When effects are global, as with the long-term buildup of atmospheric CO2, assigning causality and regulating the sources are extremely complex, both scientifically and politically (National Re- search Council, 1983c). It is clear that continued loss of communities and ecosystems will, in the long term, constitute a "cost" to society, but there is little agreement on how to express this cost. We lacl; an accepted framework for deter- mining how the cost of habitat deterioration, with resulting fragmentation, is apportioned among nonresponsible parties, because the observable ef- fects of each incremental loss are often small and local. Assigning a value to an increment of loss can be very difficult, but at some point these small and primarily local effects might be expressed in different forms on a regional scale. Knowing the nature and values of ecological thresholds would give us a benchmark for associating costs with changes, but would not solve the problem of valuing changes that do not threaten to exceed a threshold. Research that will help us to identify the threshold of rapid, observable deterioration is badly needed. SCALE AND THE RATES OF CRITICAL PROCESSES Choosing appropriate scales on which to analyze cumulative effects requires an understanding of the relative balance of important concen- trating and dispersing phenomena and of the rates of other processes that influence the removal of or recovery from individual stresses in the en- vironment being studied (see also Chapter 51. Because the rate of atmo- spheric mixing is so high and because the atmosphere is relatively un- compartmentalized, many materials that have low rates of removal from
THE SPECIAL PROBLEM OF CUMULATIVE EFFECTS 99 the atmosphere (by decomposition, chemical transformation, settling, etc.) are spread on a global scale. In the atmosphere, dispersing processes typically predominate over concentrating processes, with notable excep- tions, as when inversions and scavenging of particles by precipitation lead to local concentration. Atmospheric pollution has been modeled largely on a regional and global scale appropriate to the scale of effects. Mixing in the oceans is much more compartmentalized and less com- plete. Concentrating and dispersing processes are more evenly balanced, and the dominating process varies with local conditions. In many fresh- water systems, such as lakes, concentrating processes predominate. In- dividual lakes often behave independently of one another, so the appropriate scale for analyzing cumulative effects is likely to be an individual lake and its associated watershed. Terrestrial environments are particularly complex spatially and tem- porally. Spatial heterogeneity is marked, and the existence of many persistent and relatively impermeable structural components results in a high degree of compartmentalization. Because the basic substrate is solid, movement of materials is restricted, and many impacts remain local. Processes that affect the concentration or dispersal of materials are complex in terrestrial systems. Chemical reactions in the soil result in leaching of materials through the substrate and into groundwater or, if precipitation is less, in their concentration in different layers of the soil. Spatial scales in terrestrial systems can be particularly important when communities or habitats are increasingly fragmented by nibbling and al- teration. The sizes of habitat patches and the distance between them can profoundly influence the survival of species that depend on them. The probability of local extinction increases as population size decreases, and the chance that a given habitat patch will be recolonized depends on the dispersibility of the species, the proximity of sources of potential colo- nizers, and the ability of a population to be established by a small number of colonizing individuals (Chapters 1 and 51. Seminatural plant commu- nities on derelict lands in Great Britain often fail to re-establish themselves naturally, because the sources of seeds of desired plants in undisturbed habitats are too widely scattered (Chapter 181. Adjacent areas provide mostly seeds of introduced weedy species. To maintain natural commu- nities, managers must artificially increase the supply of propagules. The common approach of treating cumulative effects in distinct envi- ronmental compartments is artificial and often inadequate. Atmospheric phenomena, such as wind and precipitation, are intimately involved in the transport of materials into and out of terrestrial systems. Contaminants in terrestrial systems eventually find their way into aquatic systems, and
100 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS contaminants in aquatic systems often find their way into terrestrial or- ganisms through food chains. The DDT case study (Chapter 24) is a classic example of the complex interchanges that occur between environments. DDT sprayed on crops drifts on the wind and is transported by runoff into aquatic systems, where it is biologically concentrated through food chains. Eggshell thinning problems have followed in terrestrial raptors that feed on contaminated fish or in other birds that eat contaminated aquatic organisms. Whether an ecological system is adversely affected when subjected to repeated perturbations depends not only on the rate and intensity of per- turbations, but also on the rate at which their effects are absorbed by or removed from the system. If toxic chemicals are rapidly degraded bio- logically to a harmless form, a receiving system will be capable of ab- sorbing a relatively high rate of toxicant input. Ecological systems that are characterized by very low rates of growth and reproduction, such as the arctic tundra, are extremely vulnerable to repeated disturbance. MANAGING CUMULATIVE EFFECTS: BEYOND A CASE-BY-CASE APPROACH Improvement in our efforts to predict and to limit or reduce cumulative effects will require a restructuring of both scientific and institutional ap- proaches and a considerable improvement in the interchange of information between scientists and managers. If one accepted the dictum that "ev- erything in ecology is connected to everything else," the already formi- dable challenge of dealing with cumulative effects would become overwhelming. Fortunately for both scientists and managers, connections are not equally strong, and a first step in trying to predict cumulative effects is to determine which interactions are most important (to reduce the problem to as simple a set of relationships as possible) and which sources of perturbation are likely to affect the ecosystem components of concern (Chapter 101. As part of the Sustainable Development of the Biosphere project being carried out at the International Institute for Applied Systems Analysis in Austria, scientists have been developing a promising matrix-based frame- work for scoping potential cumulative-effects problems in the atmosphere (Clark, in press). The approach is designed to identify the many kinds of perturbations that affect particular valued atmospheric components and the multiple effects of particular perturbations through an analysis of the mechanisms of interaction that underlie direct and indirect pathways of cause and effect. This approach can be applied to environments other than the atmosphere.
THE SPECIAL PROBLEM OF CUMULATIVE EFFECTS 101 By indicating the relative strengths of the effects of all potential per- turbations of a system, the framework can help an environmental manager to choose control variables that offer the most promise and to avoid attempts to control a system by manipulating variables with little potential influence. Because it also identifies the multiple effects of a given per- turbation, the framework might also help to focus attention on actions that have the greatest general impact on the environment. When major effects and their pathways have been identified, the primary tasks of scientists are to track the state variables of ecological systems, to identify trends in environmental quality, and to predict critical ecological thresholds. Considerable attention has been given to tracking environ- mental trends (Chapter 7), and some refreshingly novel low-cost ap- proaches to biological monitoring have been developed (Bromenshenk et al., 19851. But methods for predicting critical thresholds are sorely lack- ing. Our understanding of the factors involved in determining population thresholds has improved (Chapters 2, 5, and 17), but much more basic research is needed. Dealing more adequately with cumulative effects will require changes in institutional approaches of both scientists and managers of cumulative effects. Assessment involves many scientific disciplines and requires both interdisciplinary and disciplinary scientists who are comfortable with and adept at working with experts in a variety of fields. Unfortunately, the reward systems for both faculty and graduate students at most universities discourage such activities. The Toronto workshop mentioned above brought together representa- tives of applied and basic science, environmental management, business, and resource agencies. The most critical immediate need in management practices identified in the workshop was the need to improve the match in scales between the ecological phenomena involved in cumulative effects and the management jurisdiction. A good start in this direction would be to improve communication between scientists and managers, and the nom- ogram used by Erdle and Baskerville (Chapter 19) to indicate the long- term benefits and costs of a variety of forestry options is a useful tool for this purpose. Comprehensive generic reviews of the scientific issues germane to par- ticular cumulative-effects problems, including a discussion of scale, would be valuable to a manager. Local decision-makers, who often lack access to the expertise and resources necessary to deal with cumulative effects, also need syntheses of regional ecological knowledge. For example, Cooper and Zedler ( 1980) have proposed a system for land-use planning that allows development projects to be analyzed on the basis of regional ecological knowledge of ecosystem sensitivity to stress.
102 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS The Toronto workshop made it clear that the project-specific nature of environmental impact assessment is inadequate for the management of cumulative effects. The management of cumulative effects must in some way match the scale of the ecological phenomena involved. When important cumulative effects have been identified in aquatic en- vironments, regional management authority has often been proposed or implemented. When valued ecosystem components have been readily iden- tified and have been recognized and generally agreed on by the public, this approach has sometimes been successful. For example, the formation of a metropolitan regulatory authority to control sewage pollution in Lake Washington was very effective, because the public readily comprehended the issues and agreed on the value of a clean lake (Chapter 204. The effort to establish regional control of multiple sources of pollution in the Del- aware River Basin was much more complex (Ackerman et al., 1974~: many issues of conflict were involved, there was less public agreement on what was most valued, the scientific questions were more difficult, the regional authority was far less successful, and attempts to expand the scope of the metropolitan authority (like that formed to protect Lake Washington) to deal with other and less clear issues, such as transportation, failed to win public support. Regional authority might need some ground swell of public support if it is to succeed. In a number of recent cases, ad hoc nongovernment organizations, such as "save the bay" groups, have helped greatly in bringing particular cumulative-effects issues to public attention and have created and supported the public debate that eventually provided the im- petus and pressure for government agencies to act. Perhaps more impor- tant, such groups can serve as intermediaries between scientists and regulators, thereby fostering exchange of information and ideas on con- troversial topics. RECOMMENDATIONS Some of the recommendations produced at the Toronto workshop for improving the scientific and institutional approaches to managing cumu- lative effects follow; they are adapted from the proceedings of that work- shop (Beanlands et al., in press). Some of them are addressed to scientific research, and some to improving institutional handling of cumulative ef- fects. · Cumulative environmental effects should be placed in readily seen time and space scales, as described for atmospheric effects (Clark, in press). This should help to identify different susceptibilities of different
THE SPECIAL PROBLEM OF CUMULATIVE EFFECTS 103 environments and ecosystems to cumulative effects caused by many kinds of perturbations. · A better match between scales of management and scales of envi- ronmental effects is needed. To this end, cases in which cumulative en- vironmental problems have been dealt with both successfully and unsuccessfully-should be reviewed to understand how the setting of management and ecological boundaries influenced their success or lack of it. · Research should be conducted to determine what rates of addition of materials to environments and harvesting of resources from them are consistent with protection of valued components of various environmental systems, as well as the systems themselves. Studies of response rates and recovery times should be included in this research. · Research should be conducted to determine the types of indicators, thresholds, and environments most likely to be useful for assessing and managing different kinds of cumulative effects. · Monitoring should be built into the design of projects, and cumulative effects taken into account where appropriate. This requires an understand- ing of the appropriate boundaries. In addition, monitoring should be fre- quent enough and carried out for long enough to detect cumulative effects. · Agreements between decision-makers, managers, and scientists with respect to the appropriate time and space boundaries for dealing with cumulative effects should be documented. This will force clear thinking about important issues and will provide a record so that procedures can be improved.