Holling, Goldberg - 1971 - Ecology and Planning

7/30/2019 Holling, Goldberg - 1971 - Ecology and Planning

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Ecology and PlanningC. S. Holling & M. A. Goldberg

Published online: 26 Nov 2007.

To cite this article: C. S. Holling & M. A. Goldberg (1971): Ecology and Planning, Journal of the American Institute of

Planners, 37:4, 221-230

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ECOLOGY A ND PLANN ING

Certain remarkable similarities can be found between theconcerns of ecologists and planners. Like complex urbansystems, ecological systems appear to be characterized byfour distinctive properties. These include their functioningas interdependent systems, their dependence on a succes-sion of historical events, their spatial linkages, and theirnon-linear structure. Both systems appear to have consider-able internal resilience within a certain domain of stability.However, programs such as insecticide spraying or urbanrenewal, that disturb the complex balance of either system,can generate unexpected and undesirable results. Use of anecological framework for planning suggests new principlesbased more on recognition of our ignorance than presump-tion of our.knowledge about the systems in which we try tointervene.

To offer an ecological view of urbansystems to a planning audience is risky. But, thepossibilities for collaboration appear to outweighthe risks. Ecologists and planners have much tolearn from each other.

Since ecology emerged from its descriptive phasein the 1920's, its emphasis has been onunderstanding the operation of complex ecologicalprocesses and ecological systems. There has beenlittle effort to develop effective appl icat ions ofecological principles. Within the last few years,however, there has been a major shift toward aninterest in application; but ecologists' first effortsin this direction have been blundering and naive.The central role of planners, on the other hand , hasbeen application and policy formulation andimplementation. Ecologists can benefit enormouslyfrom an infusion of the pragmatic realism that is,of necessity, forced upon the planning profession.Perhaps, at the same time, planners may gain someinsights about urban systems from ecologicaltheory.As a basis for dialogue between planners andecologists, we propose a conceptual frameworkbased on ecological concepts of ecosystemstructure and stability. This framework suggests anapproach for planning based on a presumption of

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HOLLING & GOLDBERG

C. S. Hol l ing and M. A. Goldberg

ignorance rather than on a presumption ofknowledge. Since the area of our knowledge ofmanlenvironment interaction is minutely small incomparison with our ignorance, this conceptualframework may have some merit for the planningprocess.

The Nature and Behavior ofEcological SystemsRather than presenting an exhaustive treatment ofecological concepts and terms, we hope to applythe philosophy of thi: ecological approach to solveproblems of a kind that recur in all complexsystems.' The key insight of this approach is thatecological systems are not in a state of delicatebalance. Long before man appeared on the scene,natural systems were subjected to traumas andshocks imposed by climatic changes and othergeophysical processes. The ecological systems thathave survived have been those that are able toabsorb and adapt to these traumas. As a result,these systems have considerable internal resilience,but we know that this resilience is not infinite. Aforest can be turned into a desert, as in the MiddleEast, or a lake into the aquatic analog of a desert.The key feature of the resilience of ecologicalsystems is that incremental changes are absorbed.It is only when a series of incremental changesaccumulate or a massive shock is imposed, that theresilience of the system is exceeded, generatingdramatic and unexpected signals of change.

This has considerable consequence for planning,since, inherent in the philosophy of planning andin te rv en ti on , is the presumption that anincremental change will quickly generate a signal ofwhether the intervention is correct or not.' If thesignal indicates the intervention produces highercosts than benefits, then a new policy and a newincremental change can be developed. But becauseof the resilience of ecological systems, incrementalchanges do not generate immediate signals of theireffect. As a result, planners can set in motion a

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sequence of incremental steps and face the realityof the inadequacy of the underlying policy onlywhen the interventions accumulate to shatter thebounds of resilience within the system. By thattime it can be too late. In order to demonstratethese features of ecosystems we will discuss twospecific case histories, each based on mansin te rvent ion . T he consequences of theinterventions reveal some of the key properties ofecosystems.

M A L A R I A L CONTROL IN BORNEOSince the Second world War, the world HealthOrganization (WHO) has developed a remarkablysuccessful malarial eradication program throughoutthe world. We wish to emphasize that, in thisexample of intervention, there is no question thatthere has been a dramatic improvement in thequality of life of people in affected regions. But,we wish to explore a specific case in which theworld Health Organization sprayed village huts inBorneo with DDT in order to kill the mosquitothat carries the plasmodium of malaria. This casehas been documented by Harrison (1965).

The inland Dayak people of Borneo live in largesingle homes or long houses with up to 500 ormore under one roof. This concentration ofpopulation allowed WHO to develop a thoroughand orderly spraying of every long house, hu t, andhuman habitation with DDT. The effect on healthstandards was dramatic with a remarkableimprovement in the energy and vitality of thepeople-particularly those remote tribes who hadnot previously had access to medical aid.Nevertheless, there were interesting consequencesthat illuminate some of the properties of ecologicalsystems.

C. S. Holling is Director of the Institute of Animal ResourceEcology at the University of British Columbia. He is a systemsecologist by training and has developed computer simulation modelsof predator-prey nteractions in ecological systems. He has ex-tended many of these functional relationships to successfullysimulate recreational land speculation in the Gulf Islands nearVancouver.M. A. Goldberg is Associate Professor of Commerce at theUniversity of British Columbia and is trained as an economist. Hiswork has included the development of the employment locationmodels for the University o f CaliforniasBay Area Simulation Study(BASS). In addition, he has studied the interactions between landvalues, land rents, and transportation in urban areas, both fromtheoretical and empirical points of view.

Holling and Goldberg are codirectors o f a large-scale environ-mental simulator of the Vancouver region. The study, to be com-pleted over the next five years, is a joint effort by the University ofBritish Columbia, the city of Vancouver, and the Greater VancouverRegional District (a prototype regional government).

There is a small community of organisms thatoccupy the thatched huts of these villages-cats,cockroaches, and small lizards. The cockroachespicked up the DDT and were subsequently eatenby the lizards. In consuming the cockroaches, thelizards concentrated the DDT to a somewhathigher level than was present in the cockroaches.The cats ate the lizards and, by eating them,concentrated the level of DDT still further-to thepoint that it became lethal. The cats died. Whenthe cats disappeared from the villages, woodlandrats invaded, and it suddenly became apparent thatt h e c at s had been performing a hiddenfunction-controlling rat populations. Now, withthe rat came a new complex of organisms-fleas,lice, and parasites, and this community presented anew public health hazard of sylvatic plague. Theproblem became serious enough that finally theRAF was called to parachute living cats in to theseisolated villages in order to control the rats.

The story isnt finished at this point, however,since the DDT also killed the parasites andpredators of a small caterpillar that normallycauses minor damage to thatch roofs (Cheng1 9 6 3 ). Th e cate rpil lar populations, nowuncontrolled, increased dramatically, causing theroofs of the huts t o collapse.

We cite this example not because it has greatsubstance, but simply because it shows the varietyof interactive pathways that link parts of anecological system, pathways that are sufficientlyintricate and complicated so that manipulating onefragment causes a reverberation throughout thesystem. In addition, this case provides a simpleexample of a food chain in which energy andmaterial moves from cockroaches to lizards to cats.Typically, in these food chains the number oforganisms at a higher level in the chain are lessabundant than those lower in the chain. This is theinevitable result of the loss of energy in movingfrom one trophic or nutritional, level to another,and the consequence is a biological amplificationthat concentrates certain material at higher andhigher levels as one moves up the chain. Acontaminant like DDT, for example, can be presentin the environment in very low, innocuous levelsbut can reach serious concentrations after two orthree steps in the food chain. Actually, thisexample is highly simplified; usually in suchsituations there is a food w eb rather than a singlelinear food chain. Several species operate at morethan one trophic level. Moreover, there arecompetitive interactions that further complicate

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and link species within an ecosystem. Even in thisexample, however, it is clear that the whole is not asimple sum of the parts and that there are a largenumber of components in a system acting andinteracting in a variety of complex ways.

A COTTON ECOSYSTEM IN PERUUnlike the preceding example, the case of cottonagriculture in many parts of the world has had amore serious outcome. Pest control practices incotton have, until recently, been both ecologicaland economic d i sas ters . A particularlywell-documented case has been prepared by Smithand Van den Bosch (1967).

There are a series of valleys on the coast of Peruformed by streams running from the high Andes tothe Pacific Ocean. Many of these valleys are underintensive agriculture and, because of the lowrainfall, are irrigated. The result is that each valleyis essentially a self-contained ecosystem isolatedfrom others by barren ridges. In one of thesevalleys, the Canete, during the 1920s, the cropshifted from sugar cane to cotton. Over the years agroup of seven native insects became significantcotton pests, including plant sucking insects, theBoll Weevil, the tobacco leaf worm, and somemoths. The pest problem, however, was essentiallymodest, and the farmers of the region lived withthe resulting economic damage. In 1949 , chlori-nated hydrocarbons like DDT, Benzene hexa-chloride, and toxaphene became widely available,and the opportunity to dramatically decrease pestdamage and increase crop yields arose. The charac-teristics of these insecticides seemed admirablysuited to achieve the goal of pest reduction orelimination in this case:

1.The insecticides are lethal to a largenumber of insects and are so mobile that theyquickly and easily concentrate within insects.

2.They are highly toxic to invertebratesand less so to mammalian forms.

3. They have a long life in the environmentso that, in theory, one application can have aneffect over some time. It has been shown, forexample, that DDT and its biologically activebreakdown products have an environmental halflife of over a decade.

4.They can be easily and inexpensivelyapplied from aircraft.

5. The contained nature of the valley eco-system made it possible to spray the entire areawith the insecticide.

We emphasize these d e t d s because the generalfeatures of this policy seem to be shared by manyof mans actions. First, the objective is narrowlydefined-in this case elimination of seven insectpests. Second, the plan developed is the simplestand least expensive means to achieve the narrowobjective. But the consequences of this approachgenerated a series of unexpected and disastrousconsequences explicitly because of the narrowdefinition of the objectives and the intervention.

The initial response to the insecticide treatmentwas a dramatic decline in pests and a one andone-half times increase in cotton production. Thislasted, however, for only two or three years, whenit was noticed that new pests were appearing thathad never been a problem during the history ofcotton production. Six new species of insectsbecame as serious a problem as the original seven.The reason for the appearance of these new pestswas the elimination of parasites and predators thatwere selectively killed because of the biologicalamplification of the insecticides through the foodweb. Within six years the original seven insect pestsbegan to develop resistance to the insecticide, andcrop damage increased. In order to control thisnew resurgence the concentration of the insecticidehad to be increased, and the spraying intervalreduced from two weeks to every three days. Asthese responses began to fail, the chlorinatedhydrocarbons were replaced by organophosphateinsecticides which deteriorated more rapidly in theenvironment. But even with this change the cottonyield plummeted to well below yields experiencedbefore the synthetic insecticide period.

The average yield in 1956 was the lowest inmore than a decade, and the costs of control werethe highest. The agricultural economy was close tobankruptcy, and this forced the development of avery sophisticated ecological control program thatcombined changed agricultural practices with theintroduction and fostering of beneficial insects.Chemical control was minimized. These new prac-tices allowed the reestablishment of the complex-ity of the food web with the result that thenumber of species of pests was again reduced to amanageable level. Cotton yields increased to thehighest level experienced in the history of cottonproduction in the valley.As in the Borneo example, this case demon-strates the complexity and the structure of oneecological system that gives the system the resil-ience to absorb unexpected changes. Applicationof the insecticide enormously reduced the com-

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plexity and diversity of the ecosystem with adramatic loss in resilience. But there is a differencebetween the two examples. In the first example,the area of intervention was local, and althoughthere was destabilization within the local region,the consequences never became serious enough todefeat the original purpose of the intervention. Inthe Peruvian example, on the other hand, theintervention was more global since the whole valleyecosystem was literally blanketed with insecticide.As a result, the short-term success of the narrowintervention led in the longer term to the completeopposite of the original goal.

N A T U R E OF ECOLOGICAL SYSTEMSThese two examples illuminate four essential prop-erties of ecological systems. By encompassingmany components with complex feedback inter-actions between them, they exhibit a systemsproperty. By responding not just to present eventsbut to past ones as well, they show an historicdquality. By responding to events at more than onepoint in space, they show a spatial interlockingproperty, and through the appearance of lags,thresholds, and limits they present distinctiven o n - h e a r structural properties. First, ecosystemsare characterized not only by their parts but alsoby the interaction among these parts. It is becauseof the complexity of the interactions that it is sodangerous t o take a fragmented view, to look at anisolated piece of the system. By concentrating onone fragment and trying to optimize the perfor-mance of that fragment, we find that the rest ofthe system responds in unsuspected ways.

Second, ecological systems have not been as-sembled out of preexisting parts like a machine:they have evolved in time and are defined in partby their history. This point does not emerge clearlyfrom the examples quoted; nevertheless, the resil-ience described in the examples is very much theconsequence of past history.

When a large area is stripped of vegetation, anhistorical process begins that leads to the evolutionof a stable ecosystem through a series of succes-sional stages. Early in this succession, pioneerspecies occupy the space, and the diversity andcomplexity are low. The species that can operateunder these circumstances are highly resistant toextreme conditions of drought and temperatureand are highly productive. Competition is low, anda large proportion of the incident solar energy isconverted to the production of bio-mass (thestanding stock of organic material). As this ac-cumulates, the conditions of the area begin to22 4

UPPER BOUNDARYOF STABILITY

O M A l N OFABIL ITY =

STABLE EQUILIBRIUM ESILIENCE

Q

LOWER BOUNDARYOF STAI I IL ITY

FIGURE 1Time

An Example of a System wi th a Stable Equil ib -r ium in which Stabi l i ty is Possible WithinDistinc t Boundariesimprove and t o permit the appearance of groups ofplants and animals that otherwise could not sur-vive. The result is a gradual increase in the varietyof species and in the complexity of interaction,and this increase in complexity is accompanied byan increase in the resilience of the system and adecrease in productivity. Under stable conditionsthis successional history can continue until a stableclimax ecosystem evolves.

Mans objective in agricultural management is tohalt this history at an early successional stage whenthe productivity is high. The price of doing this is acontinual effort to prevent the system from mov-ing to its more stable and less productive stage:hence herbicides and cultural practices eliminate orreduce those organisms that compete with man forfood. But by emphasizing high productivity as anarrow objective, man develops the simplest andmost direct policy, and the result leads to de-creased complexity-large monocultures, heavy useof chemical herbicides, insecticides, and fertilizers.For the short term, the narrow objective ofincreased productivity is achieved, but the pricepaid is a dramatic decline in the resilience of thesystem. Third, complex ecosystems have verysignificant spatial interactions. Just as they havebeen formed by events over time so they areaffected by events over space. Ecosystems are nothomogeneous structures but present a spatialmosaic of biological and physical characteristics.The differences noted above between the Borneoand Peruvian examples are explained, in part, bythe difference in the spatial scale of the interven-tion. In the Borneo example, the intervention waslocal and, in fact, increased the spatial hetero-geneity. In the Peruvian example, the interventionwas global and dramatically decreased the spatialheterogeneity. The consequence was that the resil-ience in the cotton ecosystem vanished. Finally,there are a variety of structural properties of the

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processes that interrelate the components of anecosystem. We do not wish to dwell on thesedetails other than to say they present singularproblems in mathematical analysis for they relateto the existence of thresholds, lags, limits, anddiscontinuities.

BEHAVIOR OF ECOSYSTEMSThe distinctive behavior of systems flows fromthese four properties. Together they produce bo thresilience and stability. Even simple systems haveproperties of stability. Consider the example dis-cussed by Hardin (1963). Every warmbloodedanimal regulates its temperature. In man thetemperature is close to 98.6'F. If through sicknessor through dramatic change in external tempera-ture, the body temperature begins to rise or fall,then negative feedback processes bring the tem-perature back to the equilibrium level. But we notethis regulation occurs only within limits. If thebody temperature is forced too high-above106'F., the excessive heat input defeats the regula-tion. The higher temperature increases metabolismwhich produces more heat ,which produces highertemperature, and so on. The result is death. Thesame happens if temperature drops below a criticalboundary. We see, therefore, even in this simplesystem, that stability relates not just to theequilibrium point but to the domain of tempera-tures over which true temperature regulation canoccur. It is this domain of stability that is themeasure of resilience.

In a more complex system, there are manyquantities and qualities that change. Each speciesin an ecosystem and each qualitatively differentindividual within a species are distinct dimensionsthat can change over time. If we monitor thechange in the quantity or quality of one of thesedimensions, we can envisage results of the kindshown in Figure 1. Within the range of stable

T imeF I G URE 2 An Examp le o f a Bounded L im i t CycleHOLLING & GOLDBERG

T i m e

FIGURE3 An Examp le of a Bounded Stable TrajectoryAnalogous to that Found in an EcologicalSuccessionequilibrium, if we cause a change in the quantitybeing measured, it will return to equilibrium overtime. But there is a limit to which we can perturbthese quantities, and that limit is defined as aboundary of stability.The domain of stability is contained within theupper and lower boundaries. In simple physiologi-cal and engineering control feedback systems,regulation is strong enough and conditions arestable enough that most of our attention can befixed on or near the equilibrium. This is not true ofecological systems (Holling and Ewing 1969).Ecological systems exist in a highly variable physi-cal environment so that the equilibrium point itselfis continually shifting and changing over time. Atany one moment, each dimension of the system isattempting to track the equilibrium point butrarely, if ever, is it achieved. Therefore, eachspecies is drifting and shifting both in its quantityand quality. Because of this variability imposedupon ecological systems, the ones that have sur-vived, the ones that have not exceeded the boun-daries of stability, are those that have evolvedtactics to keep the domain of stability, or resil-ience, broad enough to absorb the consequences ofchange. The regulation forces within the domain ofstability tend to be weak until the system ap-proaches the boundary. They are not efficientsystems in an optimizing sense because the pricepaid for efficiency is a decreased resilience and ahigh probability of extinction.

This view of stability is, of course, highlysimplified. There may not be just one stableequilibrium at any instantaneous point in time;there may be several. Moreover, the stable condi-tion might not be a single value but a sequence ofvalues that return to a common starting value. Thisstable condition is termed a stable limit cycle

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(Figure 2 ) . Finally, the sequence of stable valuesneed never return t o some common starting point.The earlier description of an ecological successionreally represents such a condition-a stubZe tra-jectory as illustrated in Figure 3 .

But, however th e equilibrium conditions change,they are all bounded, and what we must ask injudging any policy is not only how effectively anequilibrium is achieved, bu t also how the resilience,or the domain of stability, is changed. The twoinsecticide examples illustrate the point. The poli-cies used in these cases were character ized by threeconditions :

1.The problem is first isolated from thewhole ; that is, pests are damaging cot ton .

2. The objective is defined narrowly; thatis, kill the insect pest.3. The simplest and most direct interven-tion is selected; that is, broad-scale application of ahighly toxic long-lived insecticide.

Each of these conditions assumes unlimitedresilience i n the system. By adopting these policies,the. problem and the solution are made simpleenough t o be highly successful in the short term.So long as there is sufficient resilience to absorbthe consequences of our ignorance, then thesuccess can persist for a very long time. It issuccessful in the sense that the agriculturist canreturn his system almost instantly to an equilib-rium point of one crop and no competing pests.The price paid, however, is the contraction of th eboundaries of stability, and an equilibrium-centered point of view can be disastrous from aboundary oriented view.

It is this boundary oriented view of stabilityemerging from ecology that can serve as a con-ceptual framework for mans intervention intoecological systems. Such a framework changes theemphasis from maximizing the probability ofsuccess to minimizing the chance of disaster. Itshifts the concentration from the forces that leadto convergence on equilibrium, to the forces thatlead to divergence from a boundary. It shifts ourinterest f rom increased efficiency t o the need forresilience. Most import ant , it focuses attenti on oncauses, not symptoms. There is now, for example,growing concern for pollution, but the causes arenot just the explosion of population and consump-tion, but also the implosion of the boundaries ofstability.

The Nature and Behavior of Urban System sArguments related exclusively to the nature andbehavior of ecological systems obviously cannot be226

uncritically transferred t o urban systems. Analogiesare dangerous instruments, and in this case thetransfer should be made only when the structureand behavior of urban systems appear to be similarto the structure an d behavior of ecological systems.

Ecology is the study of the interactions betweenorganisms an d the physical environment. Planningconcerns itself with the interaction between manand the environment of which he is a part. Butdoes this analogy go deeper than a simple verbalparallelism? There are specific examples that pointto similarities between certain ecological and socialp ro c e ~ s e s . ~ut the real substance of an analogybetween ecological and urban systems lies not inthe similarities between parts or processes, but infundamental similarities in the structure of entiresystems. We earlier described four key propertiesof ecological systems which concern system inter-action and feedback, historical succession, spatiallinkage, and non-linear structu re. The same proper-ties seem to be import ant for urban systems.In the first place, both urban and ecologicalsystems are true systems functioning as a result ofinteraction between parts. Just as a narrow inter-vention in an ecological system causes unexpectedreverberations, so will it in an urban system. Afreeway is constructed as an efficient artery tomove people, but the unanticipate d social con-sequences stimulate urban sprawl and inner citydecay. A gh etto is demolished in order to revitalizethe urban core, and disrupted social interactionstrigger violence. A tax subsidy is given t o at trac tindustry, leading to environmental pollution de-teriorating the quality of life. Such narrow inter-ventions demonstrate that the whole is not asimple sum of the parts.

Second, the city region, like an ecologicalsystem, has a history. The modern cities of Nort hAmerica are, to a major extent, the product ofhistory since the industrial revolution. Th e technol-ogy of the industrial revolution removed theconstraints imposed by limitations in the environ-ment, permitting development to take place as ifthere were no environmental limitations. If, fortransient mome nts of time, the signals of theselimitations became apparent through the appear-ance of plague o r famine, the problems weregenerally resolved by looking elsewhere for thesolution. So long as there was an elsewhere-anundeveloped cont inen t, an undeveloped West-thenthis approach provided the quickest solution. Theonly constraints were placed by economic needs,hence the great emphasis on economic growth . Theresult, therefore, is an urban system with many of

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the characteristics of an early stage in an ecologicalsuccession. The system is changing rapidly in timeand is not closed. Without any apparent limitation,water and air are considered as free goods toreceive, at no cost, the wastes of the system. Onlynow is it becoming generally recognized that thereare environmental constraints, that water and airare not free goods, and that wastes cannot simplybe transported elsewhere. In a sense, the urbansystem, like an ecological one, has a memory whichconstrains it to respond to current events only as ithas been conditioned by past events. In a rapidlychanging present the responses can become danger-ously inappropriate.

Third, the urban system has significant spatialcharacteristics and interactions. Just as the city hasbeen formed by events over time, so it is affectedby events over space. The city is not a homogene-ous structure but a spatial mosaic of social,economic, and ecological variables that are con-nected by a variety of physical and social dispersalprocesses. Each individual human has a variety ofneeds-for shelter , recreation, and work. Theseactivities are typically spatially separate, and anyqualitative or quantitative change of a function atone point in space inevitably affects other func-tions at othe r points of space.Finally, the same non-linear, discontinuousstructural properties noted in ecological systemsapply to urban systems. Thresholds and limits existwith regard to city size. We know that a city of500,000 residents has more than five times thevariety of activities a city of 100,000 has. We alsoknow that below certain threshold levels, certainactivities do not occur. Thus, suburban areas andsmaller cities just do not have great art museums,operas, symphonies, and restaurants. These activ-ities appear to occur above certain population, ordensity, thresholds. Finally, such notions as ag-glomeration and per capita servicing costs are allnon-linear relationships with respect to city size.

Both the structure of the parts of an urbansystem and the whole system itself appear t o haveclose similarities to ecological systems. Since wehave argued that in ecological systems these dis-tinctive structural features account for their be-haviour, it follows that urban systems must behavein similar ways. There must be a set of urbanequilibrium conditions. But more important, theseequilibrium states must exist within a domain ofstability that defines the resilience of the urban~ y s t e m . ~nd, as in ecological systems, the con-sequences of intervention in a city can be viewedvery differently depending on whether we take anHOLLING & GOLDBERG

equilibrium conditions. But more important, theseone. If this is true, then we should be able todemonstrate, through samples of interventions, thesame kinds of effects we showed with the insecti-cide examples.

URBAN P ROGRAMS WITH UNANTICIPATEDCONSEQUENCES

For this purpose, we have selected examples ofreasonably narrow interventions. A number ofunexpected consequences emerge, and from themwe can infer that the internal dynamics of urbansystems are similar to ecological ones, and con-tinued narrow interventions can lead to effectsanalagous to those which occur in natural systems.Our examples are chosen to demonstrate thatsimplification of urban systems, through suchsimple but large-scale interventions as urban free-way programs, urban renewal, and rent control,leads to large-scale unexpected consequences and ahigh likelihood of failure even with respect to thenarrow objective of the intervention. This is thelesson from natural systems, and this, we think, isthe experience to date in urban systems.

Three examples should suffice. The first two-rent control and residential urban renewal-represent simple and direct approaches to housinglower income people. Our third example is freewayconstruction which represents a similarly simpleand direct approach to the urban transportationproblem. Each of these solutions has been carriedout on a broad scale; each represents a considerablesimplification of the real world; and each has hadeither little effect or negative effects vis-a-vis theoriginal objective.Re n t control is a reasonable starting point. As inthe insecticide examples there are three explicitpolicy conditions. First, the problem is isolatedfrom the whole; in this case it is defined asinadequate low-cost housing. Second, the objectiveis narrowly defined: to limit the price increase inrental housing during periods of rapid economicgrowth. Third, the simplest and most direct policyis proposed: to apply government control of prices.Finally, the implicit assumption is that there issufficient resilience in the system to absorb un-expected consequences.

Strong economic arguments have been presentedagainst this narrow approach to rent control byTurvey (1957), Needleman (1965) , and Lindbeck(1967) , and empirical evidence supports the thesisthat rent control in the long run can diminish thesupply of housing and therefore extend or guaran-tee a shortage (Fisher 1966 , Gelting 1967, and

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Muth 1968). Available evidence thus indicates anegative effect.

Residential urban renewal yields another in-stance where the desired result is reversed. Slumclearance programs have been aimed at revitalizingthe hearts of urban regions. In their broadestcontext, they include a multiplicity of land uses.For present purposes we are interested only inslum clearance programs aimed at providing betterhousing for low-income families and individuals.

It is fairly clear at present that these programshave not had the intended effect of providing morelow-income housing. Hartman (1964) presentsstrong evidence tha t the programs have failed. Gans(1968) and Anderson (1964) have documented theshortcomings and have demonstrated that thereverse effect has often been achieved. They claimthat the program has resulted in a decline ofhousing for low-income people and that the priceof the remaining housing has in fact increased inthe face of dubious increases in quality. In light ofthis evidence, Frieds (1963) criticisms of the socialimpact of relocation appear especially damning. Itappears, therefore, that slum clearance has failed toprovide more housing to low-income people, hasfailed to significantly upgrade the quality of theirhousing, has imposed a high psychic cost on slumresidents, and perhaps has even had the reverseeffect of removing low-income housing stock fromexistence.

Our final example concerns freeways and ismeant to illustrate a variety of unexpected con-sequences, not only those that have effects oppo-site to the desired ones, but also those thatproduce significant side effects outside thenarrow bounds of the original intervention. Ourscenario runs as follows. Current dependence onthe automobile has led to urban sprawl andcongestion. This, in turn, has induced us to treatthese symptoms with large-scale urban freewayprograms. These have induced further sprawl andchanged land use patterns which, in tu rn , havegenerated the need for more travel and thereforemore traffic. The positive feedback of freeways tocreate traffic is illustrated in almost every majorurban freeway system. Peak capacities are reachedwell ahead of design. The need for a moreintegrated and comprehensive transportation andland use planning program has been called forevery year during the 1960s. Levinson and Wynn(1962) and Wendt and Goldberg (1969) summarizethe arguments for such an integrated approach.Without it , freeway programs are bound to have aneffect opposite to that desired (that is, creation of22 8

traffic congestion rather than alleviation of con-gestion).

Freeways have also brought with them a widevariety of environmental side effects. Freewayshave changed the morphology of cities, stimulatingsprawl that typically utilizes agricultural land. Eachcity can argue, with reason, that increased ef-ficiency of agriculture and the development ofmarginal agricultural lands can partially fill the gap.But the price paid for this increased efficiency isthe consequence of yet another quick technologi-cal fix-the increased dependency on chemicals tocontrol insect pests and weeds and on chemicals tofertilize the land. Initially, the natural environmentcan absorb and cleanse these additives, but thisresilience is limited and, when exceeded, results inthe signals of pollution that are now so evident.

Interestingly enough, freeway planners haveclaimed that freeways should reduce air pollutionby increasing average vehicle speed and facilitatingthe more complete burning of gasoline. Bellomoand Edgerley (1971) provide interesting evidencethat this is no t necessarily the case. They note thatthere are three major automobile pollutants: car-bon monoxide, hydrocarbons, and oxides of nitro-gen. Both carbon monoxide and hydrocarbonemissions are reduced as a result of increased speedand more complete burning. Oxides of nitrogen,however, are produced in proportion to fuelconsumption which increases with vehicle speed.Thus, the Los Angeles freeway system, by increas-ing vehicle speeds, does reduce both carbonmonoxide and hydrocarbon emissions. Unfortu-nately, it also increases the production of oxides ofnitrogen, and it is these oxides that give LosAngeles its notorious photochemical smog. Hereagain we have the reverse of the desired state ofaffairs.

These examples illustrate the dangers inherent infocusing too narrowly on a component or symp-tom of a system problem. They have been chosento relate the ideas developed for biological systemsto urban systems to demonstrate that there arefunctional analogies that can be drawn from one t othe other. Having illustrated these relationships, wecan move on to some system oriented solutions ofthe kind that are evolving in the biological sciencesand that (again hopefully) can successfully betransplanted to urban systems.

ECOLOGICAL PRLNCIPLES AND URBAN PLANSA variety of suggestions that have been made forurban systems are analogous to ecological controlschemes in nature. They revolve around smaller

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scale interventions and decentralized efforts ratherthan large-scale monolithic approaches.

There is a common theme running throughoutthe criticisms of the approaches to urban housingand transportation problems as well as through thesuggestions for change. The criticisms concern thenarrowness of the original approach and the failureto achieve stated goals while causing a variety ofside effects. The suggestions for change areanalogous to ecological control schemes and basic-ally state that the system can cure itself if given achance. The chance is provided if our interventionsgive credence to the basic complexity and resil-ience of our urban systems. Such basic respect forthe system eliminates a host of policies like thosethat have been previously sketched out. The idea isto let the system do it, while our interventions areaimed at juggling internal system parameters with-out simplifying the interactions of parameters andcomponents.

In this vein, Gans (1969), Anderson (1964),Fried (1963), and Cogen and Feidelson (1967)argue for increased flexibility and more decentra-lized approaches to the urban housing problem.Instead of large-scale clearance and public housingprograms or rent controi, they advocate smallerscale projects of rehabilitation, rent and incomesubsidies, tax credits and subsidies for propertyowners, and so on. These solutions allow individ-uals to make decisions as they see fit, whilegovernment decisionmakers provide them withinformation in the form of subsidies and credit asto the kinds of decisions society is willing to payfor. This decentralized approach will likely bemore efficient in the long run and certainly morehumane and enduring, since individuals will bemaking decisions about their future and willnecessarily feel more a part of that future.

Completely analogous suggestions have beenmade for ,transportation planning. Again the cen-tral concept is to let individuals choose forthemselves their transportation, the locations oftheir housing relative to their jobs, and theconvenience they desire for their travel. Transpor-tation and urban planners again merely provideinformation that will guide people toward sociallydesirable ends.

Pricing is the most widely mentioned means ofachieving this end. Meyer, Kain, and Wohl (1966)describe pricing schemes both for congestion andpeak hour use of roads and for parking. Bychanging the prices the individual traveller faces, hewill more nearly bear the social costs that hecreates in the form of congestion, pollution, andHOLLlN G & GOLDBERG

dispersed land use patterns. It is then up to himwhether or not he chooses to pay these new pricesor switch his travel mode, house location, joblocation, or time of travel to a pattern that is moreconsistent with broader social goals of reasonableurban form, uncongested roads, and reasonablyclean air. At the same time a variety of modes oftravel and housing locations must be provided sothat meaningful choices exist. Provision of suchalternatives is entirely consistent with our thesissince it does not diminish the basic complexity ofthe urban system (it may well add to it) and doesnot diminish the systems resilience (again we mayscore gains where previously the system wasstretched to capacity of the preexisting mode be itfreeway, subway, or ferry).

ConclusionGiven an intuitive understanding of our complexurban system, we would hope that practicingplanners and other private and public decision-makers would draw several conclusions for them-selves about the nature of their actions in thesystem. First, and most important, is that theiractions be limited in scope and diverse in nature.Actions of this sort do preserve the complexity andresilience of the urban system and will limit thescale and potential harm of the inevitable un-expected consequences. Second, we feel that com-plexity is a worthwhile goal in its own right andshould be preserved and encouraged. Finally, andreally encompassing the above, we would hopedecisionmakers and their advisors will adopt amore boundary oriented view of the world. Weshould be much more wary of success than failure.Again, rather than asking project directors tosubstantiate the ultimate success of their projects,they should be asked to ensure that unexpectedand disastrous consequences be minimized. This isturning things around 180 degrees, but we feel thisis the only way to proceed. Success has given usfreeways, urban renewal, and public housing proj-ects. We must reduce the size of our institutions toensure their flexibility and respect for the systemof which they are a small interacting part.Authors Note: This paper is very mu ch a join t effort. Since bothour names could n otg ofi rs t , the decis ion was relegated to asimple stochastic process-we tossed a coin.NOTES

1 The reader specifically interested in an introduction to ecologycan refer to Odum (1963) and Whittaker (1970).

2 The notion of incremental (or marginal in the economistsjargon) changes is part and parcel of cost-benefit analysis. Non-marginal investments, which can change the structure of prices andthe allocation of resources, are difficult to deal with under present

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cost-benefit approaches. Thus, resilience is usually assumed byignoring changes in prices induced by large-scale projects. See Prestand Turvey (1965) for a discussion of the assumptions concerningmarginal and non-marginal projects.

3 In one example (Holling, 1969 ), a simulation model ofrecreational land use was developed. It was clear from this studythat many of the qualities and processes in land acquisition weresimilar to those found in the ecological process of predation.Moreover, the similarity between land acquisition and predationextended to the structure of the interrelations among the com-ponents of each. They differed only in the specific form of thefunctions describing the action of each component. Therefore, evenat the level of processes there is at least a structural identity that canbe usefully explored so long as it is recognized that there arefunctional differences.

4 See, Jacobs (1961, Chapter 13), for some interesting descrip-tions of the destruction of formerly diverse and stable urbansystems.

5 The most complete study of the subject to date, by the RANDCorporation in New York, is not ye t available, but preliminaryevidence supports this statement.

REFERENCESAnderson, Martin. (1964) The Federal Bulldozer (Cambridge, Mass.,MIT Press).Bellomo, S . J. and E. Edgerley. (1971) Ways to Reduce AirPollution through Planning Design and Operations. Presented at

the 50th Annual Meeting, Highway Research Board, Washington,D.C., January 1971.

Cheng, F. Y. (1963) Deterioration of Thatch Roofs by MothLarvae after House Spraying in the Course of a MalarialEradication Programme in North Borneo, Bulletin World HealthOrganization, 28: 36-7.Cogen, J. and Kath ryn Feidelson. (1967) Rental Assistance forLarge Families: An Interim Report in J. Bellush and M.Hausknecht (eds.), Urban Renewal: People, Politics G. Planning(New York: Doubleday Anchor). Especially Section VI, NewDirections, pp. 508-42.

Fisher, E. M. (1966) Twenty Years of Rent Control in New YorkCity in Essays in Urban Land Economics (Los Angeles: Uni-versity of California Real Estate Research Program), pp. 31-67.Fried, Marc. (1963) Grieving for a Lost Home, in L. J. Duhl (ed.),The Urban Condition (New York: Basic Books), pp. 151-71.

Gans, Herbert J. (1968)People andplans (New York: Basic Books).Especially Chapter 18: The Failure of Urban Renewal: ACritique and Some Proposals, pp. 260-77.Gelting, J. H. (1967) On the Economic Effects of Rent Control inDenmark, in A. A. Nevitt (ed.), Th e Economic Problems ofHousing (London: Macmillan), pp. 85-91.

Hardin, Garrett. (1963) The Cybernetics of Competition: ABiologists View of Society, Perspectives in Biology G Medicine,7: 58-84.Harrison, Tom. (1965) Operation Cat Drop, Ani m a k , 5:512-13.Hartman, Chester. (1964) The Housing of Relocated Families,Journal of the American Institute of Planners, 30 (November):266-86.Holling, C. S. (1969) Stability in Ecological and Social Systems,Brookhaven Symposia in Biology, 22:128-41.Holling, C. S. and S. Ewing. (1969) Blind Mans Bluff: Exploringthe Response Space Generated by Realistic Ecological SimulationModels, in Proceedings of the International Symposium ofS fatistical E cology (New Haven: Yale University Press). (In Press.)Jacobs, Jane. (1961) Th e Death and Life of Great American Cities

Levinson, H. S. and F. H. Wynn. (1962) Some Aspects of FutureTransportation in Urban Areas, Highway Research Board Bulle-tin, 326:l-31.Lindbeck, Assar. (1967) Ren t Control as an Instrument of Housing

Policy, in A. A. Nevitt (ed.), The Economic Problems of Housing(London: MacmilIan), pp. 53-72.Meyer, J., J. Kain, and M. Wohl. (1966) The Urban TransportationProblem (Cambridge, Mass.: Harvard University Press). Especially

Chapter 13, pp. 334-59.Muth, R. F. (1968) Urban Land and Residential Housing Markets,in H. S. Perloff and L. Wingo (eds.), Issues in Urban Economics(Baltimore: Johns Hopkins Press), pp. 285-333.

Needleman, Lionel. (1965) The Economics of Housing (London:Staples Press).

Odum, Eugene P. (1963) Ecology (New York: Holt, Rinehart andWinston).Prest, A . R. and R. Turvey. (1965) Cost-Benefit Analysis: ASurvey ,Economic Journa l, 75.Smith, Ray F. and Robert van den Bosch. (1967) IntegratedControl in W. W. Kilgare and R. L. Doutt (eds.), Pest Control

(Academic Press).Turrey, Ralph. (1957) The Economics of Real Property (London:George Allen &Unwin).Wendt, P. F. and M. A. Goldberg. (1969) The Use of LandDevelopment Simulation Models on Transportation Planning,Highway Research Record, 285:82-91.Whittaker, R. H. (1970) Comm unit ies and Ecosystems (New York:Macmillan Co.).

(New York: Random House).

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Holling, Goldberg - 1971 - Ecology and Planning