Records of the Western Australian Museum Supplement No. 68: 133-141 (2005).

Area and habitat relationships in island land snail faunas:an Aegean case study exploring the choros model

K. A. Triantis 1,2, K. Vardinoyannis1 and M. Mylonas1,21 Natural History Museum of Crete, University of Crete, Irakleio, 71409, Greece

2 Department of Biology, University of Crete, Irakleio, 71409, Greece.Correspondence: K.A Triantis, Natural History Museum of Crete, University of Crete,

P.O. Box 2208, 71409 Irakleio, Crete, Greece. E-mail: kostas@nhmc.uoc.gr

Abstract - In the present paper we study the biogeography of land snails inthe Skyros archipelago (central Aegean) using both conventional species-areaanalysis and the newly-developed choros model. The choros model is asimple mathematical relationship, which analyses species richness as afunction of area and of environmental heterogeneity, quantified by assessingthe number of defined habitats present on each island. We surveyed landsnails on 12 islands of the archipelago of Skyros in the central Aegean Sea(Greece). The different types of habitats were defined based on the ecologyand biology of each species distributed in the archipelago. Forty-two landsnail species were recorded living in 11 different habitat types. The z-valuesfrom both models, the choros and the classic species-area relationship, placethe archipelago in the "within biogeographic province" category, mainly dueto the recent formation of the archipelago. This recent formation resulted in agroup of small islands still behaving as parts of a continuous landmass. Therecorded extinction of land snails in the archipelago follows the generalpattern of extinctions of this animal group in the Aegean area. The chorosmodel gave a better fit to the data than the classic species-area model,indicating that the range of habitats available plays a significant role indetermining faunal diversity independently of area. The availability ofknowledge on the ecology of each species in most islands of the Aegeanenabled us to recognise the key environmental features on which we basedthe definition of the habitat types. Such background knowledge is essential ifisland studies are to progress beyond the simple demonstration of a species!area relationship.

Key words Aegean Sea, choros model, habitat diversity, land snails, species-area, species richness.

INTRODUCTIONThe species-area relationship not only is one of

ecology's few laws but it is also one of the longestknown. There is more evidence for the generality ofthis pattern than of any other described in ecology,but scientists are still not sure of its theoretical baseand its actual shape (Rosenzweig, 1995; Lomolinoand Weiser, 2001). Rosenzweig (1995; 2003) showedthat the processes determining species diversityproduce logarithmic species-area relationships withz-values (slopes) that vary within a specific range.For sample-area (intraprovincial) species-arearelationships z varies between 0.1-0.2, forarchipelagic ones it tends to have values of 0.25-0.55 and for the interprovincial species-arearelationships it begins at 0.6 and ranges upwards,with most about 0.9 to 1 or even higher(Rosenzweig 1995; 2003). These empiricaldifferences suggest a different balance of causes in

each case, and provide a starting point for moredetailed analyses.The two major mechanisms proposed to account

for these species-area relationships are the habitathypothesis and the area per se hypothesis (Connorand McCoy, 1979). Although there have been manyattempts to establish the superiority of onehypothesis over the other, it is generallyacknowledged that habitat diversity and area per seare not mutually exclusive mechanisms butsupplementary and may operate individually or incombination (Connor and McCoy, 1979; 2000; Kohnand Walsh, 1994; Rosenzweig, 1995; Ricklefs andLovette, 1999). Hence, at least four models havebeen developed combining the effects of area andhabitat diversity on species richness (for a reviewsee Triantis et aI., 2003). The choros model wasintroduced to reflect the actual ability of a region tomaintain a particular number of species in a certain

DOI: 10.18195/issn.0313-122x.68.2005.133-141

134

higher taxon (Triantis et al., 2003). The choros value(K), is the result of the multiplication of the totalarea (A) of the region or island by the number ofthe different habitat types found within it (H); thusK=HxA. Species richness (5) of the region isexpressed as a power function of the choros (K),namely 5 = cKZ, which is analogous to 5 = cAZproposed by Arrhenius (1921), by substituting area(A) with choros (K).In the present paper we study the biogeography

of land snails in the Skyros archipelago using boththe species-area relationship and the choros model,we evaluate the behaviour of the choros model, andwe analyse categorisation of habitats for the landsnails in the archipelago.

METHODS

Study areaThe Skyros island group lies in the central part of

the Aegean Sea (Figure 1). The largest island of thearchipelago is Skyros, which lies 19 miles east ofEvvoia Island and is the ninth largest island of theAegean Sea with a total area of 208 km2• More than20 small islets are situated around the island of

KoulouriThaliaAg. Fokas'

Lakk:lni

Skyropoula

K. A. Triantis, K. Vardinoyannis, M. Mylonas

Skyros, all inside the isobath of 200 meters. Manhas inhabited Skyros since the Neolithic (Patton,1996) but this island lies away from the maintrading routes of the Aegean. For, at least, the last200 years the total population of the island hasnever exceeded 4,000 inhabitants.The geological formations of the island of Skyros

itself consist mainly of limestone (66%), schist (22%)and neogene formations (7%) (Melentis, 1973). Thesubstrate of the neighbouring islets is mainlylimestone (Melentis, 1973). Only the islet of ExoDiavatisdoes not consist of a calcareous substrate,but of semimetamorphic clastic formations (mainlygneiss) (personal field observations).The isolation of the Aegean landmass, including

Skyros, occurred 4.5 million years ago, but theformation of the archipelago, by furtherfragmentation, is supposed to have occurred byearly Holocene (Van Andel and Shackleton, 1982;Perissoratis and Conispoliatis, 2003).The climate of the area is characterized as thermo-

mediterranean with a quite long dry period,beginning at the end of April and ending in earlyOctober (Andreakos, 1978).The southeast part of Skyros island is

mountainous and rocky covered by degraded

SarakinooI

4.5I

9I

18 kilometresI

Figure 1 Map of Skyros' archipelago with the islands included in this study and the sites surveyed.

Habitats, area and island snail faunas

Table 1 Area, number of species and number of habitattypes of the islands of Skyros' archipelagoincluded in this analysis.

Island Area No of No of(km2) species habitats

1. Skyros (Skr) 208 42 112. Valaxa (Val) 4.33 21 73. Skyropoula (Sky) 3.83 20 84. Sarakino (Sar) 3.34 16 85. Platia (PIa) 0.62 15 86. Erineia (Eri) 0.53 14 57. Koulouri (Kou) 0.29 14 68. Mesa Diavatis (MeD) 0.039 10 49. Exo Diavatis (ExD) 0.018 4 210. Lakkonisi (Lak) 0.016 7 411. Agios Fokas (AgF) 0.003 7 412. Thalia (Thl) 0.002 6 4

maquis dominated mainly by Quercus coccifera, andphrygana. By contrast, forests of Pinus halepensis,maquis mainly with Juniperus phoenicea and lushvegetation cover the northwest part. Maquis,phrygana and perennials dominate the otherislands of the group (Dafis et al., 1996; personal fieldobservations).We visited the archipelago of Skyros during late

June 2001 but mainly in early January and late April2002. We collected land snail species from 23different sites all over the island of Skyros and from11 smaller islands of the archipelago (Figure 1;Table 1). On the smaller islands the land snails weresampled from the whole area of the island. Litterand soil were removed from each site, taken frompatches likely to yield small species. All the materialcollected is deposited in the collections of theNatural History Museum of Crete.

Habitat diversityHabitat types were defined according to the

biology of each species inhabiting the archipelago,based on the proposed definition of Looijen (1995,1998) and we avoided defining habitat types apriori, based for example on the differentvegetation types of the islands. Looijen (1995,1998) proposed that biotope should be defined as"an area (topographic unit) characterized bydistinct, more or less uniform biotic and/or abioticconditions" and (realized) habitat as "the set ofenvironments in which a species lives". Habitat isassigned to the species concept and biotope to thecommunity concept.For the definition of habitats, apart from our own

research and observations on the biology of eachland snail species inhabiting the archipelago ofSkyros, we also considered a significant amount ofdata accumulated during the last decades on thenatural history of the land snail species distributedin the Aegean Sea (see Mylonas, 1982, 1985; Riedel,

135

1992; Vardinoyannis, 1994; Mylonas et al., 1995;Botsaris, 1996; Wiktor, 2001; Triantis et al., 2004).For each species, we first recorded the biotope thespecies was found in, forest, phrygana, maquis,meadows, settlements or buildings (recent andabandoned), sandy and pebbly beaches, cultivations(recent and abandoned), and salt-marshes. Then, werecorded the precise location of each species in thebiotope (rocks, stones, pile of stones, litter) (Table2). For the definition of habitat types we combinedthe above data with the special characteristics of thebiology of each species, and the presence oflimestone since the majority of land snails dependon it (HelIer, 1982; Mylonas, 1982; Goodfriend,1986). Our approach to defining and classifyingland molluscan habitats attempts to identify thespecific requirements of each species, based on ourobservations of the ecology of each species in thearchipelago and also on the available knowledge ofits ecology in the Aegean islands overall. In order tohave a simple, parsimonious and ecologicallyrealistic approach to habitat types we used the keyenvironmental features recognized by ourobservations and the available knowledge from theAegean islands (Table 2). The procedure for thedetermination of the habitat types in thearchipelago starts from specialist species towardsthe more generalist species. Based on eachspecialized species a distinct habitat type wasrecognized, except of course in the cases where twoor more specialized species occupy highlyoverlapping habitats. Then, based on the speciesthat occupy a restricted set of environments,additional key environmental features wereconsidered for the definition of new habitat types.This procedure was repeated till the most generalistspecies were included, so as to include all thedifferent habitats occupied by the land snails of theregion. With the above procedure, some species,specialists, are restricted to a particular habitat type;others are present on some of them and a numberof species are present in all the habitat typesrecognized. The key environmental features usedmay differ on scale depending on the biologicalrequirements of each species.According to our approach, the (realized) habitat

of a species can be a specific "standing place",independent of the biotope(s) the habitat is foundin. Thus, we do not use forest as habitat, since ourstudy revealed that the presence of no land snailspecies depends on forests. On the contrary, specificcomponents of a biotope, such as stones or litter,which also may occur elsewhere, are crucial for thepresence of a species. For example, the presence ofPyramidula chorismenostoma, Rupestrella philippii andR. rhodia is determined entirely by the presence ofcalcareous rocks, and not by the biotope in whichthese rocks occur. The same stands for Idylabicristata whose presence depends exclusively on

Table 2 The land snail species present on the archipelago of Skyros, the biotope where each species was found and its relative position in the biotope.~IJJ

Biotope e:tI

Relative position in biotopeSpecies Pine Maquis Phrygana Meadow Cultivation Settlements Salt Pebbly Sandy Calcareous Non- Pile of Rock Litter

forest marsh beach beach Stones Calcareous stonesStones

Helix cincta O.F. Muller, 1929 * * * *Helix aspersa O.F. Muller, 1774 * * * * * * * * *Helix aperta Born, 1778 * * * * * * * * *Eobania vermiculata (O.F. Muller, 1774) * * * * * * * * *Helicigona cyclolabris (Deshayes, 1820) * * * * * * *Theba pisana (O.F. Muller, 1774) *Lindholmiola lens (Ferussac, 1821) * * * * * * *Metajruticicola giurica (0. Boettger, 1892) * * * * * *Monacha parumcincta (L. Pfeiffer, 1841) * * * *Trochoidea cretica (Ferussac, 1821) * * * * * * *Cernuella ionica (Mousson, 1854) * * * * *Helicella sp. * * *Xerolenta oOOiaMenke, 1828 * * * * * * *Xerotricha conspurcata (Draparnaud, 1801) * * * *Cochlicella acuta (O.F. Muller, 1774) * *Albinaria cristatella O. Boettger, 1892 * * * * * * * *Idyla bicristata (Rossmassler, 1839) * * * * * * *Mastus carneolus (Mousson, 1863) * * * * * * * *Mastus giuricus (Westerlund, 1902) * * * * * *LimaxflavusLinnaeus,1758 * * * *Lehmannia valentiana (Ferussac, 1823) * * *Tandonia sowerbyi (Ferussac, 1823) * * * * * * * * *Deroceras panormitanum (Lessona et ~PolIonera, 1882) * * * * * ?>Deroceras parnasium Wiktor, 1984. * * * * * * * >-1

Daudebardia rufa (Draparnaud, 1805) * * * ...;;.Eopolita protensa (Ferussac, 1823) * * * * * * * * * ::l-Vitrea sporadica Pinter, 1978 * * * * * * * ....

!!'Oxychilus hydatinus (Rossmassler, 1838) * * * * ~Oxychilus mylonasi Riedel, 1983 * * * * * <:Oxychilus glaber (Rossmassler, 1835) * * Q>...Orculella critica (Pfeiffer, 1856) * * * * * * * * Q......

::lPagodulina pagodula (Des Moulins, 1830) * * * * 0'<Pyramidula chorismenostoma (Blanc, 1879) * * * * Q>::lGranopupa granum (Draparnaud, 1801) * * * * * * ::l....

Rupestrella philippii (Cantraine, 1840) * * * * !!'Rupestrella rhodia (Roth, 1839) * * * * ~Lauria cylindracea (Da Costa, 1778) * * * * * * * ~Truncatellina callicratis (Scacchi, 1833) * * * '$.Truncatellina cylindrica (Ferussac, 1807) * * * 0

::lCecilioides acicula (Muller, 1774) * * * * * * Q>

Cl>

Truncatella subcylindrica (Linnaeus, 1767) *Ovatella myosotis (Draparnaud, 1805) *

Habitats,areaandislandsnailfaunas

theexistenceofstones(calcareousand/ornon-calcareous)regardlessofthebiotope.Inothercases,however,aspeciesmay occurinavarietyof"standingplaces",butitisalwaysconfinedtoaparticularbiotope.Forexample,Thebapisanaisrestrictedtosandybeaches,andwithinthisbiotope,itcanbefoundalmosteverywhere,onplants,understones,etc.Inthiscase,habitatandbiotopecoincide,notasecologicaltermsbutmoreorlessastopographicalareas.Similarly,anthropophilousspecies,suchasLimaxflavus,arestronglyassociatedwithsettlements,whereitcanbefoundunderstones,inthesoil,underrottingwoodordebris.Inthiscase,settlementsaretheappropriatehabitatcategory.Maquis andphryganaareconsideredasonehabitattype"shrublands",sincecertainspeciessuchasHelixcincta,Helicellasp.,andOxychilushydatinusarestronglyassociatedwithbothofthem.Moreover, inmaquis andphryganathesamespecieswerefoundwithoutanydifferentiation(Table2).Assemblingtheinformationforeachspecies(Table2)distributedintheislandgroup,wedefined11differenthabitattypesthatdescribetheenvironmentalheterogeneityoftheareaunderstudy,inrespecttolandsnails.

StatisticalMethodsBecausebothspecies-arearelationshipandchorosmodelarepowerfunctions,byusingthelogarithmofthechorosequationandtheclassicspecies-areaequationweobtainlinearrelationsoflog(S)withlog(K)andlog(A),log(S)=a+blog(K)(eqn1)andlog(S)=c+zlog(K)(eqn2),respectively.We usetheseconventionallogarithmictransformationstoestimatetheparameterszandcforpurposes ofcomparison,sincelinearregressionistheclassicmethodused(Rosenzweig,1995).AllregressionsandtheestimationsofparameterswerecarriedoutwithSTATISTICA5(StatSoft,Inc.Tulsa,U.K).WealsoperformedaStudent'stesttocheckforstatisticaldifferencesbetweentheslopesoftheequations(Zar,1984).ThePearsonproduct-momentcorrelationcoefficient(R2)was usedforthecomparisonofthetwomodels.

137

ResultsForty-fourspeciesoflandsnailswerefoundonthesurveyedislands;42ofthemareextant,belongingto33genera,andtwoextinct.WedidnotlocateanylivingindividualsofPomatiaselegansbutonlytwosubfossilshellsonSkyrosIsland.Thus,weconsiderthatP.eleganshasbecomeextinctfromtheislandgroup.ThoughduringourthreesurveyswesearchedintensivelyinalltheareasfromwhichHelixfigulinawasreportedinthepast(Liebegott,1986)andalsoinallthepotential ~ forthespeciestooccur,noliveanimalwascollected,butonlyahugenumberofemptyshells.We believethatthisindicatesitsrecentextinctionintheislandgroup.We haveexcludedbothspeciesfromouranalyses.Intotalwemade21newrecordsfortheislandgroupandonenewforGreece(Lehmanniavalentiana).Intotalwedefined11differenthabitattypes(Table3).Thedistinctionbetweenthetwohabitattypes"calcareousstones"and"non-calcareousstones"wasmadesinceasignificantnumberofspecieswasfoundunderstonesinareasofSkyrosIslandwherelimestonewasabsent.Thesamewasnotobservedfortherocksorthepilesofstones,sothesetwohabitattypesarestronglyrelatedwiththepresenceoflimestone. Thenumberofhabitattypesineachislandwaspositivelyrelatedwiththearea,10g(H)=0.80+0.11log(A)(R2=0.69,P<O.OOl).Simplelinearregressionof10g(S)asafunctionof10g(K)and10g(A)respectively,gives:

(eqn1)10g(S)=1.06+0.1610g(K)(R2=0.91,P<O.OOl)

(eqn2)log(S)=1.19+0.1810g(A)(R2=0.88,P<O.OOl)

TheR2valueofthechorosmodel ishighercomparedtotheR2oftheclassicspecies-areamodel,indicatingthebetterfitofthechorosmodel.Theestimateoftheslopeforthechorosmodelwaslowerthanfortheclassicmodel,butnosignificantstatisticaldifferencewasfound(P>0.05).

DISCUSSION

ApproachinghabitatdiversityScientistshavelongnotedthesignificant

Table3 Habitattypesusedinthisstudyandtheislandswhereeachhabitattypeispresent.

1. Calcareousstones:Skr,VaI,Sky,Sar,PIa,Eri,Kou,MeD,Lak,AgF,Thl2. Shrublands:Skr,VaI,Sky,Sar,PIa,Eri,Kou,MeD,Lak,AgF,Thl,ExD3. Noncalcareousstones:Skr,ExD4. Pilesofstones:Skr,VaI,Sky,Sar,PIa,Eri,Kou,MeD,Lak,AgF,Thl5. Rocks:Skr,VaI,Sky,Sar,PIa,Eri,Kou6. Litter:Skr,VaI,Sky,Sar,PIa,Eri,Kou,MeD,Lak,AgF,Thl7. Cultivations:Skr,VaI,Sky,Sar,PIa,Kou8. Settlementsorbuildings:Skr,VaI,Sky,Sar,PIa9. Sandybeaches:Skr,Sar10. Pebblybeaches:Skr,Sky,PIa11. Saltmarshes:Skr

138

contributionofhabitatdiversitytospeciesrichness(MacArthurandWilson, 1967;Rosenzweig, 1995;RicklefsandLovette,1999;Triantisetal.,2003;Tewsetal.,2004).Despite this,thelackofacommonlanguageamongscientistsonwhathabitatisandonhowwe canmeasure itinacomparablewaycausesmuchconfusion.

Definitionsof"habitat"intheecologicalliteraturerefertoatleastsevendifferentconcepts(Looijen,1995;1998;Hall etal.,1997;Dennis etal.,2003).Thus,practicalguidancetotherecognitionofanorganism'shabitatislackingandconsequently,habitatsaredescribedwith alackofprecision(Rosenzweig,1995).AsWatt (1971)wrote"ifwedonotdevelopastrongtheoreticalcorethatwillbringallpartsofecologybacktogether,weshallallbewashedouttoseainanimmensetideofunrelatedinformation".Theestablishmentofthiscommonlanguagecanbeaccomplishedonlybyhavingunambiguousandmutuallyexclusivedefinitionsofterms.

EvenifthedefinitionsproposedbyLooijen(1995,1998)resolvetheproblematicuseoftheconceptsofhabitatandbiotope,definingthehabitattypesforacertaintaxonomicgroupinaregionremainsaverylaboriousprocedure.Thedifficultyoriginatesfromthefactthatthebiogeographerandtheecologisthavetopassfromthelevelofthesimplespecieslisttodetailedknowledgeoftheecologyofeveryspeciespresentintheareaunderstudy.

ThefirsteffortofdefiningandclassifyinglandsnailhabitatsinaregionisthatofBoycott(1934),whostudiedthelandsnailsofBritain.Hisapproachwasbasedonspecificcharacteristicsoftheecologyofeachspeciessuchasfood,lifehistory,dispersalandcompetition,butthehabitatshepresentedwereclosertotheterm"biotope"thanto"habitat",asaresultofthefuzzinessintheuseoftheseterms.Henoted,however,thatmanyspeciesoccupiedawiderangeofbiotopes,which containedthesamemicroenvironments. He alsopointedoutthatacollationofdatafromtheadjacentregionswasneededforthedefinitionandclassificationofhabitatsofthelandsnailsinaregion.

Inoursmalldata-set,itisevidentthatareaandthenumberofhabitatsarecorrelated.TheeffectofhabitatindependentofareacanbeseenmostclearlyonExoDiavatis,theonlynon-calcareousislandofthearchipelago.ExoDiavatis showedthelowesthabitatdiversity,andhadthesmallestfauna,despitehavingalargerareathanthreeotherislandsstudied.Itisworthnotingthatanapriorihabitatclassificationbased,forexample,onstandardplantcommunities,would nothaveexposedsucharelationshipbetweenspeciesrichnessandhabitatdiversity.

Insomestudies(e.gRicklefsandLovette,1999;FoxandFox,2000)ahabitatdiversityindex(HD)hasbeenusedtakingintoaccounttherelativeareas

K.A.Triantis,K.Vardinoyannis,M. Mylonas

ofeachhabitatpresent.We havenotusedthisapproach,andhavesimplyenumeratedthehabitattypespresentoneachisland.Thispartiallyreflectstheimpossibilityofanaccurateassessmentoftheareasinvolved,butasinsomeotherstudies(MacArthur,1958;Solemetal.,1981),italsoreflectsthefactthatmeasures otherthanarea(heightoffoliage,depthoflitter,sizeofrocks)maybeequallyorevenmore significant.Moreover, insomeofthesecasesthemeasure oftheareathatahabitattypeoccupiesis ~

Thecharasmodelandthebiogeographyofthelandsnailsintheislandgroup

Ingeneral,thechorosmodel exhibitssimilarbehaviourtotheclassicspecies-arearelationship(Triantisetal.,2003).InthecaseofthearchipelagoofSkyros,bothz-valuesofchoros(0.16)andspecies-area(0.18)models,placethearchipelagointhewithin biogeographicprovincecategory(intraprovincial)asproposedbyRosenzweig(1995).Similarzvaluesarefoundwhenwecomparethediversitiesofdifferentsizedpiecesofaprovince.They resultfromtheaccumulationofhabitatheterogeneitywithin aprovinceaswe samplelargerandlargerfractionsof it.The maindifferentiationbetweentheintraprovincialandthearchipelagiccategoryisthatinthelatter,localextinctionscannoteasilybereplaced,andthesmallertheareaoftheisland,thegreaterthechancethatsuchextinctionwillhappen,andthez-valuewillbecomesteeper(Rosenzweig1995;2003).

Thelowz-valuesforbothchorosandspecies-areamodels forthelandsnailsofthearchipelagoofSkyrosaretheanticipatedresultofthe"recent"formationofthearchipelago(VanAndel andShackleton,1982;Dermitzakis, 1990;PerissoratisandConispoliatis, 2003).This"recent"formationresultsinsmallislandsthatstillbehaveaspartsofacontinuouslandmass,·andconsequently,thereductionofareahasnotyetledtoasignificantlossofspecies(TerborghandWinter, 1980;Karr,1982),with smallislandsexhibitinghighnumberofspecies. Species such as Helix cincta areexceptionallypresentonthesesmallisletswhereasusuallytheyareabsent(VardinoyannisandBotsaris,1990).CharacteristicisthecaseofMesaDiavatis, which holdsbothHelix cinctaandH.aspersa.ThepresenceoftwoHelixspeciesinsuchasmallislandisreportedhereforthe ~ timefromtheAegean.Shortdistancesbetweentheisletsandthelongpresenceofmancontributetothis,astheprobabilityofnewimmigrantsreachingtheislandsishigh,despitethelowdispersalabilityoflandsnails.Hence, zvaluesremainintheintra-provincial,ratherthantheinter-islandrange.

TheprobableextinctionofPomatiaselegansandHelixfigulinaisindicativeofthegeneralpatternofextinctionsoflandsnailsintheAegean islands,

Habitats, area and island snail faunas

from Pleistocene to present. In general localextinctions have been reported from islands or evenislands groups, but these taxa are still present inother islands within the Aegean (Mylonas, 1982;Botsaris, 1996). According to Mylonas (1982; 1984),in the Aegean there is only one case of the totalextinction of a land snail species, that of Zonitessiphnicus. Subfossils of Z. siphnicus are found, inlarge numbers on the islands of Siphnos, Sikinosand Folegandros in Cyclades and its extinction issituated within historical times (Mylonas, 1982).This resistant character of Aegean malacofaunasseems to be the result of two factors: The effectiveadaptation of the species to their naturalenvironment, which retained a wide range ofrefugia that offered successful protection from thePleistocene climatic fluctuations, and the man-madealterations and the spread of introduced species(Mylonas, 1984).The work of Rosenzweig (1995, 2003) introduced

and demonstrated the macroecological nature of thespecies-area relationship and especially of thebiological meaning of the values of the z parameter.The value of z is not just an indication of theisolation of the island group under study butconsists of a macroscopic description of thedominant process or processes establishingbiodiversity and the time scale at which that theseprocesses act. The similar behaviour of the chorosmodel slope with the slope of the classic species-area relationship allows us to ascribe and use theslope of the choros model as a macroscopicdescription of the dominant process or processesestablishing biodiversity and the time scale at whichthat these processes act, as proposed by Rosenzweig(1995; 2003)The importance of the choros model is not just the

better fit of data in most cases compared to theArrhenius species-area relationship (Triantis et al.,2003), but relies on the fact that for the first time amathematical model that describes species richnessand includes habitat diversity is simple andeffective. Our current study demonstrates that,although the relationship between species and areais high, the choros model offers, as in other cases, afurther improvement of fit. This means that thevariation in species richness is better explained bychoros (K) rather than area alone, even though thehabitats supporting the most species are availableon most of the islands studied. Where this is not thecase, choros can be expected to account either for anexceptionally high value of z where the number ofhabitats is highly correlated with size of island, asin the Scilly Islands, south west of Britain (Holyoaket al., in press), or where there is no, or even anegative correlation, and z of the species/arearelationship is not significantly different from zero,as in the Canary Islands (Ibanez, 1994; Cameron,2004).

139

CONCLUSIONSOne of the primary difficulties in the assessment

of the relative importance of area and habitatdiversity in explaining the species-arearelationships is the inadequacy of the existingdefinitions of the term "habitat". We are proposingthat a simple, parsimonious and ecologicallyrealistic approach should be applied in order toquantify the way that each species responds to theenvironmental heterogeneity in a specific area. It isour belief that this method of defining habitatdiversity can contribute to answering questionsrelated to biodiversity, even though ouridentification of habitat types is, as yet, rather crudeand unrefined. Our goal is to refine and standardizethe categorisation of habitats still further.Use of the choros model combined with a more

ecologically realistic consideration of environmentaldiversity will contribute towards the answering ofbasic questions of conservation biology, such as theestimation of the number of species to be expected,and extinction and successful immigration ratesresulting from anthropogenic activities (Simberloffand Abele, 1976; Brooks, Pimm and Collar, 1997)and the design of nature reserves (Diamond, 1972;Soule and Wilcox, 1980; Robinson and Quinn 1992).Habitat requirements and the biological knowledgeon the species to be conserved have beenacknowledged as necessary for the effectiveapplication of island biogeography theory toconservation (Zimmerman and Bierregaard, 1986;Devy et al., 1998, Whittaker, 1998). According toDevy et al. (1998) "for the conservation andmanagement of biodiversity, especially in islandecosystems, the effects of area and habitatcharacteristics in influencing species numbers needto be determined".

ACKNOWLEDGEMENTSWe are grateful to our colleagues R.A.D Cameron,

B.M. Pokryszko, P. Lymberakis, and S.Sfenthourakis for their comments on earlierversions of the manuscript. We would like also tothank Prof. A. Wiktor for his valuable help in theidentification of the slug species. Part of the presentstudy was funded by the Natural History Museumof Crete.

REFERENCESAndreakos, E. (1978). Climatic elements ofthe Greek network

(1930-1975) (in Greek). Ministry of Defense, ClimateDepartment, Athens.

Arrhenius, O. (1921). Species and area. Journal of Ecology,9: 95-99.

Botsaris,1. (1996). The biogeography of terrestrial molluscs onthe islands and islets of Saronikos Gulf (in Greek). PhDThesis, University of Athens, Athens.

140

Brooks, T.M., Pimm, S.L. and Collar, N.J. (1997). Theextent of deforestation predicts the number of birdsthreatened with extinction in insular South-east Asia.Conservation Biology, 11: 382-394.

Cameron, RAD. (2004). From continents to quadrats:species/area relationships in land mollusc faunas.Journal ofConchology Special Publication, 3: 39-54.

Dafis, S., Papastergiadou, E., Georghiou, K., Babalonas,D., Georgiadis, T., Papageorgiou, M., Lazaridou, T.and Tsiaoussi, V. (1996) The Greek 'Habitat' projectNATURA 2000: an overview. Commission of theEuropean Communities DG XI. Directive 92/43/EECThe Goulandris Natural History Museum, GreekBiotope/Wetland Center, Thessaloniki, Greece.

Dennis, RL.H., Shreeve, T.G. and Van Dyck, H. (2003).Towards a functional resource-based concept forhabitat: a butterfly biology viewpoint. Oikos, 102: 417-426.

Dermitzakis, M.D. (1990). Paleogeography, geodynamicprocesses and event stratigraphy during the LateCenozoic of the Aegean area. In A Azzaroli (ed.),Biogeographical aspects of insularity, 8: 263-288. Atti deiConvegni Lincei, Accademia Nazionale dei Lincei.

Devy, M.S., Ganesh, T. and Davidar, P. (1998). Patternsof butterfly distribution in the Andaman Islands;implications for conservation. Acta Oecologia, 19: 527-534.

Diamond, J. M. (1972). Biogeographic kinetics: estimationof relaxation times for avifauna of southwest Pacificislands. Proceedings of the National Academy of ScienceUSA, 69: 3199- 3203.

Fox, B.J. and Fox, M.D. (2000). Factors determiningmammal species richness on habitat islands andisolates: habitat diversity, disturbance, speciesinteractions and guild assembly rules. Global Ecologyand Biogeography, 9: 19-37.

Goodfriend, G. A (1986). Variation in land snail shellform and size and its causes: a review. SystematicZoology, 35, 204-223.

Hall, L.S., Krausman, P.R and Morrison, M.L. (1997). Thehabitat concept and a plea for standard terminology.The Wildlife Society Bulletin, 25:173-182.

Heller, J. (1982) Natural history of Theba pisana in Israel(Pulmonata: Helicidae). Journal of Zoology, 196: 475-487.

Holyoak, G.A, Turk, S.M. and Cameron, RAD. (in press).Land molluscs on the Isles of Scilly: species richnessand composition related to area. Journal ofConchology.

Ibanez, M. (1994). Los moluscos terrestres endemicos delachipielago Canario. Actas X Congreso Nacional deMalacologia, Barcelona, 23-27.

Karr, J.R (1982). Avian extinction on Barro ColoradoIsland, Panama: a reassessment. American Naturalist,119: 220-239.

Kohn, D.D. and Walsh, D.M. (1994). Plant speciesrichness the effect of island size and habitat diversity.Journal ofEcology, 82: 367-377.

Liebegott, A (1986). Die land und Siisswassermolluskender Nordlichen Sporaden (Aegaeis). Mittheilungen desdeutsches malakologische Geselschaft, 39: 1-28.

Lomolino, M. V. and Weiser, M. D. (2001). Towards amore general species-area relationship: diversity on

K. A. Triantis, K. Vardinoyannis, M. Mylonas

all islands, great and small. Journal ofBiogeography, 28:431-445.

Looijen, RC (1995). On the distinction between habitatand niche, and some implications for species'differentiation. Poznan Studies in the Philosophy of theSciences and the Humanities, 45: 87-108.

Looijen, RC (1998). Holism and Reductionism in Biologyand Ecology. The Mutual Dependence ofHigher and LowerLevel Research Programmes. PhD Thesis, University ofGroningen, Groningen.

MacArthur, RH. (1958). Population ecology of somewarblers of Northeastern coniferous forests. Ecology,39: 599-619.

MacArthur, RH. and Wilson, E.O. (1967). The theory ofisland Biogeography. Princeton University Press, NewJersey.

Melentis, E. (1973). The geology of the island of Skyros(in Greek). Bulletin of the Geological Society of Greece,10: 298-322.

Mylonas, M. (1982) The zoogeography and ecology of theterrestrial mollusks of Cyclades (in Greek). PhD Thesis,University of Athens, Athens.

Mylonas, M., (1984). The influence of man: a specialproblem in the study of the zoogeography of theterrestrial molluscs on the Aegean islands, Greece. InA. Solem and AC Van Bruggen (eds), World-wideSnails, 249-260. E.J. Brill, Leiden..

Mylonas, M. (1985) The distribution and ecology of thespecies Helix godetiana Kobelt (Gastropoda:Pulmonata). Biologia Gallo-Hellenica, 10: 139-142.

Mylonas, M., Botsaris, I., Sourdis, I. and Valakos, E. D.(1995). On the development, habitat selection andtaxonomy of Helix (Jacosta) siphnica KOBELT(Gastropoda, Helicellinae). Zoological Journal of theLinnaean Society, 115: 347-357.

Patton, M., (1996). Islands in time. Island sociogeography andMediterranean prehistory. Routledge, London and NewYork.

Perissoratis C and Conispoliatis N. (2003). The impactsof sea-level changes during latest Pleistocene andHolocene times on the morphology of the Ionian andAegean seas (SE Alpine Europe). Marine Geology, 196:145-156.

Ricklefs, RE. and Lovette, I.J. (1999). The roles of islandarea per se and habitat diversity in the species-arearelationships of four Lesser Antillean faunal groups.Ecology, 68: 1142-1160.

Riedel, A (1992). The Zonitidae (sensu lato) (Gastropoda,Pulmonata) of Greece. Fauna Graeciae V. HellenicZoological Society, Athens.

Robinson, G. R. and Quinn, J. F., (1992) Habitatfragmentation, species diversity, extinction and thedesign of nature reserves. In S.K. Jain and L. W.Botsford (eds), Applied Population Biology, 223-248.Kluwer Academic Publishers, Netherlands.

Rosenzweig, M.L. (1995). Species diversity in space and time.Cambridge University Press, Cambridge.

Rosenzweig, M.L. (2003). Reconciliation ecology and thefuture of species diversity. Oryx, 37:194-205.

Simberloff, D. and Abele, L.G. (1976). Islandbiogeography theory and conservation practice.Science, 191: 285-286

Habitats, area and island snail faunas

Sokal, RR and Rohlf, F.J. (1981) Biometry, 2nd edn, WHFreeman and Company, New York.

Solem, A, Climo, F.M. and Roscoe, D.J. (1981). Sympatricspecies diversity of New Zealand land snails. NewZealand Journal ofZoology, 8: 453-485.

Soule, M.E and Wilcox RA (1980). Conservation biology:its scope and its challenge. In M.E. Soule, and B.AWilcox (eds), Conservation Biology: An evolutionary-ecological perspective, pp. 1-8. Sinauer Associates,Sunderland, Massachusetts.

Terborgh, J. and Winter, B. (1980). Some causes ofextinction. In M.B. Soule, and B.A Wilcox (eds),Conservation Biology: An evolutionary-ecologicalperspective, pp. 119-134. Sinauer Associates,Sunderland, Massachusetts.

Triantis, KA., Mylonas, M., Lika, K and Vardinoyannis,K. (2003). A model for species-area-habitatrelationship. Journal ofBiogeography, 30: 19-27.

Triantis, KA., Vardinoyannis, K and Mylonas, M. (2004).Contribution to the knowledge of the terrestrialmalacofauna of the island group of Dionysades(Crete, Greece). Journal ofConchology, 38: 47-56.

141

Van Andel, T. H. and Shackleton, J. C. (1982) LatePalaeolithic and Mesolithic Coastlines of Greece andthe Aegean. Journal ofField Archaeology, 9: 445-454.

Vardinoyannis, K (1994). The biogeography of land snails inthe south Aegean island arc (in Greek). PhD Thesis,University of Athens, Athens.

Whittaker, RJ. (1998). Island biogeography. Ecology,Evolution, and Conservation. Oxford University Press,Oxford.

Wiktor, A. (2001). The slugs ofGreece (Arionidae, Milacidae,Limacidae, Agriolimacidae Gastropoda Stylommatophora).Fauna Graeciae, vol. VIII. Hellenic Zoological Society,Iraklion.

Zar, H.J. (1984). Biostatistical analysis, 2nd edn, PrenticeHall, New Jersey.

Zimmerman, B. 1. and R O. Bierregaard, Jr. (1986).Relevance of the equilibrium theory of islandbiogeography with an example from Amazonia.Journal ofBiogeography, 13: 133-143.