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Society for Conservation Biology Biological Consequences of Ecosystem Fragmentation: A Review Author(s): Denis A. Saunders, Richard J. Hobbs and Chris R. Margules Reviewed work(s): Source: Conservation Biology, Vol. 5, No. 1 (Mar., 1991), pp. 18-32 Published by: Wiley-Blackwell for Society for Conservation Biology Stable URL: http://www.jstor.org/stable/2386335 . Accessed: 11/10/2012 13:03 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. . Wiley-Blackwell and Society for Conservation Biology are collaborating with JSTOR to digitize, preserve and extend access to Conservation Biology. http://www.jstor.org Review BiologicalConsequences ofEcosystem A Review Fragmentation: DENIS A. SAUNDERS CSIRO Divisionof Wildlifeand Ecology LMB 4 P.O. Midland WesternAustralia6056, Australia RICHARDJ.HOBBS CSIRO Divisionof Wildlifeand Ecology LMB 4 P.O. Midland WesternAustralia6056, Australia CHRIS R. MARGULES CSIRO Divisionof Wildlifeand Ecology P.O. Box 84 Lyneham AustralianCapitalTerritory2602, Australia Abstract:Research on fragmentedecosystemshas focused mostlyon thebiogeographicconsequencesof thecreationof habitat "islands" of different sizes, and has provided littleof practical value to managers.However,ecosystemfragmentation causes large changes in thephysical environmentas well as biogeographicchanges.Fragmentationgenerallyresults in a landscape thatconsistsof remnantareas of native vegetationsurroundedby a matrixof agriculturalor other developed land. As a result,fluxes of radiation; momentum (i.e., wind), water,and nutrientsacross the landscape are altered significantly.These in turncan have importantinfluences on thebiota withinremnantareas, especiallyat or near theedge betweentheremnantand thesurroundingmatrix. The isolation of remnant areas by clearing also has importantconsequencesfor the biota These consequences varywiththetimesince isolation; distancefrom otherremnants,and degreeof connectivitywithotherremnants.The influencesofphysical and biogeographicchanges are modified by the size, shape, and position in the landscape of individual remnants,with larger remnantsbeing less adverselyaffectedby thefragmentationprocess. The dynamics Paper submittedMay 2, 1990; revisedmanuscriptacceptedSeptember 20, 1990. 18 ConservationBiology Volume 5, No. 1, March 1991 Resumen: La investigacionsobre los ecosistemasfragmentados se ha enfocadoprincipalmenteen las consecuencias biogeograficasde la creacion de "islas" de habitat de diferentestamaniosyha sido de muypoco valorpracticopara los manejadores del recurso.Como quiera que sea la fragmentacion de los ecosistemascausa grandescambios en el medio ambiente fisico asi como en el ambito biogeografico.La fragmentacionresultageneralmenteen terrenosque consisten de areas remanentesde vegetacion nativa rodeada de una matriz de tierrasagricolas u otrasformas de uso de la tierra Como un resultado de esto,el flujo de la radiacion; del momentum(ej. el viento),del agua y de los nutrientesa travesde la tierrason alterados significativamente. Esto en su turno,puede influenciare la biota dentro de las areas remanentes,especialmenteen o cerca de los limitesentrelos remanentesy la matriz que los rodea El aislamiento de las areas remanentesporla tala tambientieneimportantesconsecuenciaspara la biotay estas consecuencias varian con, el tiempodesde el momentodel aislamiento, la distancia hasta los otrosremanentesy el grado de coneccion entreellos. La influenciade los cambios fisicos y biogeograficoses modificada por el tamanio,la formay la posicion en el terrenode remanentesindividuales siendo los remanentesgrandes los menos afectados adversamantepor el proceso de fragmentacion. La dinamica de las areas remanentesson dirigidas Saunders etal. Consequences ofFragnentation 19 of remnantareas are predominantlydrivenbyfactorsarising in thesurroundinglandscape. Management of,and research on, fragmented ecosystemsshould be directed at understandingand controllingtheseexternalinfluencesas much as at thebiota of theremnantsthemselves.Thereis a strongneed to develop an integratedapproach to landscape managementthatplaces conservationreservesin thecontext of theoverall landscape. predominantementepor factores que surgen en el terreno circundante.El manejoy la investigacionde los ecosistemas fragmentadosdeberi'ade dirigirsetanto al entendimientoy controlde estas influenciasexternascomo a las biotas remanentesen si. Hay unafuerte necesidadpara el desarrollo de un enfoque integradoen el manejo de tierrasque coloca a las reservasparala conservacionen el contextodel terreno en general. Introduction the originalvegetationhas been removed(Saunders et al. 1987a). Remnantsoftenhave been called habitatislands and the changes thatresultfromthe isolationof these islandshave been the subject of considerabledebate in the literature.This debate has centeredmostly on the equilibriumtheoryofislandbiogeography(MacArthur& Wilson 1963, 1967) and its applicabilityto conservation. In particular,the importance of size, shape,and designofsinglereservesand reservesystems, extinctionand colonizationrates,and species-arearelationshipshave been much discussed (Wilson & Willis 1975; Diamond 1975, 1976; May 1975; Terborgh1976; & Abele 1976a4 1982; Whitcombet al. 1976; Simberloff Pickett& Thompson 1978; Game 1980; Marguleset al. 1982; Boecklen & Gotelli 1984). Particularattention has been paid to the question ofwhetherone largereserve could preserve more species than several small reservesaddingup to the equivalentarea of the larger reserve(the so-calledSLOSS,or "singlelarge or several small"debate; Simberloff & Abele 1976b, 1984; Gilpin & Diamond 1980; Higgs& Usher 1980; Jairvinen 1982; Willis 1984). These questionshave been reviewedelsewhere(Simberloff1988) and are notexaminedin detailhere.While of theoreticalinterest,most of these issues are of little practicalvalue in managingfragmentedsystems(Zimmerman& Bierragaard1986; Margules 1987; Hobbs 1988; Margules& Stein 1989). The species-areaequation,forexample,may give a managera roughidea of how manyspecies will be maintainedon a remnantofa on givenarea, but will yield absolutelyno information thepracticalissue ofwhich habitatscontributemostto species richnessor on which species are mostlikelyto be lostfromtheremnant.Simberloff ( 1986) stated,"It is also sad thatunwarranted focuson thesupposedlessons of island biogeographytheoryhas detractedfromthe maintaskofrefugeplanners,determining what habitats are importantand how to maintainthem." Margules (1986), in a discussionabout two conservationevaluationexercises,notes that,"no panel membersin either exerciseconsideredthe species-arearelationshipor the equilibriumtheoryof islandbiogeographyin theirevaluations;at least not explicitly."In addition,the debates about reservedesignand SLOSS are oflimitedrelevance because,withveryfewexceptions,managersof conser- the naturalvegeSince the developmentof agriculture, tationcover of everycontinentexcept Antarcticahas been extensivelymodified.A cycle of agriculturaldeoftheland has velopmentfollowedby overexploitation been repeated throughoutrecorded history.Forman (1987) quotes Plato's(ca. 2350 BP) descriptionofhow an area ofancientGreece was strippedofitssoil following clearingand grazing,leaving"the mere skeletonof and theuse ofinappropriate theland."Overexploitation practiceshave led to desertencroachment, agricultural as in the Sahel regionin NorthAfrica(Le Houerou & Gillet 1986; Ehrlich& Ehrlich1987), and to extensive loss of soil, oftenwith disastrousand dramaticconsequences, as in the Dust Bowl ofthe UnitedStates(Hudson 1981). The process of land clearingand the consequentenvironmentaldegradationis continuingrapidlyin many regionssuch as SoutheastAsia and SouthAmerica,particularlyin areas of tropicalrain forest(Myers 1988). deAustraliaprovidesan exampleofrecentagricultural velopment,with vast areas being cleared over the last 100 yearsforcereal croppingand stockgrazing(Saunders & Hobbs 1989; Hobbs & Hopkins 1990; Saunders et al. 1990). In some regionsover 93% of the native land vegetationhas been removedand the agricultural thathas replaced it is subject to extensivewind and withconsequentpolwatererosionor soil salinization, lutionofwater suppliesfordrinkingand irrigation. One legacy of the extensiveremovalof nativevegetationis thatthe remainingvegetationis usuallyin fragmentedpatches across the landscape.These patchesor positionsin the landremnantsare situatedin different soil types,possess different vegscape and on different etationtypes,and varyin theirsize,shape,isolation,and typeof ownership.Over much of the world,conservationofregionalbiotasdependsentirelyon theretention and managementofthese remnants.Conservationmanfacedwiththedual issuesofwhether agersare therefore the remnantshave any practical conservationvalues, and iftheydo, of how theymustbe managedto retain these values (Saunderset al. 1987a). In thispaper we use theword remnantto defineany patch of nativevegetationaroundwhich most or all of ConservationBiology Volume 5, No. 1, March 1991 20 Consequences ofFragmentaton vationreservesare facedwith a faitaccompli. Conservation considerationshave rarelybeen taken into account duringthe developmentof areas foragriculture, or othersuch uses. Conservationmanmining,forestry, agersmustworkwiththe remnantsleftfollowingthese neverhave the opportunity developmentsand virtually to design a reserve networkbefore an area is fragmented.There is an increasingneed to utilize design .criteriato improveconservationnetworksin already fragmented areas,but thisrequiresa clear understandin thefirst ingoftheproblemscreatedby fragmentation place. Hence we believe thatresearchand discussionshould focus on practicalissues relatingto the impactof fragmentationon naturalecosystemsand managingremnants for conservation(Saunders et al. 1987b). We sharethe fearofNoss and Harris(1986) thatconservabiological tionagencieshave not realizedtheimportant and have consequences of ecosystem fragmentation thereforenot developed policies to managetheirremnantsto maintainconservationvalues. The aim of this paper is to point out the physicaleffectsof fragmentation, the biological consequences for naturalecosysand the optionsavailableforcontemsof these effects, servationresearchand management. Characteristics ofFragmented Ecosystems Fragmentationof the landscape produces a series of remnantvegetationpatches surroundedby a matrixof different vegetationand/orland use. Two primaryeffectsofthisare an alterationofthemicroclimatewithin and surrounding the remnantand the isolation ofeach area fromother remnantpatches in the surrounding landscape. Thus, in a fragmentedlandscape there are as well as biogeochangesin the physicalenvironment graphicchanges.Most discussionsofhabitatfragmentation have concentratedon the biogeographicaspects, and the physicalchangeshave receivedlittleattention. All remnantsare exposed to thesephysicaland biogeographicchangesto a greateror lesserdegree,but their effectsare modifiedby the size, shape, andposition in thelandscape of individualremnants.We examinefirst and then discuss the physicaleffectsof fragmentation factors. the operationof the modifying Changesin Microclimate ofthelandscaperesultsin changesin the Fragmentation physical fluxes across the landscape. Alterationsin fluxesof radiation,wind,and watercan all have importanteffectson remnantsof nativevegetation. 1. RADIATIONFLUXES The energybalance ofa fragmented landscapewill differ fromone witha completecoverofnativevegmarkedly Conservation Biology Volume 5, No. 1, March 1991 eta). Saunders etation, especially where the native vegetation was dense beforeclearing.Removingnativevegetationand architecreplacingit with crop species with differing ture and phenologyaltersthe radiationbalance by increasingthesolarradiationreachingthe groundsurface duringthe day,changingthe albedo, and increasingreradiationat night.These featuresvary depending on time of year; ploughing,crop growth,and harvesting produce an alternationof bare groundand varyingdegrees of vegetationcover (Geiger 1965; Milthorpe& Moorby 1974). In cleared areas, in general,daytime temperaturesare higherand nighttemperatureslower thanin naturallyvegetatedareas. This leads to greater temperature rangesbothat thesurfaceand in theupper layersof the soil, and an increased incidence of frost (Geiger 1965). These changes in the cleared partsof the landscape impingeon the remnantnativevegetation,especiallyat theedge betweenthetwo. Except near the equator,the orientationoftheedge affectsthedegree to whichsolar timesof radiationincreaseswithina remnantat different year (Geiger 1965; Wales 1972; Ranneyet al. 1981). Latitudealso affectsradiationinput,and at highlatitudes especially,where solar angles are low, a remnantedge can receive significantly more solar radiationthanunfragmented areas receive (De Walle & McGuire 1973; at the Hutchinson& Matt1976,1977). Airtemperatures higherthan edge ofa forestremnantcan be significantly thosefoundin eitherthe interiorof the remnantor the surroundingagriculturalland (Geiger 1965; Kapos 1989). The consequences ofincreasingsolar radiationat the edges of remnantsare not clear. The indicationsare, however,thata different suite of species may come to occupy thisalteredhabitat.Lovejoyet al. (1986) report in tropicalforestsresultsin therapid thatfragmentation growthof vines and other secondaryvegetationin a 10-25 m striparound the remnantedge. This may effectivelyseal offthe remnantand maintainan environment withinthe remnantsimilarto that that existed priorto fragmentation (F. Crome,personalcommunication). This also occurs to some extentin temperateregions(Gysel 1951; Trimble& Tryon1966; Ranneyet al. 1981). Shade-tolerant species maybecome restrictedto the interiorpartsof the remnant,withdifferent species redistancesfromthe edge. The compoquiringdifferent sitionof remnantedges maybe affectedby edge aspect (Wales 1972; Palik & Murphy1990). Distinctsets of "interior"and "edge" species have been recognizedin fora long time, landscapes thathave been fragmented forinstancein the easternUnitedStates(Ranney et al. 1981). Nutrientcyclingprocesses may be affectedby increasedsoil heatingand itseffecton soil microorganism Saunders etaW. and invertebratenumbers and activity(Klein 1989; Parker1989), on litterdecomposition,and on soil moisture retention.Changes in the radiationbalance also affectlargerfaunaboth directlyand indirectlythrough altering resource availability(via changes in plant growthand phenology).Lovejoyet al. (1986) attribute changesin butterfly community compositionin tropical forestfragments partlyto increased insolationwithin smallremnants. Increasedradiationload and dessication ratesmaylead to reduced foragingopportunities. Saunders (1982) consideredthatelevated temperaturesin fragmented landscapesreduced the foragingtimeavailable to adultCarnaby'scockatoos(Calyptorbynchusfunereus latirostrus)and contributedto theirlocal extinction.Alternatively,some species dependent on temperature thresholdsmaybe able to forageforlonger periods.For instance,the stronglydominantantsof the Iridomyrmexgenus are knownto forageonlywhen insolationand temperaturesare high,and otherant species forageonlywhen Iridomyrmexis absent (Greenslade & Halliday 1983; Andersen 1987). Increased insolation and ambient temperaturescould then increase the foragingtimeavailableto Indomyrmexbut reduce it forsubordinatespecies. Alteredtemperature regimescan also have the effect of destabilizingcompetitive,predator-prey, and parasiticinteractions. Geiger(1965) quotes the exampleof the timberpest Ocneria monacha, which lays eggs on treetrunks.Elevatedspringtime temperatures on trunks at the forestedge allow larvae to emergebeforetheir parasites,which emerge fromthe cooler forestfloor. This givesthepest a head startand resultsin population buildups. 2. WIND Withthe removalof surrounding vegetation,the entire patternof momentumtransfer over the landscapeis altered.As airflowsfromone vegetationtypeto another, the wind profileadjuststo the new roughnesscharacteristics.When air flowingover one vegetationtype comes to a boundarywith a new vegetationtype,the upper partof the wind profileinitiallyretainsthe characteristicsofthepreviousvegetationtype,butthelower part takeson new characteristics reflectingthe roughness of the new vegetationtype.The wind profiledoes not fullyequilibratewith the new vegetationforsome distance.Rulesofthumbforwindprofilemeasurements give the minimum"fetch"(i.e., the minimumdistance fromthe vegetationboundarythatwill ensurethatthe ofthevegetation profilehas takenon thecharacteristics understudy)as 100-200 timesthe heightof the vegetationunderstudy(Monteith1975; Grace 1983; Jones 1983). Turbulenttransfer forthe transport is important of atmosphericgases, and gas fluxesabove vegetation are controlledby theseprocesses.It follows,then,that Consequences ofFragmentation21 these processes mustbe significantly modifiedin remnantvegetationareas,where thewindprofilewill notbe in equilibriumwith the remnantvegetation(Jarvis& McNaughton1986). Given the fetch requirements,a woodlandwith20-m-talltreeswould need to be at least 2-4 km wide before wind profileswould resemble thosein an unfragmented situation.The implicationsof thisforplant gas exchange and growthhave not been examined,but could be significant. A more obvious effectof landscape fragmentation is thatremnantsare subjected to increased exposure to wind. This mayresultin damage to the vegetation,eitherthroughdirectphysicaldamage(Moen 1974; Grace 1977), or by increasingevapotranspirationwith reduced humidityand increaseddessication(Tranquillini 1979; Lovejoyet al. 1986). Direct physicaldamage can take the form of wind pruning (Caborn 1957) or windthrowof trees.Distinctedge structureshave been foundto develop at theedges oftreeplantations(Fraser 1972), and thisis likelyto be the case forremnantareas also (see Ranneyet al. 1981 for a discussion of edge structures).Trees near the edge of recentlyisolated remnantsare particularlyat risk of windthrowsince they have maturedwithin a closed canopy and have therefore developed in the absence ofstrongwinds and lack the necessarysupport mechanismsto deal with such winds. Windthrowof dominanttrees results in changesin thevegetationstructureand increasedavailabilityof regenerationgaps, allowingrecruitment, particularlyof pioneer or light-demandingspecies. Increased litterfallthroughtree damage is likelyto alter soil surface characteristicsand hence the habitat of ground-dwelling fauna.Similarly, increasedexposureto wind mayremove loose barkand reduce the substrate availableforbark-dwelling and hence also invertebrates, reduce theiravailabilityas a food resource. Increased wind turbulencedue to clearing has been shown to affectthe breedingsuccess of birdsby creatingdifficulties in landingdue to wind shear and vigorouscanopy movement(Brett 1989; Revilleet al. 1990). In the case of tropicalforests,fragmentation can resultin hot,dry windsblowingintoremnantareasfromthe surrounding cleared areas,withtheprobableresultof increasedtree mortality (Lovejoy et al. 1986) and preventionofregenerationof species whose successfulestablishmentrequires humid conditions or persistentsoil moisture (anzen 1986). This mayalso be importantin the regeneration of species in other areas, such as those with mediterranean climates,where successfulestablishment requires adequate soil moisture (e.g., Gordon et al. 1989; Williams& Hobbs 1989). This effectwill be lessened in cases where theedges ofremnantsare sealed off by rapidsecondarygrowth. Increasedwind speeds at remnantedges have thesecof materialsuch ondaryeffectof increasingthe transfer as dust and seeds in fromthe surroundingmatrix.Gei- ConservationBiology Volume 5, No. 1, March 1991 22 etal. Saunders ofFragnentation Consequences ger (1965) gives an example where particulatematter depositionat the edge of a forestremnantincreasedby 40% over that in the open. Transferof nutrientsby saltationof surfacesoil particlesis also possible, and stronggradientsin soil nutrientlevels have been found at theedges ofremnantareas(Muir 1979; Cale & Hobbs 1991). Wind can deposit seeds of nonnativespecies over considerabledistancesintoremnantareas (Hobbs & Atkins1988). Transferof insectsand disease organismsinto remnantareas mayalso be increased. 3. WATERFLUX of the landscape resultsin modification Fragmentation of the local water regimeby alteringthe variouscomponents of the hydrologicalcycle. Removal of native vegetationchangestheratesofrainfallinterceptionand and hence changes soil moisture evapotranspiration, levels(Kapos 1989). The pathwaysbywhichwaterpenetratesthe soil mayalso be markedlyaltered(Bormann & Likens1979; Nulsen et al. 1986; Peck & Williamson 1987; Sharmaet al. 1987; Bell 1988). Replacementof deep-rootedperennialspecies withherbaceouscrop or pasturespecies leads to greatlyreduced evapotranspiration and increased surface-and groundwaterflows. The hydrologicalsystemin generalbecomes muchless events(Hornbeck bufferedwith more extremerun-off 1973; Simons1989). Increasedsurfacewaterflowslead ofparticulatematter to increasederosionand transport (e.g., Bormannet al. 1974). Topsoil removedfromhigh in the catchmentends up as sedimentin the riversystem.Transportof nutrientsinto streamsalso increases (Likens et al. 1970). Rises in water tables can bring storedsaltsto the surfaceand cause secondarysalinity, with considerableimpactson both remnantvegetation and the surroundingagriculturalmatrix(Peck 1978; Williamsonet al. 1987). Movementof storedsalts,nutrients,and pesticides washed fromcleared land can have significantimpacts on river systems(Kendrick 1976; Karr& Schlosser1978). The impactofthesechangeson an individualremnant depends greatlyon itspositionin the landscape(Swanson et al. 1988). Remnantsat thetop ofa catchmentcan be expected to be relativelylittleaffectedby changesin water flux,whereas remnantson midslopesand valley areas Remnantsin run-off bottomswillbe moreaffected. can be expected to experience more erosion,while those in run-onareas will experiencemore soil deposition,especiallyon the up-slopeedge. Furtherimpactson remnantareas can be expected followingmanagementoperationsin the surrounding matrixthatalterhydrologicalprocesses.Thusirrigation, waterstorage,or drainagemayaffectremnantareas.An extremeexample of thisis foundin thefensof eastern England,wheredrainagehas led to peat shrinkageand a drop of4 m in land level in 130 years.Remnantareas of ConservationBiology Volume 5, No. 1, March 1991 naturalwetlandnow requirepumpingsystemsto retain adequatewaterlevels(Hutchinson1980; Rowell 1986). Changes in water fluxes and associated particulate and nutrientfluxescan have importantinfluenceson thebiotaofremnants. Alteredpatternsoferosionlead to changesin drainagepatternsand theproductionofnew substratesforplant colonization.Of particularimpormaterialin runtance is the depositionof nutrient-rich on areas,whichcan act as a focusforinvasionby species requiringdisturbanceand/ornutrientenrichmentfor successful establishment (Hobbs & Atkins 1988). Changesin surfaceand soil moisturelevels could also lead to changes in decompositionrates,altered seedand changes in habitatforgroundbed characteristics, dwellingfauna. Isolation Landscape fragmentationhas two importantconsequences forthe biota. First,thereis a reductionin the total area of habitatavailable,with possible increased and second, densitiesofsurviving faunain theremnants, the habitatthatis leftis brokenup into remnantsthat are isolated to varyingdegrees (Lovejoy et al. 1984, 1986; Haila & Hanski 1984; Wilcove et al. 1986). The timesince isolation,thedistancebetweenadjacentrembetweenthemare nants,and the degreeofconnectivity all importantdeterminantsof the biotic response to fragmentation. 1. TIME SINCE ISOLlATON Upon isolation,a remnantis likelyto have morespecies and species will thanit will be capable of maintaining, be lost as the changesbroughtabout by fragmentation take effect(Miller & Harris1977; Miller 1978; Wilcox 1980; Harris1984). Thisprocess of "species relaxation" is consideredan inevitableconsequence of area reductionand isolation,on thebasis ofislandbiogeographical predictions. However, the various mechanisms by whichlocal extinctionsoccur will resultfromthephysical changes discussed above and resultingchanges in biotic interactions.The rate of species relaxationwill differamongdifferent taxa. The mostrapid extinctions are likelyin species thatdepend entirelyon nativevegand those etation,those that require large territories, thatexist at low densities.Dispersal behavior and dewill determinetheresponseofindividualspemography cies to fragmentation (Karieva 1987). Populationsthatare too smallto be viablemaypersist forlong periods simplybecause of the longevityof individuals.For example,in remnantsin theWesternAustralianwheatbelt,femaletrapdoorspidersAnidiops villosus can live for at least 23 years (Main 1987), and smallAustralianpasserinesof about 25 g maylive over 20 years (AustralianBird BandingScheme records). It may take several hundredyears to lose some species Saunders etal. such as long-livedtrees,especiallysince adultplantsare oftenless sensitiveto changed environmentalconditions than seedling and juvenile stages.Alterationsin disturbanceregimein remnantareas maypreventsuccessfulregeneration(Hobbs 1987; Bond et al. 1988). Presenceof a species in a remnantis thusno guarantee of its continuedexistence there;successfulreproduction and recruitment are required.Managerstherefore may need to examine the age structureof species on remnantsto identifyvulnerablespecies to be targeted forspecial management. Time since isolationwill therefore determinehow far down the"relaxationtrack"anygivenremnanthas traveled. Recentlyisolated remnantscan be expected to continuelosingspecies; thisprocess may continuefor relativelylong periods in the absence of interventive management(Soule et al. 1988; Saunders1989). Longisolated remnantscan be expected to have lost a proportionof the species originallypresent,and gainedan additionalcomponentofinvadingspecies thatare capable of establishingin the fragmented system.It is thus wrongto consideronly species numbersand not species compositionwhen discussingspecies diversityin remnantareas:species numberscan potentially increase in fragmented systemswhere invasiveand edge species can establish,but the numbers of species originally foundin thearea maycontinueto decline(Verner1986; Murphy1989; Webb 1989; Harris& Scheck 1991). It is commonlyassumedthatat some stage the remnantwill reequilibratewiththe surrounding landscape. It is, however,questionablewhethera new stableequilibriumwill be reached since the equilibrationprocess is liable to be disruptedby changingfluxesfromthe surrounding matrix,disturbances,and influxof new invasive species. The finalequilibriumcan be likenedto an idealizedendpointthatis neverlikelyto be reached, in much the same fashionas the climaticclimaxis now conceptualized in succession theory.Managementof remnantareaswill thusbe an adaptiveprocessdirected at minimizing potentialfuturespecies losses. 2. DISTANCEFROMOTHER REMNANTS The abilityofspecies to colonize a remnantdepends to some extenton the distanceofthe remnantfromother areas of nativevegetation,be theyother remnantsor nearbyunclearedareas. Colonizingabilityis relatedto dispersalmode,withwind-dispersedand vagilespecies more likelyto arriveat isolated remnants.However, whethersuch species become successfulcolonistsdepends on physicaland biotic factorssuch as nutrient and competitiveinteractions availability (Vepsalainen& Pisarski1982). Animalspecies may have the physical abilityto disperselongdistances,butlackthebehavioral repertoireto traversethe matrixsurrounding the remnant;the matrixbecomes an effectivebarrierto move- Consequences ofFragmentation23 ment.Organismsize is also important, and 100 m over agricultural fieldsmaybe a completebarrierto dispersal forsmallorganismssuch as invertebrates (Mader 1984) and some species ofbird(Saunders& de Rebeira1991). The persistenceof such species on a remnantwould then depend entirelyon the retentionof enough suitable habitatto maintainsufficient numbersto withstand therisksofextinction(Soule 1987a, Ewens et al. 1987). Some evidence existsthatfragmentation of largepopulations into subpopulationsmay decrease the risk of overallspecies extinctioneven thoughlocal extinctions may occur (Higgs 1981; Quinn & Hastings 1987). It seems likelythatdifferent species will respond differentlyto the creationof subpopulationsand thatknowledge of the details of an organism'sbehaviorwill be necessaryto predict its response (Karieva 1987; Merriam 1991). 3. CONNEClvmY Associatedwith the effectsof distanceis the degree to which individualremnantsare connected in some way to adjacentareas.The issue ofconnectivityand theusefulnessof corridorsconnectingremnantshas received increasingattentionin theliterature(MacClintocket al. 1977; Wegner& Merriam1979; Baudry 1984; Forman & Baudry1984; Merriam1984; Harris1984, 1985; Fahrig & Merriam1985; Noss & Harris1986; Bridgewater 1987; Simberloff & Cox 1987; Noss 1987; Soule et al. 1988), and was the subject of a recent symposium (Hobbs et al. 1990; Saunders& Hobbs 1991a). Corridors are generallybelieved to providebenefitssuch as enhanced biotic movement,extra foragingareas, refuges duringdisturbances,and enhancementof the aestheticappeal of the landscape. In some areas theysignificantlyadd to the area of native vegetation left followingfragmentation. Simberloff & Cox (1987) pointedout thatmostofthe workon thevalue ofcorridorshas not been sufficiently controlledto demonstratean unequivocal role in increasingimmigration and/ordecreasingextinctions.An increasingnumber of studies,however, now indicate thatcorridorsare of value formovement,at least fora subset of the biota (papers in Saunders & Hobbs, 1991a). Simberloff and Cox (1987) also discussed potentialdisadvantagesof corridors,which include facilitationof the spread of disease,pests,and fireand other disturbances,increased predation,and high costs of linearremnantswithhighedge:arearatios. maintaining The relativemeritsofcorridorsand theirrequiredcharacteristics(i.e., width,composition,etc.) will varyfrom place to place and will depend on the targetspecies. Detailed predictionsof corridorvalue in reducingisolationof remnantareas are not possible withoutinformationon the movementof individualspecies across the landscape. Such information is, however,difficult ConservationBiology Volume 5, No. 1, March 1991 24 Saunders etal Consequences ofFragnentation to collect (Saunders& de Rebeira and time-consuming 1991). Nevertheless,while such data are being gathered, we need to take the approach thatcorridorsdo have value forbioticmovementand attemptto retaina good corridornetworkwhereverpossible (Harris & Scheck 1991; Saunders& Hobbs 1991b). IANDSCAPE 4. CHANGESIN THE SURROUNDING a Removalof the vegetationfromthe area surrounding remnantleads to theremnantbecomingtheonlyarea of suitablehabitatremainingforbiota displaced by clearing. This may lead to the concentrationof mobile elementsofthebiotain theremnants(Lovejoy et al. 1986). Thisconcentrationor crowdingeffectmaybe rapidand of the remnantby some speresultin supersaturation interaccies. Crowdingcan alterintra-and interspecific tions.Competitionand predation,forexample,can be increased,resultingin changesin fecundityand thepotentialultimatecollapse of the population. Resource availabilityis also affectedby overexploitation;forinstance,increasedherbivoryby largeherbivoressuch as the elephantin Africanreservescan lead to quite dramaticchangesin habitat(Spinage & Guiness1971; Laws et al. 1975; Walker1981). resultsfromthe influxof species naSupersaturation tive to the area,but thereare also potentialinfluxesof new suitesof species thathave increasedin abundance or established.in the surroundinglandscape following Such species include those that have fragmentation. developbeen introducedin the process of agricultural ment (pasture and crop plant species and livestock), and naother deliberateand accidentalintroductions, tive or migrantspecies thatcan take advantageof the new habitatconditionscaused by fragmentation. Naturalcommunitiesvaryin theirsusceptibilityto invasion,althoughthereis stilldebate overwhich characteristicsrenderone communitymore invasiblethan another(Fox & Fox 1986; MacDonaldet al. 1986; Crawley 1987; Usher 1988; Rejmanek1989). For vegetation, establishmentof nonnativespecies seems to be enhanced by some formof disturbance,especiallyif this increasesthe availabilityof a limitingresource(Hobbs 1989; Panetta& Hopkins 1991). Thus the opening of light gaps in dense forestswhere lightwas limiting syscould enhanceinvasion,whereasin nutrient-limited increasesthe perfortems,nutrientinputsignificantly mance ofnonnatives, especiallyin conjunctionwithsoil disturbance(Hobbs & Atkins1988). Invasionmay be restrictedto the edge ofremnantsifdisturbancefactors decline with distancefromedge (Cale & Hobbs 1991; Panetta & Hopkins 1991), but species with wind- or animal-dispersedseeds can establishin suitable areas withina remnant, awayfromthe edge. Invadingspecies can establish,forexample,fromseeds carriedin by,or depositedin feces of,animalsthatfeed in the area sur- ConservationBiology Volume 5, No. 1, March 1991 roundingthe remnantbut use the remnantforshelter. An examplefromWesternAustraliais the Wedge-tailed Eagle (Aquila audax), which breeds or roostsin remnantsbut forageson carrionin the surroundingfarmland,bringingpartsofsheep carcassesback to the roost to consumethem.Wool fromcarcassescarriesseed that is dropped under the roost tree. Nutrientinput from eagle droppingsand the disturbancecaused by other scavanginganimalsprovides a focus for the establishmentof nonnativespecies (Saunders,personalobservation). Thus, even in the absence of deliberatedisturbance withinremnantareas,invasionsmayoccur. impactson the Invasivespecies can have significant plantcommunitieswithinremnants;forinstance,invadalterthefuelstructure ingplantspecies can significantly and hence fireregime,and can inhibitthe regeneration of native species (Wycherly 1984; Macdonald et al. 1989; Panetta& Hopkins 1991). Nonnativeherbivores, includingstock,can also dramatically changevegetation structureand prevent regeneration.Species that increase because of landscape modificationcan also have significant impactson the restofthebiota.For instance, in NorthAmericaincreasedpressurefromnest predators and parasitessuch as the Brown-headedCowbird (Molothrus ater) has affectedpasserine bird populations in fragmentedsystems(Brittingham& Temple 1983; Wilcove 1985; Andren& Angelstam1988). Similarly,the Galah (Cacatua roseicapilla) has moved into all agricultural areas in Australiaas a resultof the developmentof cereal croppingand the provisionof wateringpointsforstock(Saunderset al. 1985). It now roosts in remnantwoodland areas,oftencompetingwithother indigenous hole nesters (Saunders & Ingram 1987; Saunders1990), and can damage tree foliageand bark, in extremeexamples causingtree mortality. Influences Modifying 1. REMNANT SIZE The smallera remnantis, the greaterthe influenceexternal factorsare likely to have. In small remnants, ecosystemdynamicsare probablydrivenpredominantly by externalratherthan internalforces.Of importance here are "edge effects"(Williamson1975; Janzen1983; Harris1988; Yahner 1988). Largerremnantshave a bigger core area that is unaffectedby the environmental and biotic changesassociatedwith edges. Here,we rethisis opposite gardedge effectsas mainlydetrimental; to the traditional view thatedges and ecotones are beneficialto wildlife(Harris 1988). The differenceis that remnantedges are createdby removalof the surroundingvegetationand place the remainderin juxtaposition witha completelyalteredsurroundingmatrix. Noss and Harris(1986) have pointed out thatwe do notknowthe minimumcriticalsize an ecosystemneeds Saunders etal. and to be to preserveitscharacteristic species diversity species composition(Lovejoy & Oren 1981). In factthe "minimumdynamic area" of Pickett and Thompson (1978) or "the smallestarea witha naturaldisturbance regime which maintains internal recolonization sources" would probablyexist only in the largestconservationparks. Largerremnantsusuallycontaingreaterhabitatdiversitythansmallerones. A collectionof smallerreserves may,however,cover a greaterarrayof habitatsthana single large one simplybecause a single large reserve will not containall of the habitatslikelyto occur in a region.These argumentshave been discussed in detail elsewhereand will notbe pursuedhere.It is important, however,to recognizethattheprocessoffragmentation is generallynot random(Usher 1987). Land clearance usuallyoccurs on a selective basis, with the best soil types being cleared first.For example, in southwest Western Australia,settlersselectivelycleared woodlands because theyoccurred on the heaviersoils best suitedforagriculture.As a result,woodland communities are now poorly representedin conservationreserves and most woodland remainingon farmsis in a highlydegraded state (Saunders & Hobbs 1989). Few reservesin the area are large enough to containrepresentativesamples of all preexistingvegetationtypes. Kitcheneret al. (1980a b, 1982) found,however,that even relativelysmallreserves(i.e., 30 ha) could be rich in some groupsoffauna,butwhetherthesepopulations are viable in the long termis debatable (see Saunders 1989). Remnantsize determinesthe potentialsize of populations of componentspecies. Clearly,the numberof individualsof any particularspecies thata given remnantwill supportdependson organismsize and requirements.The largerthe remnant,the morelikelyit is that populations will be large and more likely to resist chance extinctions(Gilpin & Soule 1986; Soule 1987a). Retainingpopulations in the long term may require largepopulationsizes (of the orderofhundredsor very much greater;Shaffer1987), althoughthe actual numbers requiredwill depend on the lifehistoryand populationgrowthrateof the species involved.Pimmet al. (1988) confirmthatover a fewdecades extinctionrisk does decrease withpopulationsize, but theyfoundno extinctionsamongBritishislandbirdsnumberingover 18 pairs.The issue ofminimumpopulationsize has been discussed extensively in the literature (see Soule 1987b), but thereis stillno real resolutionas to what constitutesa minimumviable population. There has been extensivemodelingof the concept,but littleexperimentalwork. Largerpopulationstend to have higherlevels of heterozygositythan small isolated populations. Current is beneficial.Species that is thatheterozygosity thinking Consequences ofFragmentation25 have gone throughgenetic "bottlenecks"are likelyto suffera reductionin heterozygosity with consequent loss of abilityto adapt to changingconditions.Species isolatedon remnantsmaygo throughsuch geneticbottlenecksbecause of smallpopulationsizes, and deliberate transfer of individualsbetween populationsmaybe required(Boecklen & Bell 1987). However,the general assumptionthatheterozygosity is essentialforlong term populationviabilityis stillopen to debate. Largereservesmayhave some disadvantages;in particular,the possibilityof disease spreadingthroughentirepopulationson a large reservehas been discussed. However,thereis a wide rangeof species' lifehistories and distribution patternsin the biota,and the effectsof reserve size are largelyspecies-specific.Species that have large area requirementsor thatrequire combinations of different habitatsare likelyto surviveonly in relativelylargeareas,whereas organismswithsmall,localized populationsand simplehabitatrequirementscan surviveon smallerremnants.However, all will be affected by the disruptionof physical ecosystem processes that result fromfragmentation and were discussed earlier. 2. SHAPE The shape of a remnantis importantonlyforrelatively smallareas;thereis some size beyondwhichshape does not reallymatter.However,forsmall remnants,shape determinesthe perimeter:core(or edge:interior)ratio. Long, thin remnantshave proportionallymuch more edge than square or round remnants(Diamond 1975; Wilson& Willis1975), and are more open to detrimental edge effects.However,some vegetationtypes,such as riparianstrips,are naturallythin,and corridorsare by definitiongenerallylong thin remnants(although the widertheyare themoreusefultheymaybe to aid movement of biota; Saunders & Hobbs 199 lb). Long, thin remnantsmay also, dependingon theirorientation,lie along environmentalgradientsand thus contain more vegetationtypesand habitatsthan a square reserveof similararea. Linearfeaturesare thuspartof the natural and fragmented landscape,and thereis no point in tryingto develop optimaldesignprinciplesthatdo nottake themintoaccount;the mostimportantquestionis how to managethe remnants, whatevershape theyare,so as to minimizeexternaleffects. 3. POSITION IN LANDSCAPE The positionof a remnantin the landscape has important influenceson all the featuresso fardiscussed. It affectsprefragmentation patternsof geomorphology, soil,and vegetation,and hence determinesthestructure and vegetationcompositionof any givenremnantarea. It also significantly affectspostfragmentation processes. ConservationBiology Volume 5, No. 1, March 1991 26 Consequences ofFragnentation For instance,thereis an importantdistinctionbetween run-onversus those remnantsthat are predominantly This influencesnot only run-off. thatare predominantly the hydrologicalregime of the remnant,but also the and seeds into and out of movementof soil, nutrients, the remnantarea. Lessons forManagement Managementof fragmentedecosystemshas two basic components:(1) managementof the naturalsystem,or the internaldynamicsof remnantareas, and (2) Managementof the externalinfluenceson the naturalsystem.Forlargeremnantareas,theemphasisshouldbe on managingthe internaldynamics,including,forinstance, the disturbanceregimeand populationdynamicsofkey on the otherhand,manorganisms.For smallremnants, at controllingthe agementshouldbe directedprimarily Janzen(1983, 1986), however,has externalinfluences. pointed out thatexternalinfluencescan be important whateverthe remnantsize. Since most impactson remnantareas originatefrom the surroundinglandscape, there is clearlya need to departfromthe traditionalnotionsof reservemanagement, and look instead toward integratedlandscape management.It will become increasinglydifficultto maintainremnantsof nativevegetationif the managementpracticesin the surroundingmatrixhave continuous adverseimpactson them.Traditionalreservemanagementstops at the reserveboundary;fluxesofwater, particulates,and organismsdo not. Placingthe conservation reservesfirmlywithinthe context of the surto develop compleroundinglandscape and attempting mentarymanagementstrategiesseems to be the only way to ensure the long termviabilityof remnantareas (Hobbs & Saunders1991). This has importantimplicationsforland managerssince it involvesa radicallynew way ofviewingmanagementand requiresthatneighborlandowners,interingland uses, and hence neighboring butnotimpossible, act in a positiveway.Thisis difficult, and thereare encouragingexamplesof attemptsat this type of integratedmanagement(e.g., FitzgeraldBiosphereProject 1989; Bradby1991). The landscapeapproachto managementis also essentialsince severalremnantareas takentogethermayrepresent a systemover which componentsof the biota travelto meet habitatand food requirements.The loss ofa singlecomponentofsuch a networkcould severely affectthe capabilityof the remainingremnantsto carry ifforinstancea particularspeout the same functions, cies or habitatwas lost.Such a networkconsistsnotonly ofthe designatedreserves,but also otherremnantareas and linkagesbetween them. The goal of conservationmanagementusuallyis to and the methodof achieving maintainspecies diversity, ConservationBiology Volume 5, No. 1, March 1991 Saunderset al. thisis to attemptto maintainrepresentative examplesof each ecosystemor communitytypepresentbeforefragmentation.To do this,we need to know the distributionsof species and communitiesand thenselect areas thatrepresentthem.In general,thereare two possible scenarios.In thefirst, we have a systemthatis about to be fragmented and we have to design the ideal set of reservesforthearea. In the second,we have an alreadyfragmented systemthatwe need to make the most of. Most theoriesin conservationbiology,includingvirtuallyall thediscussionsofislandbiogeographyin relation to reserves,have dealtwiththefirstscenario,whereasit is the second scenariothatwe mostfrequently have to confront. Herewe presenta seriesofguidelinesformanagementin thissituation. 1. The initialstepmustbe to determinetheminimum subsetoftheexistingremnantsthatare requiredto representthe diversityof a given region (Margules et al. 1988; Margules& Stein 1989). This requires thatwe have some knowledgeof the distributionof species or ecosystemtypes.Clearly,it would be desirableto have all existingremnantsavailable forthispurpose,but in manycases this is not achievable,and there must be prioritiesforreserveretentionor acquisition. 2. The systemmustthenbe managedto maintainthe diversityof species or ecosystems. The question of whethermanagementshould be forindividualspecies or whole ecosystemsis largelyirrelevant, because individual species require functioningecosystemsto surbut vive. Managementguidelineswill be area-specific, the need to manageexternalinfluencesis universal. 3. Prioritiesfor managementmust be established. Clearlythereare manyproblemsto be tackled,and usually there are limitedresources available for the job. There mustthereforebe a clear priorityrankingto ensure thatresources are deployed optimally.Problems thatare likelyto disruptecosystemprocessesand hence threatenthe viabilityof a remnantarea should be given highpriorityfortreatment. 4. Continuous managementis needed to maintain remnantareas in theircurrentstate,due to the constant pressureof altered internaldynamicsand externalinfluences.Here again,the allocation of scarce management resources must be considered. Effortshould go into maintainingsome remnant areas in as near a "natural"stateas possible,but it will not be feasibleto do this for all remnants.There is a strongcase to be made for lettingsome areas degrade so that theybecome less naturalbut are easier to manage and stillretainsome conservationvalue (Bridgewater1990). This is not as radical as it may sound, since the process is ongoingin manyremnantareas anyway.Once we accept thatmanyremnantsnow contain"synthetic"communitiesthatare not likelyever to returnto theirpristinestate,managementprioritiesbecome easier to set. Saunders etal. ResearchRequirements Research to date on fragmentedecosystemshas providedfewanswerson theissuesofpracticalimportance to management.It is just as importantto set priorities forresearchas formanagement,and in the same way, research costs must be taken into account since resources forresearchare also limited.For instance,is it betterto concentrateon single-speciesstudiesthatcan produce resultswithdirectpracticalapplicationbut are verycostlyand time-consuming, or shouldwe concentrate on the community/ecosystem approach that is cheaper but mayyield more equivocal results?Clearly, balanced use of both approaches is needed. We have identifieda numberof priorityareas that require research effort (see also Soule & Kohm 1989). 1. A majorpriorityis to understandthe effectsof externalfactors.Comparisonsof pre- and postfragmentation systemswill be particularly useful(Lovejoy et al. 1986; Margules 1987). Effectsof changes in radiation and water fluxes are particularlyimportant,as is the biotic invasionprocess. 2. Changesin internalprocesses since fragmentation also requirefurtherinvestigation. In particular,the interactionbetween internaland externalprocesses is likelyto be of criticalimportance. 3. Isolationfactorsneed to be betterunderstood.In particular,rates of genetic change in isolated populations require study,as does the question of whether reductionin genetic variabilityis important.We also requiremoreand betterdata on therole ofcorridorsin allowingbiotic (and hence genetic) movementin fragmentedlandscapes. 4. Whiletheoreticalstudieshave theirplace, thereis an urgentneed forfieldexperimentation in both managementand restoration.While such experimentsare costly to set up, it is possible to make use of many situationsthatofferready-madeexperiments.Our unofsuccessionhas benefitedgreatlyfromthe derstanding studyof abandonedold fields,and in the same way,we can use currentor past managementactivitiesas largescale experiments;thereis plentyof experimentalmaterialaround(McNab 1983; Hopkins& Saunders1987; Pimm1986;Jordanet al. 1987; Hobbs & Hopkins1990). Research has much to gain froma close liaison with management, especiallyifmanagementoperationsactuallycan be carriedout as designedexperimentsand the resultssuitablymonitored. Conclusions Emphasisin the literaturehas been on the design of naturereserves,but we are usuallytoo late to do anythingexcept tryto maintaintheremnantsleftfollowing Consequences ofFragmentation27 fragmentation. 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