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Urban landscape networks: an ecological planningframeworkEdward A. Cook aa Associate Professor of landscape architecture in the Department of Planning , ArizonaState University , Tempe, Arizona, U.S.A.Published online: 03 Jan 2007.

To cite this article: Edward A. Cook (1991) Urban landscape networks: an ecological planning framework, LandscapeResearch, 16:3, 7-15, DOI: 10.1080/01426399108706345

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Urban Landscape Networks: an ecologicalplanning framework

Edward A. Cook

It is now recognised thatnatural areas need to beincorporated into the planningof the environment as a wholerather than just in isolatednature reserves (Tubbs andBlackwood 1971). Anappropriate planningframework to facilitate suchintegration does not yet exist.Some progress is being madewith agricultural landscapes(Gelderblom et aL 1988,Kerkstra and Vrijland 1990) inthe Netherlands and othermember countries of theEuropean Community (EC). Asthe EC moves towardsunification in 1992, severalcountries have looked atagricultural reallocation schemesthat would take some land outof agriculture and convert it tonatural areas (Dessylas 1990).The Dutch government has alsorecently adopted a policyestablishing a 'main ecologicalinfrastructure' (Lammers 1989).

Towns are generally deficient inareas with significant ecologicalvalue for a number of reasons.The urban development processis usually highlyanthropocentric in orientationand natural areas, if they persistat all, are relegated to remnantpatches or corridors. They oftenexist because they areconsidered less suitable forurban development because offlooding, severe slope conditionsor other limiting characteristics(Spirn 1986). Previouslyfunctioning ecosystems aresevered from the supportingstructure and becomedysfunctional. Natural areas,that do persist often becomeover used. This leads toaggressive management andfurther disruption of naturalsystems. In many cases,remnant patches or corridorsacquired by municipalities orretained by developers asprivate open space systems aredeveloped into cultivated parks.These are suitable for human

use, but lack the characteristicsrequired to support the speciesthat originally inhabited theland. Other corridors areretained or managed for floodcontrol. Patches or corridors in apredominantly unmodifiedcondition have becomeincreasingly rare and wholefunctioning ecosystems are evenmore so.

Fragmented, dysfunctionalecosystems will not providelong-term sustainabOity norbenefit society and the globalenvironment Through a processof preservation, management,reclamation and establishmentof man-made nature areas, amultiple-use system can beestablished that will bebeneficial in an urban setting.This system can be viewed as anatural or ecological network.The network would consist of asystem of interconnectedpatches and corridors woveninto an urban landscape matrixand connected to external andinternal source areas (largenatural areas with significantnative species populations).Corridors and patches would bedominated by native plant andanimal species. Numerousfunctions can and must occur inorder for such a network to bedeemed justifiable. Generally,urban land economics createextreme pressure for the mostprofitable use of the land so thevalidity of an ecologicalnetwork must be wellestablished from asocio-economic perspective.Also, the idea that a network beecosystem-based, implies thatmultiple functions occur if it istruly ecological.

The history of comprehensiveapproaches to preserving orestablishing networks in urbancontexts is somewhatinconsistent (see Little 1990 for areview of the 'greenwaymovement' in the United Statesand Groome 1990 for a review

of 'green corridors' in theUnited Kingdom). Beginningwith Loudon's greenbelt planfor London and Olmsted's planfor The Fens and Riverway inBoston, far reaching proposalshave been implemented tovaried degrees. The work ofNorth American landscapearchitects, most notably AnneSpirn (1988,1986 and 1984) andMichael Hough (1990 and 1989),in urban contexts has provideda foundation upon which plansare now being implemented toreconnect natural areas. InEurope, plans for the GreaterCopenhagen Area (Asbirk andJensen 1984) include a system ofnature corridors. Other lesscomprehensive efforts thatinclude an inventory of natureareas (Rogers and Rowntree1988), isolated preservation(Sukopp and Weiler 1988), orreclamation (Bradshaw 1983)occur more frequently in urbanor urban fringe landscapes.More research is neededspecifically dealing with urbanareas, however, to determinethe effectiveness and feasibilityof various initiatives andtheories. Implementation intowns becomes more complexbecause of competing interests,multiple ownership orjurisdiction and intensive urbandevelopment

A number of benefits can beviewed as objectives whendeveloping an urban landscapenetwork though not all of thesemay be achieved totally. Forexample, recreation in someareas may inhibit use by somewildlife because certain speciesare very intolerant ofdisruption. If both functions aredeemed necessary in an area,physical separation or bufferingmay be required. This increasesthe land area needed and mayrequire the establishment ofminimum criteria for eachfunction. A brief description ofa number of functions follows.These represent only a limited

KeywordsEcologyUrban PlanningLandscape -USA-ArizonaRecreation

AbstractThis paper presents aframework for theplanning of urbanlandscape networks inan attempt to havenature areas integratedinto traditional urbanland-use planning. Thebiophysical andsocio-economic benefitsof such a network arediscussed to providejustification for thisincorporation. Aframework isdescribed, buildingupon ecologicalplanning processes andincorporating theoryand methodology fromlandscape ecology. Theframework includes anassessment of naturaland cultural resources,formulating the spatialstructure of thenetwork andexamination ofnetwork components.Implementationstrategies are discussedwithin the context oftraditional urbanland-use planningpractice.

AuthorEdward A. Cook is anAssociate Professor oflandscape architecturein the Department ofPlanning at ArizonaState University,Tempe, Arizona,U.S.A. where he isengaged in teachingand research onlandscape ecologicalplanning and design.

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ReferencesAdamus, P. R. and G. C.dough, 1978. Evaluatingspedes for protection innatural areas. BiologicalConservation, 13:165-178.

Agger, P. and J. Brandt,1988. Dynamics of smallbiotopes in Danishagricultural landscapes.Landscape Ecology, 1:227- 240.

Ambuel, E. and S. A.Temple, 1983. Areadependent changes in thebird communities andvegetation of SoutheasternWisconsin forests. Ecology,64:1057-1068.

Asbirk, S. and S. M. Jensen,1984. An example ofapplied island theory anddispersal biology. In P.Agger and V. Nielson(eds.). Dispersal Ecology.Copenhagen:Naturfredningstradet ogfredningsstyrelsen. pp. 49-54.

Balling, R. C. and S. W.Brazel, 1987. Time andspace characteristics of thePhoenix urban heat island.Journal of the Arizona-NevadaAcademy of Science, 21:75-81.

Balser, D., A. Bielak, G.DeBoer, T. Tobias, G.Adindu and R. S. Domey,1981. Nature reservedesignation in a culturallandscape, incorporatingisland biogeography theory.Landscape Planning,8329-347.

Barbier, E. G., 1987. Theconcept of sustainableeconomic developmentEnvironmental Conservation,14:101-110.

Bradshaw, A. D., 1983. Thereconstruction ofecosystems. Journal ofApplied Ecology, 20:1-17.

Brux, V. H., 1989. On thenecessity of natureconservation in urbangreens. Landschaft and Stadt,21:144-147.

Budd, W. W., P. Coen, P. R.Saunders and F. R. Steiner,1987. Stream corridormanagement in the PacificNorthwest I. Determinationof stream corridor widths.Environmental Management,11:587-597.

Cook, E. A., 1991:Ecosystem modeling as amethod for designingsynthetic fluvial landscapes:A case study of the Salt

number relevant to applicationin the arid context of theSouthwestern United States.These are described asbiophysical and socio-economicbenefits.

Biophysical: an overall goalfrom a biophysical perspectiveshould be to maintain bothspedes and ecological processes.Several functions can beconsidered as objectives whendeveloping an ecologicalnetwork. They can be dividedinto three general categories:biodiversity, sustaining .hydrological processes andameliorating climate. Theseobjectives are particularlyimportant in towns becausethese conditions are so oftenlacking or severely altered there.While there are numerous otherfunctions worth noting, theserepresent those that may haveparticular importance in an aridregion.

Maintaining biological diversityrequires a level of ecologicalfunctioning of the ecosystemswhich indicates health andminimal human intrusion. Anurban landscape network cancontribute to greater diversityby providing areas where nativespecies may dominate. Intowns, native species are oftendriven out or areunder-represented. Whenestablishing objectives forbiological diversity, speciescomposition becomes criticaland native, rare andendangered species should beweighted more heavily (Hobbs1988, Noss and Harris 1986,Noss 1983). Networks cancontribute to the diversity ofboth plant and animal species.Corridors and patches can besuitable for wildlife habitat andmigration between biotopes.Plant species are also dispersedthroughout the network. Often,because these areas are conduitsfor energy, nutrient and speciesflow, they are the only possiblemeans of maintaining biologicaldiversity in this context. Islandbiogeography theory(MacArthur and Wilson 1967)plays an important role inpatterning natural or ecologicalnetworks to enhance biologicaldiversity. This theory and its

role in designing an urban,landscape network will bediscussed later in this paper.

Hydrological processesgenerally have a significantinfluence in the development oflandscape structure and,ultimately, future landscapefunction. Drainage corridors cancomprise a major portion of anetwork. In towns, flood controlis always a high priority.Drainage corridors or wetlandpatches serve to containfloodwarers. Kept in a naturalstate, the corridors act as afilter, catching contaminantsfrom adjacent lands thusparticipating in the purificationprocess. Well-vegetatedcorridors also help to reduceerosion by stabilising banks.

A well vegetated network canmake a significant contributionto climatic amelioration at microand macro scales. The 'urbanheat island effect' (Balling andBrazel 1987) and 'globalwarming' are two significantproblems that can be partiallymitigated by the preservation ofexisting vegetation andextensive new planting. Urbanlandscape networks are idealplaces to promote extensivepreservation and plantingschemes. On a micro scale, moreextensive vegetative coverprovides shade, windprotection, and cooling throughevapotranspiration. Theseconditions provide betterenvironments for wildlifehabitat by providing relief fromextreme elements. Retention ofsoil moisture is also increased,and is particularly important inhot, arid regions.

Socio-economic In an urbancontext, undeveloped patches 'and corridors often existbecause of their socio-economicrelevance but rarely are theyviewed as a network or acomprehensive system. Ifdeveloped holistically they canprovide more significantbenefits. The most frequentlyarticulated function is passiverecreational use. Bicycle, hikingand equestrian trails arecommon in corridors andpicnicking, camping and natureobservation may occur inpatches. If nature preservation

and rehabilitation are integral tothe network, educational andhuman psychological benefitsmay be derived as welLReinforcing a human/naturerelationship in the dry may bean important element inestablishing an environmentalethic in sodety (Spirn 1988,Jacobs 1986, Barbier 1987).

These areas may also beeffective land use buffers andmarkers. Less compatible landuses may be separated veryeffectively with patches orcorridors of a network.Corridors may also be useful fordelineating propertyboundaries, changes in use orother cultural phenomena. Thisis quite common in agriculturallandscapes (Agger and Brandt1988) and may also be foundoccasionally in urban areas.There may also be several directeconomic benefits of an urbanlandscape network. Studieshave shown that natural areasprovide additional aestheticqualities and increase amenityvalue; effectively used, thesequalities may be translated intoincreased property value andtax income. Natural systems canalso be self-sustaining and assuch they require lessmaintenance or money.

Many other benefits may beattributed to the development ofecological networks in urbanlandscapes. The most criticalissue, however, is integratingvarious ecological objectives intothe normal planning process.The value of a multiple-useapproach is demonstrable in anurban sodo-economicenvironment and can begin toraise the importance oftraditionally less valuedecological functions.

Establishing a PlanningFrameworkOccasionally, large scale plansare made for urbandevelopment that incorporatenatural systems in a holistic andcomprehensive way, thusproviding the ability tointegrate healthy functioningecosystems into an urbanenvironment. In most cases,

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remnant corridors and patchesmust be used to establish anetwork within an existingsocio-economic context Theprocess described in this paperassumes the latter situationbecause most forms havedeveloped with littleconsideration for maintainingnatural ecological functions. Themetropolitan area of Phoenix,Arizona will be used as anexample to clarify some of theconcepts. Phoenix is well suitedbecause it is a low density drywith patches and corridors ofseveral types. It is also adeveloping area, so someaspects of preservation in theadvance planning process arepossible. There has also beenincreasing interest in 'adapting'existing corridors (dry rivercorridors, drainage washes andcanals) and in protectingpatches (mountain preserves,natural desert parks). Some ofthe existing research, asdescribed in the literature,focusing on patch, corridor andnetwork dynamics may not becompletely relevant in thiscontext since it generallyassumes that remnant naturalpatches are woodlands. In thelower Sonoran desert, remnantnatural patches are often lessvegetated than the surroundingurban matrix. Corridors tend tohave the greater abundance ofvegetation and bioh'c diversity.For the purposes of this paper,however, these patches areassumed to have-equivalentvalue and play a vital role indeveloping a functioningnetwork. More research on thistopic, specifically, will berequired to determine if thesepatches in fact perform a similarfunction to the woodlands. Theprocess for establishing aplanning framework willaccommodate varying regionalcharacteristics. It builds upon atypical ecological planningprocess (McHarg 1969, Steineret aL 1988) but incorporatestheory from landscape ecology(Forman and Godron 1986,Neveh and Iieberman 1984,Vink 1982). The processincludes an assessment ofnatural and cultural resources,the formation of the network'sspatial structure and an

examination of the network'scomponents.

Assessment of Naturaland Cultural ResourcesAny defensible planning processmust include an inventory andassessment of existing andpotential resources. Approachesto recording remnant naturallandscape elements or biotopeshave been described by severalauthors (Agger and Brandt1988, Sukopp and Weiler 1988,Rogers and Rowntree 1988).Methods for performingecological assessments, fornumerous purposes, have alsobeen well documented (Wathernet al. 1986, Steiner et al. 1987,Greer 1982, Goldsmith 1975,Peterken 1974, Tubbs andBlackwood 1971).

Inventory and description: anappropriate assessment beginswith an inventory anddescription of existing andpotential resources. These canbe classified as corridor or patchelements according tocharacteristics relevant tocontextual criteria and thenassessed for relative ecologicalworth. Inventory andclassification must span severalscales, recognizing thatecological functioning ishierarchical but interconnectedbetween scales. Finally, quality,quantity and location ofcorridors and patches areevaluated while consideringinterrelationships with thesurrounding matrix.

Sukopp and Weiler (1988) andAgger and Brandt (1988) haveestablished methods forrecording biotopes in developedlandscapes. Sukopp and Weiler(1988) present two methods ofbiotope mapping: selectivemapping, which includes onlybiotopes that are worthy ofprotection, and comprehensivemapping, which includes allbiotopes found in the city. Thelatter method is mostappropriate for establishment ofan urban landscape network.This allows for the reclamationof areas that are not currentlyconsidered valuable (Wittig andSchreiber 1983). Agger and

Brandt (1988) have devised aclassification system anddistribution analysis that isuseful as a model. Corridor andpatch biotopes are divided andcharacteristics relevant to thecontext, in their case Danishagricultural landscapes, areused to distinguish the biotope.Descriptive characteristics mayinclude whether they aredominated by man-made ornatural elements, vegetationdensity, current use and so forth.

Scale and hierarchy becomecritical if a network is tofunction most effectively.Numerous authors have writtenabout hierarchy and theconnection between scales(Wiens 1989, O'Neill et aL 1988,Urban et aL 1987, Noss andHarris 1986, Noss 1983, Bakeret aL 1981). A biogeographicclassification system describedby Balser et aL (1981), based onDorney and Hoffman's (1979)system includes a range ofscales for mapping 'islands'(patches or corridors). Thisseven-unit scale accommodatesthe finest scale eco-elements(0.01-0.02 ha), mapped at1/2|50-1/10,000 scale, to therealm, mapped at 1/8,000,000-1/30,000,000. Interrelationshipsat all scales and between scalesare important, but the context ofeach situation will dictate therange that is appropriate. It isalso important to consider scalein terms of species diversity,spatial relationships andmanagement units. Noss (1983)recommends at least three scalesin an attempt to integrate theseperspectives. A community orsingle habitat, correspondingwith alpha scale speciesdiversify (Whittaker 1972), is anarea less than a few hectareswith uniform vegetativestructure. Dorney and Hoffman(1979) call this an ecosite and itmay correspond to anindividual management unitsuch as a park. A series ofhabitats or communities (betadiversity) is the middle scaleidentified by Noss (1983). Thismay correspond roughly withan ecosection or ecodistrict(Dorney and Hoffman 1979)and a city may be acorresponding managementunit A third level is a large

River in Arizona, Landscapeand Urban Planning.20291-308.

Dessylas, M. D., 1990. Theadaptation of social andstructural policy of the EECto the changed marketsituation: The protection ofthe countryside. Landscapeand Urban Planning, 18:197-207.

Diamond, J. M., 1975. Theisland dilemma: Lessons ofmodern biogeographicstudies for the design ofnature reserves. BiologicalConservation, 7:129-146.

Diamond, J. M. and R. M.May, 1976. Islandbiogeography and thedesign of natural reserves.In: R. M. May (ed.)Theoretical Ecology. London:Blackwell Scientific pp.163-186.

Dorney, R. S. and D. W.Hoffman, 1979.Development of landscapeplanning concepts andmanagement strategies foran urbanizing agriculturalregion. Landscape Planning,6:151-177.

Eagles, P. J., 1984. Planningand Management ofEnvironmentally SensitiveLands. London: Longman.

Fahrig, L. and G. Merriam,1985. Habitat patchconnectivity and populationsurvival Ecology,66:1762-1768.

Franklin, J. F. and R. T. T.Forman, 1987. Creatinglandscape patterns by forestcutting: Ecologicalconsequences andprinciples. LandscapeEcology, 15-18.

Forman, R. T. T. and M.Godron, 1986. LandscapeEcology. New York: JohnWiley & Sons.

Forman, R. T. T., 1983.Corridors in a landscape:their ecological structureand function. Ekologiya,2:375-385

Gaines, D. A., 1980. Thevalley riparian forests ofCalifornia: Their importanceto bird populations. In A.Sands (ed.). Riparian forestsin California, their ecologyand conservation. Davis,California, Division ofAgricultural Science -University of California, pp.57-85.

Game, M. and G. F.Peterken, 1984. Nature

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reserve selection strategiesin the woodland of centralLincolnshire, England.Biological Conservation,29:157-181.

Gelderblom, L., R. Juystersand L. Meyssen, 1988.Mogelijkheden voorIandinrichting binnen eenecologische infrastructuurvan houtsingels.Landinrichting, 28:75-82.

Goldsmith, F. B., 1975. Theevaluation of ecologicalresources in the countrysidefor conservation purposes.Biological Conservation,3:89-96.

Greer, D. M., 1982. Urbanwaterfowl population:Ecological evaluation ofmanagement and planning.Environmental Management,6:217-229.

Groome, D., 1990. 'Greencorridors:' a discussion of aplanning concept. Landscapeand Urban Planning,19:383-387.

Harris, L. D., 1984. TheFragmented Forest: IslandBiogeography Theory and thePreservation of BioticDiversity. New Haven: YaleUniversity Press.

Helliwell, D. R., 1978.Survey and evaluation ofwildlife on farmland inBritain: an indicator speciesapproach. BuiologicalConservation, 13:63-73.

Hobbs, E. R., 1988. Speciesrichness or urban forestpatches and implications forurban landscape diversity.Landscape Ecology, 1:141-152.

Hough, M., 1989. City Formand Natural Process. London:Routledge.

Hough, M., 1990. Out ofPlace. New Haven: YaleUniversity Press.

Jacobs, P., 1986. Sustaininglandscapes: Sustainingsocieties. Landscape andUrban Planning, 13:349-358.

Johnson, R. R., 1985.Desertification of wetriparian ecosystems in aridregions of the NorthAmerican southwest In: EWhitehead, C. F.Hutchinson, B. N.Timmennan and R. Varady(eds.). Arid Lands Todayand Tomorrow: Proceedingsof a symposium. Tucson,Arizona, pp. 1383-1393.

region with total speciesdiversity (gamma) (Whittaker1972). Dorney and Hoffman(1979) would classify this as anecoregion. A watershed ornatural region may be the mostsuitable management unit atthis scale but, in the U.S. feworganisations manage with thisunit For classification purposes,the three scales, as suggested byNoss (1983), were determined tobe valid for application in anurban landscape network.Useful terms for integrating theideas of species diversity, spatialrelationships and managementunits are site (finest scale), localand regional.

Assessment: Determination ofthe relative worth or quality ofbiotopes is important whenconducting ecologicalassessments. Wathern et al.(1986) have provided a reviewof ecological assessmenttechniques. They categorisethese into system-based,indicator-species and inventoryapproaches. Depending on thespecific problem at hand or theexpertise of the individualsconducting the assessment, anyor all of these approaches maybe valid.

A system-based (also calledecosystem or site evaluation)approach uses criteria includingrarity, diversity, naturalness andstructural differentiation. Theseare the most widely adoptedapproaches for ecologicalassessments. Aggregate valuesfrom the established criteria areused to form a quantitativedescription of relative worth.Sargent and Brande (1976) andWright (1977) have alsoincorporated management, ornon-scientific, criteria inaddition to those noted. Thisapproach may be particularlyappropriate for applicationwithin an urban context sincenon- scientific criteria are usedfor most land-use decisions. Toensure the integrity of theprocess, it may be appropriateto develop two aggregatevalues, one using only scientificcriteria and anotherincorporating non-scientificcriteria.

An indicator-spedes approachuses individual species which

can serve as surrogates for morecomprehensive or complex data(Walthern et aL 1986). Oftenthis approach is based largelyon detailed knowledge ofparticular habitats. The presenceof higher species, which may beused as indicators, wouldsuggest that an area has greaterrelative worth. Adamus anddough (1978), Peterken (1974),Game and Peterken (1984), Rose(1974), Ward and Evans (1976)and Heffiwell (1978) havedocumented more genericevaluation techniques based onindicator species to eliminatethe need for detailed knowledgeof specific habitats.

Inventory approaches arevaluable in certaincircumstances becauseclassification can be separatedfrom assessment or evaluation.Wathern et al (1986) have notedthat classification can usuallyremain objective leaving theassessment process moreexposed to be scrutinized forsubjectivity. Through theprocess of classification, naturalversus man-made distinctionscan be made, which providevaluable insight into theinherent quality of a biotope.Natural corridors or patcheswithin a town often exist asevidence of the 'deep structure'(Spim 1988) in the landscape.Deep structure refers to theprevailing patterns inhuman-dominated landscapesthat are formed by geomorphicprocesses. Most landscapeelements of human creationtend to be ephemeral,consequently natural elementsmay have greater stability andvalue. Spirn (1988) notes thatthis structure should provide aframework for landscapeplanning for natural areas.

Spatial Structure ofNetworkA spatial structure of a networkcan be developed using aminimum of three scales;regional, local and site. Islandbiogeography theory(MacArthur and Wilson 1963)and subsequent researchfocusing on nature reservedesign (Balser et aL 1981,

Diamond 1975, Fahrig andMerriam 1985, Kuiper 1987,Gelderblom et al. 1988) provideguidelines for an ideal spatialstructure from a biologicalperspective. In every case, localresearch is needed to determinespecific criteria whichincorporate variations inregional landscapes: Achievingan ideal spatial structure isvirtually impossible, particularlyin towns. However, the goalshould serve to determine therelative importance of variouscomponents. Compromise toaccommodate competing humaninterests will be inevitable.

There is considerable debate(Margules et aL 1982, Hobbs1988) over the principles ofnature reserve design (Diamond1975, Wilson and William 1975,Diamond and May 1976) basedon the area- relations of speciesand the equilibrium theory ofisland biogeography(MacArthur and Wilson 1967).From a biological perspectivethe questions raised suggest thatcaution be used, and moresubstantive research beundertaken before applying therecommendations of theseauthors. However, it has alsobeen noted by many authorsthat have been critical of theseprinciples, that some of therecommendations are valid. Theentire discussion of naturereserve design is mostimportant, from a multiple-useperspective, because it raisescritical questions that must beaddressed to maximisebiological diversity. Biologicaldiversity is an importantmeasure of ecological healthand represents a usefulstandard. While this should be agoal in designing urbanlandscape networks, thepracticalities of implementationare always so great that it isvirtually impossible to achieveideal designs. A basic principleof traditional urban planningpractice is that ideal solutionsare rarely achieved. Theplanning process is inherentlyone of compromise. The bestthat can be expected is asolution that meets numerousobjectives and clarifies areas inwhich less than ideal conditionsexist An understanding of the

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factors considered whendeveloping ideal nature reservedesigns, however, will increasethe potential for incorporatingbiological criteria whendeveloping open space systems.

There are a number of criteriathat affect the spatial structureof the network. Connections tospecies-rich source areas,network to contextrelationships, and networkstructure and content are allareas in which specificrecommendations can be madeto enhance the functioning of anetwork. In each case, theoriesand methodologies are beingdeveloped.

At all scales, but particularly atthe largest or regional scale,species-rich source areas shouldbe connected using corridorswith stepping stones to facilitatemigration of species. Harris(1984) suggests that existingparks, wilderness areas andreserves will serve as sourcepools from which species willmigrate. In urban landscapes,source pools for native spedesmay be limited, so connectionsto other more natural sourceareas become critical. Steppingstones, islands or patchesenhance the connectivity wherecorridors are not continuous.These 'regeneration respites' areimportant even when corridorsmay exist, because somecorridors are not suitable forlonger term habitation for somespecies. Landscape elementssuch as river corridors are oftenthe most effective as naturetransmission zones (Kuiper1987). In the arid southwesternU.S., riparian zones oftenrepresent the habitat types withgreatest diversity (Gaines 1980,Johnson 1985) and may also beprimary source areas. Harris(1984 p. 148) provides a model,using the Mount Jeffersonregion of the Cascade Range inthe Northwestern U.S., forspatial and size-frequencydistribution of old growthpatches along riparian corridors.Harris's description provides auseful foundation fordeveloping other models at aregional scale in other landscapetypes. More research is needed,on an individual species basis,

to determine the requirementsfor migration through networksin various regions of the world.

Network-to-context relationshipshave been identified by Nossand Harris (1986) and Harris(1984) as an important factorwhich influences speciesdiversity. Several importantquestions must be answered tounderstand the impact of thecontext for every landscapetype. The extent of utilisation ofthe surrounding area for habitatwill influence such things aspatch and corridor size, distancebetween patches, andpercentage of total matrixdevoted to ecological patchesand corridors. Within an urbanlandscape matrix, 'edge effect'has significant impact on thequality of corridors or patches.Native plant and animal speciesoften struggle for existencebecause of competition withedge species that are usually notnatives and considered weedy(Ambuel and Temple 1983).Ranney (1981) has noted thatedges extend 10- 15m into apatch or corridor. Consequently,the surrounding matrixdramatically affects (maybeeven dominates) smaller patchesor narrow corridors. The extentof human influence will requirevariations in network structure(White and Bratton 1980) andconstant management andcooperation in land-useplanning (Harris 1984 and Nossand Harris 1986).

Network structure and contentvary but the goals of enhancingconnectivity (Merriam 1984,Macdintock et al. 1977, Fahrigand Merriam 1985 and Nossand Harris 1986) and ofproviding suitable habitats canbe promoted by appropriatestructure and content Somethings are known and othersare theorized about the spatialdistribution and size-frequencyrelationships of corridors andpatches, habitat structure andthe impacts (positive ornegative) of disturbances asthey relate to enhancingbiological integrity. These needto be tested, but may be used toguide development of thespatial structure.

Harris's (1984) spatial

distribution model of islandsand corridors uses ripariancorridors as the skeleton towhich patches of varied sizesare linked. In urban settings,riparian corridors are frequentlyundeveloped and managementpractices and extensive humanuse frequently deplete theirquality. With alternativemanagement riparian corridorsmay represent the bestpossibility for building the basicskeleton for urban landscapenetworks. The spatialarrangement of corridors andpatches can facilitate the goalsof connectivity and qualityhabitat in a number of ways. Ingeneral, patches will generallybe more important as habitatand corridors as migrationroutes. Simerloff and Cox(1987), Gaines (1980) and Buddet al. (1987), however, have 'noted that corridors have valueas habitat for many species.Recognizing that patches havethe greater potential as qualityhabitat they are located atimportant points throughout thenetwork. Harris suggests apossible distribution of patchesalong corridors at progressivelygreater distances from a speciessource pool. Nearer to the poolthe patches are smaller andcloser together. Patches are alsofrequently located at junctionsof corridors where they functionas nodes (Noss and Harris1986). They also suggest thateach landscape should have oneor more multiple-use-module(MUM). A MUM is a wellprotected habitat of sufficientsize to support interior species.A concentric buffer shouldprovide decreasing effects fromexternal influences to theinterior of the MUM. Forman(1983) also suggests providingalternative loops within thenetwork to promote migrationand decrease the potential ofbarriers which inhibitmovement Often barriers in theform of disturbances (humaninduced or otherwise), predatorsand human influence willinhibit migration.

Patch and corridor size andfrequency are interdependentvariables. Generally speaking,increased patch size may reducethe number of patches needed

Kerkstra, K. and P. Vrijland,1990. Landscape planningfor industrial agriculture: Aproposed framework forrural areas. Landscape andUrban Planning, 18:275-287.

Kuiper, J., 1987. Eenecologische infrastructuurvoor het centralerivierengebeid. Landsdiap,130-40.

Lai, R. T., 1988. Law inUrban Design and Planning:The Invisible Web. NewYork Van NorstrandRheinhold.

Lammers, W., 1989. Deecologische hoordstructuurin het natuurbeleidsplan.Landschap, 3239-247.

Levenson, J. B. 1981.Woodlots as biogeographicislands in SoutheasternWisconsin. In: R. L. Burgessand D. M. Sharpe (eds.).Forest Island Dynamics inMan Dominated Landscapes.New York: Springer-Verlag.pp. 13-39.

Little C. E. 1990. Greenwaysfor America. Baltimore: JohnsHopkins University-Press.

Lord, J. M. and D. A.Norton, 1990. Scale and thespatial concept offragmentation. ConservationBiology, 4:197-202.

Lovejoy, T. E. and D. C.Oren, 1981. The minimumcritical size of ecosystems.In: R. L. Burgess and D. M.Sharpe (eds.). Forest IslandDynamics in Man DominatedLandscapes. New York:Springer-Verlag, pp. 7-12.

MacArthur, R. H. and E. O.Wilson, 1967. The Theory ofIsland Biogeography.Princeton: PrincetonUniversity Press.

Macdintock, L., R. F.Whitcomb and B. L.Whitcomb, 1977. Evidencefor the value of corridorsand minimization ofisolation in preservation ofbiotic diversity. AmericanBirds, 31:6-12.

Margules, C, A. J. Higgsand R. W. Rafe, 1982.Modern biogeographictheory: Are there anylessons for nature reservedesign? BiologicalConservation, 24:115-128.

McHarg, I. L., 1969. Designwith Nature. New York:Doubleday & Company.

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Merriam, H. G., 1984.Connectivity: Afundamental ecologicalcharacteristic of landscapepattern. In: J. Brandt and P.A. Agger, (eds.).Methodology in LandscapeEcological Research andPlanning. Volume 1: pp.5-15.

Moss, E (ed.), 1977. LandUse Controls in the UnitedStates. New York: The DialPress/James Wade.Naveh, Z. and A. S.Iieberman, 1984. LandscapeEcology: Theory andApplication. New YorkSpringer-Verlag.Noss, R. F. and L. D.Harris, 1986. Nodes,networks and MUMs:Preserving diversity at allscales. EnvironmentalManagement. 10299-309.

Noss, R. F., 1983. Aregional landscapeapproach to maintaindiversity. BioSdence,33:700-706.O'Neill, R. V., B. T. Milne,M. G. Turner and R. H.Gardner, 1988. Resourceutilization scales andlandscape pattern. LandscapeEcology, 2:63-69.

Opdam, P., G. Rijsdijk andF. Justings, 1985. Birdcommunities in smallwoods in an agriculturallandscape: Effects of areaand isolation. BiologicalConservation, 34:333-352.

Peterken, G. F., 1974. Amethod for assessingwoodland flora forconservation using indicatorspecies. BiologicalConservation, 6239-245.

Ranney, J. W., M. C Brunerand J. B. Levenson. 1981.The importance of edge inthe structure and dynamicsof forest islands. In: R. L.Burgess and D. M. Sharpe(eds.). Forest Island Dynamicsin Man DominatedLandscapes. New York:Springer-Verlag. pp. 67-96.

Fig. 1 Existing urbanlandscape networkcomponents in theYheonix urban area •

although minimum distancesbetween patches should bemaintained. Minimum patchsizes to achieve an acceptablelevel of species diversity havebeen documented (Van Dorpand Opdam 1987, Opdam et aL1985, Helliwell 1976, Lovejoyand Oren 1981 and Levenson1981) for particular landscapes.Many researchers have shownthat larger patches will supportmore species and that thelargest possible sizes arepreferable. However, therealities of urban land-useplanning are such that extensiveareas cannot be devoted entirelyto nature conservation. Idealshapes or configurations havebeen proposed by Diamond(1975) and evaluated myMargules et al. (1982). Minimumcorridor widths have also beenidentified by Budd et al. (1987),MacOintock et al. (1977),Merriam (1984) and Fahrig andMerriam (1985) for variousconditions. At a local scale,preferred sizes and shapes ofpatches and corridors becomemore important because themagnitude of the resource ismuch smaller.

Habitat structure (the internalcomposition of patches andcorridors) also directlyinfluences the spatialdistribution and size-frequencyrelationships. Clearly, if a patchor corridor comprises materialunsuitable as habitat, it makeslittle difference how large it is.MacClintock et al. (1977) andVan Dorp and Opdam (1987)have noted that high densitycorridors result in greaterconnectivity. O'Neill et al. (1988)have addressed issues ofconnectivity in relation tocontent within the framework ofpercolation theory (Stauffer1985). They have determinedthat organisms limited tomovement within a land type(e.g. forest) require certaindensities to be able to utilise theavailable resources. This densityfor corridors and patches maynot be the same and differentorganisms have differentrequirements so it is difficult toestablish simplerecommendations to achievemaximum utilisation by thegreatest number of organisms.

Existing natural landscapes canbe studied to provide modelsfor designing the landscapestructure of corridors andpatches.

Disturbance is an importantprocess that replenishes anecosystem in many cases, suchas frequently flooded areas.Interruption of this type canhave serious negative impactsand cause significant change instructure and function. On theother hand, certain humandisturbances can deplete habitatquality. Hobbs (1988) notes thatmaintenance activity, such asmowing, significantly reducesspecies richness with unmownstands having significantlylarger numbers of nativespedes. He also notes thatnative species seem moresusceptible to other humandisturbances such as trampling.Urban natural areas have faced,and will continue to face,pressure from human use.Recognising disturbanceregimes that influence habitatquality and managing theirnegative impacts will help toensure that the resource is notseverely depleted.

Network ComponentsNumerous authors havedescribed the many types oflandscape elements that can beclassified as patches orcorridors. Each has inherentcharacteristics that make it moreor less valuable to performimportant functions in an urbanlandscape network. Thecomponents will vary withcontext but may have some:

common characteristics. Forexample, a hedgerow in an

agricultural setting has some ofthe same properties as alandscaped boulevard in anurban context Both are narrowstrips of fairly dense vegetationwithin an intensively used,cultivated landscape. Both mayfunction as windbreaks,providing shelter and shade forvarious organisms andproviding separation of differentland areas. There are numerousother parallels that could bequoted, as well as somesignificant differences. The valueof comparing these elements isto link research conducted inother contexts on landscapeelements to an urban setting.Each locality may have aslightly different set oflandscape elements, but asimilar process can be followedto classify them and assess theirpotential value. In this case,elements from the Phoenix,Arizona urban area will be usedto demonstrate a simplifiedassessment process.

In the Phoenix area, existingnetwork elements fall into fivegeneral categories: 1) transportand utility corridors, 2) drainagecorridors and canals, 3) linearparks and trails, 4) cultivatedparks and 5) open spacereserves. The present quality ofmany of these elements is lessthan desirable thoughimprovement and reclamationmay result in significant change.Phoenix is probably bettersuited than most cities to aprocess of fitting an ecologicalnetwork in to an existing urbanfabric It is a very low densitycity and has several large openspaces within the urban area.There are also numerous linearelements that are being studied

regional analysisphoenix metropolitan area

legend

•*—_

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Fig. 2 Transport corridor (top) with native vegetation and utility corridor(bottom)

for their multiple-use potential.Figure 1 illustrates variousexisting components in thePhoenix area that may be usedin establishing an urbanlandscape network.

Transport and Utility Corridors:streets (including Rights ofWay), railways, power lines andother utility corridors are allman-made elements that couldbe adapted to provide somelimited value as networkcomponents (Figure 2). Thegreatest potential lies in the useof native vegetation tolandscape these corridors.Although it is unlikely thatthese corridors will be utilizedby many of the most valuedspecies, even limited use willcontribute to the overallefficiency of the network from abiological perspective. Perhapsthe greatest value for thesecorridors is forhuman-dominated activity.Functions such as aesthetics,climatic amelioration andrecreation may beaccommodated along withlimited biological functions.

Drainage Corridors and Canals:perennial and ephemeralstreams and rivers are amongthe most important networkcomponents (Figure 3). Theseelements often span scales andprovide links as transmissioncorridors from species sourcepools. Normal fluvial regimesshould be maintained orreinstated to ensure continuednutrient and gene flow.Biological functions shouldreceive highest priority in thesecorridors though. Canals maybe less critical from a biologicalperspective than streams orrivers because they areman-made and usually subjectto regular maintenance. Theycan, however, make importantlinks within a network ifdeveloped in a way thatsimulates the landscapestructure of a natural watercourse. In areas wherealternative loops may belacking, 'synthetic' drainagecorridors may suffice usingcanal rights of way.

linear parks and trails arebecoming increasingly popular

Rogers, G. and R. A.Rowntree, 1988. Intensivesurveys of structure andchange in urban naturalareas. Landscape and UrbanPlanning, 1559-78.

Rose, F. R , 1974. Theepiphytes of oak. In: M. G.Morris and F. H. Perrin(eds.). The British Oak ItsHistory and Natural History.Favingdon: E. W. Classey.pp. 250-273.Sargent, F. O. and J. H.Brande, 1976. Classifyingand evaluating uniquenatural areas for planningpurposes. }ournal of SOU andWater Conservation,31:113-116.

Simerloff, D. and J. Cox,1987. Consequences andcosts of conservationcorridors. ConservationBiology, 1:63-71.

Spim, A. W., 1984. TheGranite Garden. New York:Basic Books.

Spim, A. W., 1986.Landscape planning andthe city. Landscape andUrban Planning, 13:433-441.

Spirn, A. W., 1988. Thepoetics of city and nature:Towards a new aesthetic forurban design. LandscapeJournal, 7:108-126.

Steiner, F. R., R. W.Dunford and N. DosdalL1987. The use of the landevaluation and siteassessment system in theUnited States. Landscape andUrban Planning, 14:183-199.

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Tubbs, C R. and J. W.Blackwood, 1971. Ecologicalevaluation of land forplanning purposes.Biological Conservation,3:169-172.

Fig. 3 Drainage corridor (top)and native vegetation, and canal(bottom) within residentialdevelopment areas.

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Urban, D. L., R. V. O'Neilland H. H. Shugart Jr., 1987.Landscape ecology, ahierarchical perspective canhelp scientists understandspatial patterns. BioScience,37:119-127.

Van Dorp, D. and P. F. M.Opdam, 1987. Effects ofpatch size, isolation andregional abundance onforest bird communities.Landscape Ecology, 159-73.

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Ward, S. D. and D. F.Evans, 1976. Conservationassessment of Britishlimestone pavement basedon floristic criteria. BiologicalConservation, 9217-233.Wathem, P., S. N. Young, I.W. Brown and D. A.Roberts, 1986. Ecologicalevaluation techniques.Landscape Planning,12:403-420.

White, P. and S. P. Bratton,1980. After preservation:Philosophical and practicalproblems of change.Biological Conservation!18241-255.

Whittaker, R. H., 1972.Evolution and measurementof species diversity.Taxonomy, 21213-251.

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Wilson, E. O. and E. O.Willis, 1975. Appliedbiogeography. In: M. L.Cody and J. M. Diamond.Ecology and Evolution ofCommunities. Cambridge:Belknap Press, pp. 522-534.

Wittig, R. and K. F.Schreiber, 1983. A quickmethod for assessing theimportance of open spacesin towns for urban natureconservation. BiologicalConservation, 2656-64.

Woodbury, S. R., 1975.Transfer of developmentrights: A new tool forplanners. AIP Journal,123-14.

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as recreational elements inmany cities (Figure 4). Mosthave been developed in ahighly cultivated manner, butsome have retained the originalvegetation and character.Although they may be modifiedin varying degrees, linear parksand trails can function in alimited way as biologicalcomponents. Perhaps more thanany other component, theseelements may be well suited tobalanced biological and humanfunctions. Reclamation effortsmay be necessary in someseverely altered areas or alaissez-faire maintenancephilosophy may be suitable ifan area is capable of selfregeneration. The value ofhighly cultivated or extensivelyused areas should be examinedto determine if managementmay help to return some of theoriginal biological functions.

Cultivated parks generally havelarge numbers of speciesutilising the existing habitat(Figure 5). The majority,however, are usually lessvalued species associated withthe urban matrix. Varioustechniques are being researchedto make cultivated parks moresuitable for other species (Hobbs1988 and Brux 1989).Management, maintenance andrevegetation programmes holdthe greatest promise fortransforming existing cultivatedparks while normal recreationalactivities can still beaccommodated. The biologicalvalue of these componentswould be somewhat limited inmost areas though some parksmay be well suited to significantimprovement as a biologicalnetwork component. Alternativemaintenance programmes mayalso result in reduced operatingcosts.

Open Spaces: the Phoenix areahas at least six open spacesranging in size fromapproximately 150 - 2,000hectares (figure 6). They consistof largely undisturbed nativevegetation and terrain. Smallerand more centrally locatedreserves are experiencing somepressure from continued humanuse. These are important

Fig. 4 Linear park (Indian Bend Wash, Scottsdale, Arizona) withcultivated landscape treatment.

Fig, 5 City park (Scottsdale, Arizona) with cultivated and laissex-fairemaintenance zones.

Fig. 6 Urban open space (Phoenix South Mountain Preserve) with nativevegetation.

patches that will function asnodes or MUMs. Primaryobjectives for these areas shouldbe to continue preservation andlimited passive recreation.Buffers do not currently existbut could be very useful for

protecting core areas of thereserves. Active conservationmanagement is critical if thesereserves are to retain theirbiological integrity.

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ConclusionThere are several strategies thatcan be employed to implementthe concept of an urbanlandscape network. One of thefirst and most important is tocreate an appropriate politicalclimate for receiving the ideas.This must be done with fullrecognition of thesocio-economic context in whichit is being proposed, andcreation of urban landscapenetworks should proceed withfull participation from thepeople. Popular support willensure that appropriateconsideration is given in theland-use planning process.Clearly, the validity of thisconcept must be justified withthe realities of economics andsocial responsibility as well asecology. Longer term measuresmay be the most appropriate tomake this case, but short-termbenefits must also bedemonstrated to ensure that itsurvives political expediencies.

Preservation is currently beingemployed for several verysignificant patches of naturallandscape in the Phoenix area.Continued developmentpressures are testing the resolveof local government and grassroots preservation groups. Insome cases, reserves are wellprotected legally as dedicatedopen space, though its qualityhas very limited protection.Specific plans, ordinances andother mechanisms can help toensure that quality ismaintained. Drainage corridorsare generally more difficult topreserve in a similar fashion.They are often subject toaggressive management becauseof the potential flood hazardthey pose. Specific mechanismsneed to be developed at thelocal level to ensure that thesecritical elements are notmanaged into extinction.

Reclamation is rapidlybecoming a common methodfor establishing landscapes withecological value. It is also

expensive. Specific methods forre-creating nature through aprocess of ecological landscapedesign have been discussed byBradshaw (1983). Ecosystemmodelling, a process in whichreclamation plans are designedsimilarly to functionalecosystems with similar climatic,geomorphological anddisturbance regimes, is useful toensure that re-establishednetwork components arebiologically appropriate (Cook1991). In urban landscapes itwill frequently be necessary tocreate synthetic networkcomponents, either to reclaimthose previously destroyed or toprovide substitute or extraconnections within the network.In any case it is important touse natural ecologicalprecedents as models for design.

One of the most direct ways tobegin an urban landscapenetwork is through landacquisition. Land acquisition isalso very costly, particularly inurban areas. Consequently,acquisition should be reservedfor the most important cases inwhich critical corridors orpatches are required tocomplete the network. Wherenecessary, creative approachesother than outright purchasemay be employed. Varioustechniques including landtrades, the transfer ofdevelopment rights (Woodbury1975) and public/privatedevelopment efforts can lead topublic ownership or control thatwill facilitate appropriate longterm management activitywithout the excessive cost ofpurchase. Various privatenon-profit organizations, suchas the Nature Conservancy inthe U.S., are actively engaged incoordinating activities that leadto ownership or control ofcritical natural areas for thesingle purpose of preservation.Land-use controls (Lai 1988,Moss 1977) are becoming morecommon as a planning tool.Only recently, however, havethese techniques been usedextensively for the purposes of

nature conservation.Conservation easements,overlay zoning, andenvironmentally sensitive landordinances (Eagles 1984) arenow frequently used in localplanning activity to protectsensitive areas.

This paper has only begun toidentify issues central tocreating an urban landscapenetwork. Much of this has builtupon unproven theories andresearch related to specificgeographic regions. Theprimary intent has been toidentify topics for which moreresearch is needed, but also tobuild a conceptual planningframework that can begin tointegrate important ecologicalissues into a traditional urbanland-use planning process. Ifimplementation is dependent onvarious research questions beinganswered, the opportunity toimplement any semblance of anecological network may be lost.It may be appropriate to use theplanning process as amechanism to test the validityof the concept of an 'ecological'network. Various schemes orcomponents could be put intouse and evaluated andimproved over time. Uponexamination of many planningconcepts that have been testedthrough implementation in thepast, it appears thatimplementation of this conceptis less of an 'experiment' andmore of a safe hedge forsustainable developmentpractices.

AcknowledgementsI would like to thank ProfessorHubert van lier of WageningenAgricultural University and tworeviewers for their helpfulcomments. I would also like toacknowledge the input of JohnPennel Philips who preparedthe drawing of the urbanlandscape network componentsin the Phoenix area.

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