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OCR for page 93
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
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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).
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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
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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.
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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
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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
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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
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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.
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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.
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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
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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.
Representative terms from entire chapter:
cumulative environmental
