toward an ecological definition of an island: a northwest european perspective haila, yrjo

8/13/2019 Toward an Ecological Definition of an Island: A Northwest European Perspective Haila, Yrjo

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Toward an Ecological Definition of an Island: A Northwest European PerspectiveAuthor(s): Yrjo HailaReviewed work(s):Source: Journal of Biogeography, Vol. 17, No. 6 (Nov., 1990), pp. 561-568Published by: Wiley-BlackwellStable URL: http://www.jstor.org/stable/2845140.

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Journal ofBiogeography (1990) 17, 561-568

Toward an ecological definition f an island:a northwestEuropean perspectiveYRJO HAILA Department fZoology,UniversityfHelsinki, . Rautatiekatu3, SF-OO100Helsinki, inland

Abstract.The equilibrium heory f slandbiogeography fMacArthur nd Wilson aimed at providing a unifyingframework or studying nsular ecology. However, thetheory oes not elaborate he conceptof an 'island' in anecological framework. suggest thatdifferentypes ofislands be ecologically defined n the basis of processesrelative o which some ecologically nterestingntities nthose islands' actually are isolated from heir urround-ings. Four main island scales can be discerned,namely,the cales of (1) individuals, 2) populationdynamics, 3)population ifferentiation,nd 4) evolutionary ivergence.I illustrate hese differentsland types nd their nterfaces

with examples mainly fromnorthwestem urope. Theequilibriumhypothesis eems relevantprimarily n thepopulation dynamics scale. A research programmefollowing uch differentiationould include two phases:first, ssessment f factorswhich make the environmentstudied 'insular', and second, construction f specificmodels for analysing ecological processes in thoseparticularnsular onditions.Key words. Islands, island biogeography, isolation,immigration,xtinction, cological scaling, conservationbiology, ennoscandia.

1. INTRODUCTIONArchipelagoes,be they composed of islands at sea orhabitat patches on land, present both theoretical ndempirical hallengesto ecological research.Theoreticallythe challenge is to understand processes structuringpopulations and communities in environments thatcomprisediscrete patialunits. n practice uch an under-standing ught ohelp preservingcological diversityn en-vironmentsragmentedy human ctivities.The theoryof island biogeography y MacArthurWilson 1967) is the usual starting ointfor slandstudies.This is no wonder:The theory iewed slandcommunitiesas dynamic units and offered a unifyingconceptualstructure or island ecology, based on such apparentlystraightforwardoncepts as immigration nd extinctionratesof organisms, ssumed to be functions fisland areaand solation.Aftertspresentationhe heory as hailed asa major breakthroughn ecological biogeography e.g.Simberloff,974). Thetheory as become a firm art fthecollective consciousness of ecologists, and has also acentralrole in conservation iology (Soule, 1986). It isdifficulto overlook he theory n insular tudies-islandsexist n the ecological science largely n the ight f inter-pretationsnd reinterpretationsf theMacArthur-Wilsontheory.

The legacy of the theoryhas problematicfeatures,however, as has been pointed out by numerouscritics

(Simberloff, 976; Gilbert, 1980). The theorycan beinterpretedn two ways Haila Jarvinen, 982), either sa strict xplanatorymodel that predicts the number ofspecies in a given taxon on islands, or as a deductivescheme hat oints ut variouspotentiallymportantactorsaffectingnsular ommunities. ccording othe atter iewthe theory s a researchprogramme ather han a strictexplanatoryhypothesis, nd the appropriateness f thetheory s assessed indirectly y evaluating he fruitfulnessof the pecific esearch roblems roposed ythe heory.The narrow nterpretationas dominated hescene. Thisis explainableby a semiotic imension f scientificheories(Haila, 1986). By a semioticdimension refer o the rolesymbolshave in scientific ommunication. ymbols notonlyrefer o external bjectsbut also carrywith themasignificance f another ype,namely, hat of being signs('signifiers'; Barthes, 1967) of more abstractvalues. Aparticular alue of the MacArthur-Wilson odel is that tformulateshe relationships etween the basic variables,speciesnumber nd sland rea, s a simple nalytic ule.Applications fthetheory ave rangedfrom ostplantsas islands for herbivorous nsects Janzen, 1968), or theGalapagos as islands for Darwin's finchesPower, 1975),topieces of laboratory lass as islands for quaticdiatoms(Patrick, 1975). These are all 'islands'-but of vastlydifferent inds.This is not a matter f semantics, ut amatter f biology:These differentnvironmentsre insularwithrespect o quite differentcological processes.On the

561


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562 YrjoHailaGalapagos hefocus s onspecies hat re endemicn thearchipelago,nd existnowhere utsideone or a fewislands. n theglass lides olonized ydiatomshe ocusis on local assemblagesf sessile eproducingndividualsthat renot solated tall fromopulationsnthe urround-ings. n otherases he ocus s on somethingn-between.Formally he situationsan be described y similarvariables 'immigration'nd 'extinction'), ut such anequationmay e substantiallynvalid.It seems hat sland cology eeds clarificationf thevery oncept f an island'. suggest hat necologicallymeaningfulefinitione soughtby startingrom heecological rocesseselativeowhichan sland' ctuallysan sland.2. ISLANDS N SPACEANDTIMESpace and timeare mutuallyelated n ecology, nddifferentcological rocesses ave differentharacteristicspace-timecales as discussed y Wiens, 981;Decourt,Delcourt Webb,1983; Birks,1986; O'Neill etal., 1986).In any particularituationhe space and time calesinwhich cological rocessesake lace re oupledogether:the argerhe patial cale, he argerhe ime cale.Thismeans hatisolation',hedifferentiapecificaf islands',is to be viewed nrelation o specific rocesses nd thecharacteristicpace-time atrixf hose rocesses.InFig.1 I reproducehebasic cheme f Wiens 1981),but dding suggestions tothemost elevantlternativescales n whichan island' anbe viewednanecological-evolutionaryerspective.n thefollowingillustrateheschemewith xamplesmainly rom orthernuropeanarchipelagoes; irds dominatemy examplesbecause

TIME

Evolutionary ndbiogeographic

dynamicsPopulations:

demography,dynamics

Individualbehaviour,

SPACE

(1) (2) (3) (4)FIG. 1. A schlcinicf diflerent ime-spacescales of ecologicalprocessesndcriteriahat efineorrespondingcalesof nsularity.1)Individual cale; (2) Population cale 1: dynamics; 3) Populationscale 2: differentiation;4) Evolutionarycale.

quantitativeata on island distributionsf othergroupsoforganisms re scarce. The scales form continuum,ndtheir nterfaces re also interestingnd provide mportantproblems or nsular cology.2.1. Individual caleOn the individual cale a patchof land is an 'island' ifsomecrucialphaseofthe ife ycleof ndividual rganismsobligatorilyakesplace withintsboundaries. ither epro-ductionor survivalmaybe important.or instance,mostbreeding pairs of passerines on the Skokholm island(Williamson,1983), on islands n Lake Malaren AhlenNilsson, 1982), on the Aland Islands (Haila, JarvinenKuusela, 1983), or on islands n Lake Inari Haila, 1983b)completetheir eproductive hase on a single island,butspendthe winter omewhere lse. The fact hat heybreedon real slandsmaybe importantor heir eproduction,uttheir ife cycle is influenced y a varietyof additionalfactors.In contrast, arge nonpasserines, .g. predators,mayinclude several islands in their foragingterritoriesnnorthernrchipelagoes Ahlen Nilsson, 1982; Haila etal., 1983; Haila, 1983b), and no single sland s an 'island'for hem.Birds that resedentarynthenorth epresent differentcase. For themwinter urvival s a majorproblem,ndtheymay demand larger areas of high-qualityhabitatsforsuccessfulwinteringhan forbreeding.For such speciesforestfragmentsmay be 'islands' primarily uringthewinterHelle, 1984; Virkkala,1987). We are still on theindividualscale, but the processesdefiningislands' aredifferentrom hoserelevant or reeding airs.If an island is too small for successfulreproductionrsurvival f ndividuals f a particularpecies,the pecies slikelyto be absent.This makes theconceptof 'minimumarea requirements'mportantn the evel of individual e-productive nits Haila, Hanski Raivio, 1989). Insularityon the individual level may also lead to ecologicallyinteresting consequences in terms of behaviouraladaptationsuchas searching orpreyor forhostsfornestparasitismWilcove, 1985).However, tcan hardly e expected hat nsularityf theenvironmentn the ndividual cale has directpopulationorcommunityevelconsequences, xceptthrough eingofinferioror superior) ualityas reproductivenvironmentfor ndividual eproductivenits.ndividuals iving n suchislands are an integral art f theregional opulationnthesurroundingreas.Thiswayofdefiningislands' is plausiblefor fficientlydispersing irds in northernonditions.A correspondingmodel of islandcolonization y land birds can be derivedfrom an analogue thatbird communities n small non-isolated slands resemblerandom amples drawn' by theisland from he surroundingvifaunal universe Preston,1960; Haila, 1983a). The spatial dynamics of birdsbreedingn smallnon-isolated orest ragmentsn southernFinland greewith hisview (Y. Haila, I. K. Hanski and S.Raivio, npreparation).

A samplingview seems less plausible for most othergroups of organismsfor two main reasons. First,many


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activelydispersing rganisms, .g. flying nsects,movewidely roundduring hereproductiveeriod ncontrastoterritorialirds.Consequently,n any single spatialunit'rescue effect' (Brown Kodric-Brown, 1977) mayfollow. Individuals may invade (and disappear) sofrequently hat no 'islands' can be discerned on theindividual scale. Second, local populationdynamics iscrucially importantfor species that do not disperseefficiently, hich lead, e.g., to aggregated dispersionpatterns even within continuous, uniform habitats(Southwood,1978). The spatialdynamics f individuals fsuch species may resemble 'sampling' on appropriatespatial ndtemporal cales,but theecologicalrelevance fthisphenomenonequires eparate ssessment.Quantitativedata on the colonization dynamics ofarthropodson islands are extremely scanty, but thethoroughly documented cases of small mangrove(Simberloff,969), and salt marshRey, 1985) islandsoffthe Floridan coast as well as sand isletsoffthe PuertoRican coast (Heathwole Levins, 1973) overwhelminglydemonstrateheimportance f other han ndividual-levelprocesses.Distributional ata on carabidbeetles Niemela,Ranta Haila, 1985; Niemela,Haila Halme, 1988), ants(Vepsalainen Pisarski, 1982) and land snails (BaurBengtsson,1987) in the archipelagoesof the Baltic Seasimilarly emonstratehat n individual-levelnterpretationwould be inadequate. We enter another cale on whichpopulationsre nvolved.2.2. Population cale 1: dynamicsWhen slandsbecome arger nd more solated, heymaysupportpopulations hat are dynamically ndependent fpopulations n otherslands ndonthemainland.Small islandsin theBaltic are not islandsfor birds onthisscale. There is no evidencethatbirdpopulations nsingle slands nthe Alandarchipelago, p to some tensofkm2 n area, are dynamically ndependent Haila et al.,1983). Indeed, his eemsto hold true or majority f andbirdsbreeding nthemain slandsoftheAlandarchipelagoas well, despite their substantial rea (970 kM2) andisolation c. 40 km from he Swedishand 70 km from heFinnishmainland). Both species turnovernd populationchanges documented n the archipelagosince the 1920sparallel trends bservedon the Finnishmainland Haila,Jarvinen Vaisanen,1979).However,for fewsedentary irdspecies the arge andmost isolated Baltic islands such as Gotland and Alandclearly re slandson thepopulationevel.For nstance, hegreen woodpecker Picus viridisL.), European nuthatch(Sitta europaea L.) and marsh it Parus palustrisL.) areabundant n the Swedishmainlandbut absentfrombothGotland and Aland (Svardson, 1949), although suitablehabitats abound on the islands. Furthermore,he grey-headedwoodpecker Picus canus Gmelin), and crested it(Parus cristatus .), havecolonizedtheAland slands sincethe1920s (as well as capercaillie TetraourogallusL.) andhazel hen Tetrastes onasia (L.)), due to successful ntro-ductions) (Haila et al., 1979). The thriving Alandpopulations f thesespecies are presumably solated fromthemainland opulations.

Definitionf n island 563Compensatoryhangesn thehabitatelection,oragingbehaviournd evenmorphologyf some sedentaryirdspecies n the argeBaltic slands eemto be connectedwith he bsence f potentiallyompetingpecies. his ssuggested y abundancedata on large woodpeckers(Svardson, 949;Haila Jarvinen,977)anddetailed b-servational ata on forest its Alatalo,Gustafsson

Lundberg,986).Data on thedistributionf carabid eetlesntheAlandarchipelago uggest hatpopulation-levelrocesses reimportant or their abundances n differentslands.Populationizes on small slands fewkilometresff hecoast deviategreatly rom xpectationserivedfromsimilar amples n similarhabitats n the mainland(Niemela t al., 1985).A comparisonf carabid amplesfromhree . 0.5 km2slands -15 kmoff he oastwitheachother nd with amples rom hemainland evealedmore ronouncedifferencesNiemelat l., 1988).A fewspecies bundantn themainlandPterostichuselanarius(Ill.) andPterostichusersicolorSturm)) ere ompletelyabsent romimilar abitatsn the slands. even pecieshad a 'mosaic' distributionmong the islands.beingabundantn one or two of them ut bsent rom imilarhabitatsnthe ther(s). urthermore,ome pecies particu-larly alathus uscipesGz.)) were ound n the slandsnmuch higher umbershanon similarhabitats n themainland, hich uggests compensatoryopulation-levelresponse.Thepatternsndicatehat ocalcarabid opulationsnislands fewkilometresff he oastmaybedynamicallyindependentrommainland opulationsn the borealconditionsf theBaltic.Mostof themainlandpecies reableto colonize slands, ut heir olonizationuccess shighly ariable rom pecies o species.Unfortunately,nthe basis of distributionalata it is impossibleo inferwhetherheislandpopulationshowreal immigration-extinctionynamicsntime.Immigration-extinctionynamicsontrolhe ersistenceof shrew opulationsn small slandsn lakes neasternFinland (Hanski, 1986). The abundant nd sociallydominantommonhrewSorex raneus .), isconsistent-lypresentn slandsarger han ha,but emographicto-chasticity ay ause ocal extinctionsfsmall'populationson slands2 ha. ThesmallermaskedhrewS. caecutiensLaxmann), nd pygmy hrew (S. minutus .), areapparentlyore ensitivehan he ommonhrew o envi-ronmentalariation.heyhavemore atchy istributionsandareatany inglemomentbsent rommany elativelylarge islandswithsuitablehabitats. he immigration-extinctionynamicsn these slands re presumablyotindependentfmainlandopulations.Dynamic istributionsf smallmammalsn archipela-goesare often onnected ith opulationluctuationsnthemainlandHanski, 986).This s amply emonstratedinpopulationynamicsfthe ield ole Microtusgrestis(L.)) inTvarminne,outherninland Pokki, 981). Thenumberf slands nhabitedvthefield ole varied romtwenty-eightofifty-twona 6-year eriodout f eventy-one islands ncluded n the study).Demographic haracter-isticsof the colonizing mall mammalsmay nfluence heir


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564 YrjoHailaability opersist n islands, s demonstratedxperimentallyfor he bank vole (Clethrionomus lareolus Schreber))byEbenhard 1987) inSweden.For an island to be an island on the population ynamicscale, twocriteria eedto be fulfilled. irst, he whole lifecycleof theorganisms oncernedmustbe confinedwithinthe sland.Second,the slandpopulationmust nprincipleget along for several generations independentlyofpopulations n surroundingreas. When the timespan ofdynamic solation becomes longer, he island populationbecomes increasingly ndependentlso as a geneticunit.We enter henext evel of nsularity.2.3. Population cale 2: differentiationVery little s knownabout island populationsof anyorganisms in the north in terms of their geneticcomposition.We do not know whether hepopulations fthecarabidCalathusfuscipes, ay,aregenetically ifferen-tiated n the above-mentioned.5 km2Aland islandsfromthoseon the main islands of Aland. The timetheyhavebeen solated s fairlyhort, utthepossibilityfdifferenti-ation due to genetic drift, r even to adaptive 'nichevariations'Van Valen, 1965) inthe slandenvironmentssnot excluded. The same guess can be made for thesedentarywoodpeckerand tit populationson the largeBaltic slandsGotland nd Aland.Morphological hangeshave been demonstratedn thepopulation fpygmy hrew n Gotland omparedwith heSwedish mainland. Individuals on Gotland have largerjaws, possibly s a character isplacement onnectedwiththe absence of common shrew, a larger competitor(Malmquist,1985). Gotland s an islandfor he shrews nthe cale ofpopulation ifferentiation.Birds in the boreal zone, being mostlymigratory,reunlikely andidatesfor howingpopulationdifferentiationon islands. In contrast, urtheroward the south a largerproportion f island bird populations re sedentary,ndgeneticdifferentiationecomes more mportant.he wren(TroglodytesroglodytesL.)) has evolvedinto at least sixdistinct ubspecies in different orth-Atlanticrchipela-goes (Williamson,1981). The largeMediterraneanslandshost wo endemicbird pecies, theCorsicannuthatchSittawhiteheadi harpe)andCypruswarblerSylviamelanotho-raxTristram)Blondel, 1985). Similarly, variety f smallmammalshave differentiatedntogenetically istinct ormson the slandsnorth f BritainBerry, 986).Detailed populationgeneticstudies on invertebratesnnorthernslands re scarce.Differentiation,ossiblydue tofounderprinciple, s highly likely in species that areentirely dependent on human transportationn theirdispersalto distant slands, e.g. lumbricids n the FaroeIslands Enckell et al., 1986; see also Enckell,BengstonWiman, 1987). Jarvinen Ranta -(1987) summarizedrecently esults rom opulation enetic tudies onductedon small slands nthenorthernaltic;founder rinciplesimportant, ut some differentiation ue to selectivepressureshas been documented as well (primarily yHalkka and his coworkers, .g. Halkka Halkka, 1974;Halkka Mikkola,1977).In some cases differentiationf archipelago opulations

compared with mainland ones can be traced to thedynamics of environmental changes in the wholearchipelago ather han o conditions n any single sland.For nstance, hewaterstridererris horacicus chumm. sin southern inlandparticularlydapted to the unpredic-table environmentf rock pool 'archipelagoes' of Balticskerries but not to any single rock pool) (Vepsalainen,1974; Jarvinen Vepsalainen, 1976; Kaitala, 1987).Another example is providedby carabid beetles. Theproportion f brachypterousndividuals s high amongcarabidbeetles ntheAlandarchipelago omparedwith heScandinavian mainland (Lindroth, 1949). This ispresumablydue to populationprocesses in the wholearchipelago ather han o conditions n single slands.TheBaltic is a land-upliftrea, whichproducesa continuouschange in island configurationss well as in habitats nindividual slandsdue toprimaryuccession,which nducespronounced aunal ynamicsHaila et al., 1982;JarvinenRanta, 1987). Abouttwenty lant species have taxonomi-cally distinct ubspecies in the outerarchipelagoof theBaltic, and several species in the taxonomicallyverycomplex generaof HieraciumL. and Taraxacum Webermay have undergone onsiderablegeneticchanges in thearea sincethe atest laciationPalmgren, 948).The scales ofpopulation ynamics ndpopulation iffe-rentiationan usuallybe separated rom ach other ecausethe respectivetime-scalesdiffergreatly. n populationdynamics elevant imeperiods remeasuredby singlere-productive ycles. In population differentiationelevanttimeperiodsare longer nd highlyvariable from ase tocase. Often the scales interact, owever. For instance,continuous mmigration-extinctionynamics may bringforth ifferentiationue to founder ffect. uch seems to bethe case in shrewpopulations n small islandsin easternFinnishakes Hanski,1986).The status of the equilibriumhypothesis s differentrelative opopulation ynamics ersuspopulation ifferen-tiation. On the populationdynamiclevel immigration-extinction ynamics re a reasonableconcept,which hasbeen documented in several cases, albeit mainly onsouthernatitudes Simberloff, 969; Rey, 1985; Hanski,1986). However, herelevanceofthe concept s less clearinconnectionwith opulation ifferentiation.2.4. EvolutionarycaleThe broadest cale in Fig. 1 is constitutedy evolution-ary processesthat ead to thedivergence f species. TheGalapagos and otheroceanic islands are 'islands' on thisscale for a greatnumber f taxa. A comparable xamplefromnorthernatitudess thefreshwater island' of LakeBaikal, which hosts numerous endemic species, bothvertebrates nd invertebratesKozhov, 1972). However,endemicforms re scarce on northernslands comparedwith slands closer to the equator n all taxa (Carlqvist,1974). Thismaybe partly ue to differencesngeologicalhistory, ut there is also real variation n the spatio-temporal cale ofevolutionary rocesses ndifferentioge-ographicregions.An island needs to be larger nd moredistant rom hemainlandn thenorth han n the ropics obe an island' on the volutionarycale.


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The monograph f MacArthur Wilson (1967) dealswith evolutionary rocesses, the dynamic oncept n thisconnection being the 'taxon cycle' (Wilson, 1961).However, ogical ties betweenthe equilibrium ypothesisand evolutionaryonclusions re loose (Haila Jarvinen,1982). Furthermore,mpirical evidence to support theclaim that speciation nd extinction n islandswould beequilibrialprocesses is only suggestive t best (FaethConnor, 1979). It is doubtfulwhether he equilibriumhypothesis s actually applicable on the evolutionaryscale.3. INTERFACESIt seems that he domain of theequilibrium ypothesissrestricted ainly o the second,population ynamics cale.On this scale, empirical evidence on the realism ofimmigration-extinctionynamics has been obtained ininsular species assemblages. In contrast, as regardsindividual scale islands the equilibrium hypothesis sirrelevant,ndfor hetwo broadest cales it s an empiricalmatterodecidewhethermmigration-extinctionynamicsarerealand relevantnnatural onditions. tpresent irectpositive vidence s lacking.However, he scales grade from ne to another. ariouscombinations of different cological processes mayinfluence opulations nd communitiesn islands nd,as aconsequence, the definition f an 'island' relative toecological processes s often ntologically mbiguous. hisimplies thatthe equilibrium ypothesismaybe a fruitfulstarting ointfor tudies n a variety f insular ituations;this onclusion greeswith heviewof,e.g.,Brown 1986)and Lomolino (1986) on the relevanceof the theory orstudies f mammals n islands.A particularlymportantnterfaces between islands' onthe ndividual nd population ynamics cales. Singlecowdroppingsre islands' fordungbeetles Hanski, 1980) andso are carcasses for carrion flies (Hanski, 1976) on theindividual scale. The developmentcycle of individualbeetles or flies s confined o singleresourcepatches,butsinglepatches upport 'population'onlyfor part f onegeneration.However,distancesof thepatchesfrom achother ffect reatly eetles ndflies nthepopulationevel.When distancesbetween thepatches ncrease t becomesmoreandmoredifficultordispersingndividuals o findnew suitable patch, and a population level responsefollows: hepopulation oes extinctHanski, 1982).Populationsize of colonizing species in source area(mainland rnearby arger slands) s an importantariablein islandecologyas has beendemonstratedarticularlyormammals (Lomolino, 1986; Peltonen et al., 1989).Propagules re morefrequent rom argethanfrom mallsource populations,which increases the persistenceofisland populations, nd decreases the likelihood of theirgenetic ifferentiation.The land uplift archipelagoesof the northern alticexhibitmixedpatternss well.Vegetation f the slands scharacterizedy directional,uccessional hange, nd bothplants and animals have far higher immigration hanextinction ates nsingle slands.Population esponses uch

Definition f an island 565as differentiationf plant populations Palmgren, 1948)occur on the evel of the whole archipelago ather hanonthe evelof single slands Jarvinen Ranta,1987).4. ISLANDSANDCONSERVATIONThe theory f islandbiography as been applied to conser-vationproblemsmainlynthecontext fhabitat ragmenta-tion (Soule, 1986). Insular habitat patches produced byfragmentationre often t the interface etween the firsttwo island scales. The space isolatinghabitat atchesfromeach others usuallynotequallyhostile otheorganismsswater s formost terrestrialrganisms; his s the case forcarabidbeetles n thetaiga (Niemela Haila, 1986). Theconsequenceis thatdispersalrates from ne fragmentoanother ncreasewhichmakestheequilibrium rameworkinadequate. t is necessary o investigate opulation istri-butions and habitatrequirements n the regional scale(Haila Hanski,1984; Haila et al., 1989).A crucial conservation hallenge onnectedwithhabitatfragmentations the problem of minimum viablepopulation ize (Gilpin Soule, 1986). In theframeworkof Fig. 1, theproblem s mainlyconnectedwith the twolargest cales, that s, with situationswhere an endemicspecies or population s endangeredbecause of habitatdestruction. ritical additional effectscan be producedthrough he spatial structure f the population.Lande(1987) appliedthemetapopulation odel of Levins (1969)to territorialrganisms uch as birds and concluded thatincreasingdistance betweensuitableterritories,ossiblycombinedwith decrease n their verage uitability, ightdrive populations to extinction. n such a case, the'insularity' f the environments only on the individuallevel but populationevelconsequencefollows.However, the equilibrium hypothesisdoes not givedetailedguidelinesforelaborating he extinction isk ofindividual species. Even though average extinctionprobabilityna habitat ragment ould be a function fthetotal numberof species present nd fragment rea, thespecific processes intensifyinghe risk of extinction fendangered pecies (the 'extinctionvortices', GilpinSoule, 1986) need study n each case. Habitatfragmenta-tion is likelyto increase extinction isk,but the specificmechanisms must be separately assessed (SouleSimberloff,986).

5. CONCLUSIONS: RESEARCH ROGRAMMEORISLAND IOGEOGRAPHYA particular atchof and s an island' in a differentensefordifferentpecies.This causes seriousrestrictionsn thedomainofvalidity fequilibriumslandbiogeography.t isusually not ecologicallyrealistic to include all differentspecies present imultaneously n an island in a singlevariable speciesnumber'.Immigration-extinctionynamicsare oftenreal withrespect o individual opulations n islands,buttheir xtra-polation to the communityevel may be invalid.This isbecause all species in any given taxon do not necessarily


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566 Yrjo ailaparticipatenthe mmigration-extinctionynamics. s aconsequence,hedynamicsmaynot be reflectedn thecommunityevel t ll.Carabidbeetleson smallBaltic slands erve as anexample (Niemela et al., 1988). The number f speciesshows a regularncreasewith sland area,but this sstronglyonnectedwithhabitatheterogeneityf theislands.A number f species are good colonizers fislands nd eem o bevirtuallylways resentn islandsclose to the shore, ome of thempresumably eingspecialistsof shore-line abitats.Consequently,hespeciesthatpossiblyhave a dynamic alancebetweenimmigrationndextinctionn the rea areconfoundednthe otal pecies ists; heirnfluencen the otal peciesnumberfthe slands snegligible.Thesame s true fcarabid eetles nterrestrialabitatfragmentsn southern inland.Distributionalata onforestarabidsnsmall orest atchesurroundedy agri-culturalndsuburbanabitatsnthevicinityfHelsinki(Halme, 1987) suggest hat fewforest pecieshavedifficultiesn establishing iable populations n thepatches.However, heir ommunityevelcontributionsconfoundedy generalistpecies ndspecies avouredyhumannfluence.The equilibrium odeldoes notprovide heunifyingconceptualcheme or nsulartudiestpurportsobe. Isuggesthat he onstructionf a new,more ealistic iewof insularecology than the one providedby theMacArthur-Wilsonrameworknclude womain teps:First,indingut nwhich articularcologicalense heislands tudied re 'islands' forthe organisms ne isinterestedn.

Second,developingmodels hatreflect heparticulardynamicshatre mportantnthe ystemtudied.Differentypesof questionsneed to be asked indifferentituationsor, oput tdifferently,he bservedpatternseedto be relatedo differentcontrastpaces'which defineexcludedalternativesDyke, 1988). Insmallersland cales nappropriatecontrastpace'wouldbe based onvariablesnfluencingolonizationropensityof ndividualpeciese.g.Simberloff,981;VepsalainenPisarski, 982;Hailaet al ., 1983), nd nbroaderslandscales n variablesnfluencingheirong-termersistence.Whateverhedetailedtructurefthe nsuingmodels,idiosyncracys not heonly ptionhat ollows romhedisintegrationf heMacArthur-Wilsonramework.slandstudiesdiversify,ut lower-level eneralizationsndmodels an bedevelopedhatover pecifiednd cologi-cally ealisticubsets f llpossible inds f rchipelagoes.Generalismndrealism an be combinednvariouswaysinconstructingcologicalmodelsLevins, 966).Williamson 1989) evaluated the legacy of theequilibriumheory ythe tatementtrue ut rivial'.Myconclusion s different.he domain f validity f theequilibriumypothesiss more narrow hanoriginallysupposed inmanyituationshehypothesiss simplyn-appropriate.owever, hen he cale of slandsmatchesimplicit ssumptionsf the hypothesisnd relevantimmigration-extinctionynamics an be demonstrated,hehypothesiss highly on-trivial.

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toward an ecological definition of an island: a northwest european perspective haila, yrjo

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