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Selection of Islands for Conservation in the Urban Archipelago

of Helsinki, Finland

Ranta, P.

Blackwell Publishing

1999

Ranta, P. et al. 1999. Selection of Islands for Conservation in the Urban Archipelago of

Helsinki, Finland. Conservation Biology 13: 1293-1300.

http://hdl.handle.net/1975/257

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, Selection of Islands for Conservation in the urbanArchipelago of Helsinki, FinlandPERTTI RANTA,*$ ANTTI TANSKANEN,T JARI MEMEki,* AND ARTo KURTTo+*Depaftrnent of Ecology and systematics, P.o. Box 17, universiry of Ilelsinki, FIN-OO01,i, Helsinki, FinlancliMetsitihti Ltcl. Flitsaaiankatu l, FIN-0081O, Helsinki, Finlancl*Rotanical Musettm, Finnish Museum of Naruril History, P.o. Box 7, University of Helsinki, FIN{O0I.(, Helsinki, Fi.land

Abstract: Tbe occurrence of uascular plants u,as sttrtteyecl on 207 islancl-s (size range O.O I -390.2 lta, numberof pla't species I -449) olfsbttre from tbe city ttJ Hetsinki in the Baltic Sea to examine tbe conseruation uolueof these i-slancl-s. We ceilculatecl a rariet score for eacb species (1,/nrtmber of istancts occupied by tbe species)ancl a biorlit'ersity score for eacb islancl (sum of tbe rarity scores of eacb species present on the istand). posi-tiue correlations betzueen species number ancl biotliuersitl score (r, - 0.97, p < 0.OOI) ancl betuteen biocliuer_sity score ctncl islancl area (r, : o.87, p < 0.0o1) inrticated tlrat tbese paranreters are bectuily depenclent on is-land size- Vitlt tbe goctl of including at least one occurrence (island) of atlplant species, an iteratit)e selectioneilgoritbnt chose a set of 1l klands ubose auernge size (29.3 ba) uctsfour times tbe auerage size of alt exisring iskrncls (7.o bct). strong nesteclne.ss (N < 54) explctirts tbe concentration of phnt species diuersity on largeislanrls. An operatioltctl strate5gt for selection of sites fc,tr protection is to complement tbe set proctucecl by a se-lectictn algoritbm uith target species not ).tet includecl (e.g., enctangerecl species uitlt seueral occtrrences).('itmprebensiue mc.ppirtg and analysis of a taxonomic gror.tp ruitl belp integrate conserrtcttion biotoglt insolancl-use plrtruting ctnd increase tbe qualiql of tbe netuorks of protected areas.

Selecci6n dc Islas para C-onservaci6n en el Archipielago lJrbano cle Flelsinki, f inlandia

Resurnen: Lluestreattos la presencict de ptctntas uasculares en 2O7 islas (rctrtgo de tamaiio 0.01-390.2 lta,ntimero .le plant.$ I 7tr9) en las axtas de Ia ciuclacl cte llelsinki, en el ntctr Btiltico para examinar el t,alorrle cr-tnseruaci6n cle estcts islas. Calculctntos un inclice cle rareza [tctra cada especie (I/ntintero cle islas gc1pa-rlas prr la especie) y un fnclice de bioeliuersitlael para cacla islet (suma cle los inclices de rareza cle catla especiepresertte en let i'sltt) Cctrrelacictnes positittct.s entre ntinrcro de especies )/ el inclice cle bir,tcliuersidact (r, : 0.97,p .< O.00 1) -y el inclice de biodiuersiclact y el drea de kt islct (r,:0.g7, p < {).001) inclicaron que estosparaimetros srtrt ctltctttente dependientes clel tcttrtttiio cle la isla. Con la meta cle tnc[ttir al tnenos Lrne {)crftTen-cict ('iskt) para todas las especies cle plantas, un algoritmo cle selecci6rt escogi6 1l islas, cuyo tamailo proTne-clio (29.3 hct) fue cuutro ueces el tanaiio promedio de todas los islcts (7.0 bet). Lafuerte aniclaci6n (N : 5i)e'rplicct ltt cctncetilraci6n cle Ia cliuersiclact de e-species cle plantos rttt islas granrles. [Jnct estrategia operaciortctlpcrt'et la selecci'n de sitirts porct su protecci1n cottplententarfct et jttego cle sitios pr(.)ducido prtr Ia selecci6n 4ealgoritmo cort especies ltletnr:et arin no inclufrlets (e.g., especies arnertazculcts cttn ttctrias octrrrencias). Eltnapeoc()mf)ret.siLto y el uncilisis cle ttn gntpo tctxctn6ntico pt>clria ayuclc:tr a integrar h biotr,tgia tle Ia conseructcionen lo pl.tttedL'itin de t.rco del -suelo e incrententdr Ia cnlitlad cle Iss recles cle tir.eas protegiclas.

\ettail pcrtt i rant({,Ailt"tsat(rltt i inetfi, jari.tr ietneluQDl.telsinl: iJiI '(tper sul)tnittedJtttte tl, l9()l l : ret,tsed ndnuscript accepted ,lnr(b I0

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1294, le lectknof lsknulsf tn 'Consertut ior t .

Introduction

A cnrcial issr.re in planning representative networks of prc>tected areas is how to select the sites. Several selection al-gorithms have been published (e.g., Nicholls & Margules1993; Pressey et al. 1993, 1997;Underh-ill 1994; Viilliams etal. 1996; Freitag etal. 199D, but usually the selection unitshave been gpid cells of equal size and the algorithms havebeen mclstly applied at national or regional level. Only afe.w local-level applications have been offered (Game & Pe-terken l9tt4; fkpoport et al. 1986). Furtl-rennore, selectionmeth()ds have been developed and tested in floristicallyrich areas with many endemic species, such as southern Af:rica and Ar-rstralia (Margules et'nl. 1988; Rebelo & Siegfried7992, L<>mbard et al. 1997; Pressey et al. 1997).

In the species-poor northern boreal region, on theother hand, there are no globally important hotspots ofendemism. For instance , the entire flora of Finland doesnot include a single endemic vascular plant species. Ourfocus is on protccting nationally or locally sig.nificantbiodiversity. To achieve this there is a need to developmethods and tools for local conservation-oriented plan-ning because municipalities, the basic land-use planningunits, frequently include biocliversify conservation intheir planning principles.

The purpose of our study was to establish which is-lands of a group of 2O7 in an archipelago would need tobe protected to maintain the diversity of vascular plants.This represents a fypical planning situation in which realland units instcad of grid cells are used. The selection wasbasecl on a comprehensive database of vascular plants.

Methods

Study Area and Databases

Thc archipclago offshore from the ciry of Helsinki in thee astern Baltic consists of 315 islands (Ciry of Ilelsinki Ur-ban Facts 1998). The 207 islands included in our study(size range O.Ol-390.2 ha) comprised practically all thc"real" islands inside Helsinki city limits; small rocks protruding fiom the sca were excluded because they lackvascular plants. Most of the islands studicd are uninhat>itecl, but scveral large ones closc to the mainland havepermanent human settlement. The t>ccurrence (pres-ence ahrsence) of vascular plants on the islancls wasmappccl by the author A.K. and his collaborators be-twe( 'n | 99O rrnd 1995.

Islands in the Baltic have risen from the sea since thelast g,f aciati<.rn. Land uplift of approximately 2.5 mm/yearrnakcs ncrv islands appear continuollsly ancl existingoncs expancl. 'fhe islands stuclied consist of Precambrianbedrock with a thin layer of topsoil. The natural yegeta-

tion on ttre larger islancls rypically consists of lorv Scr>tsp i ne ( P i r t t.ts .rylrre.s/rz.s) f orest with heather (C ct I I tt tt ct t, t.t I -

( - ( ) n s ( ' r ! r t r ( ) n l J i o l o g !

\ o l u r n c I J N o ( ) . I ) c ( : c m b c r 1 9 9 9

Rnnto et al.

gctris), bilberry (Vacciniunt myrtilhts), and corvberry(.Ycrccinium uitis-iclctea) as dominant spccie s in the fielcllayer. In protectecl bays, common alder (Alttus gluti,nosa) groves are found with more luxuriant vegetation.Small islands lack fbrests and are characterized by stonyshore-meadows and patches of meadow veEietation indepressions and rock fissures. Bare rock sudhces arecommon on exposed shores on both large and small is-lands. The ecological f'eatures of the stucly area are com-parable to those of other archipelagos in tl"re Baltic (As etal. 1992).

Calculations

We calculated a rarity score for each plant species: l/cr,{ci ( ) O, 1 < i - nl, where c, is the number of islandsoccupied by species z. The biodiversity score of an is-land was the sum of the rarity scores of all species occur-ring on it.

Several algorithms have been developed for reserveselection (Saetersdal et al. 1993; Lombard ct al. 1995;Csuti et il. 1997; Pressey et al. l99D- Common to allthese algorithms is that they start from an empfy selec-tion, adding new areas in the most effective way untilthe predetermined goal is reached. For instance, thegoal may be to include all species at least once.

We used an inverted method to select a near-minimumset of islands that includes all vascular plant speciespresent in the archipelago. The algorithm "mt)st com-mon" (MC) starts with all the islands includecl and eliminates one island in every cycle until the selected stoplevel has been reached. Initially, MC calculates the nlrm-ber of occurrences-the number of islands on whichthe species occurs-for every species (step l) ancl iden-tifies the rarest species on each island (step 2; Fig. l).The algorithm then ranks the islands according to thenumber of occurrence of the rarest species. Other spe-cies on each islancl are ignored (step 3). At this stalae,each island is represented by onty one species, the rarest.Thercafter, MC linds the island with the rarest specieswith the highest mrmber of occurrences (stcp 1r). Thc rar-est species on each island is the species with fewest oc-currences. The MC then starts eliminating islancls wherethe number oi occurrences of the rarest species is high-est (condition 1). This island may be interpreted as leastveluable. The eliminating order follows the number ofocclrrrences of species as the number of islands in-cluded graclually decreases (step 5). The MC recalculatcsthe frequencies of all species on the rcmahing islirndsand ag,ain identifies thc rarest species on each islancl.

Cyclical elimination continues until the rarest specieson e:rch of the remirining islands occurs on cinly one is.lancl. This means every island in the set has at least oneunique species (conditi()n l). Setting, the stop level tOzero procluces a ntrmeric:rl orcler <>f all islends. -fhis

is an

Ranta et al.

Step ' l

Step 2

Step 3

Rank islands according to

Step 4Select the island that haslargest A/

l t oI

End

F-igure l. Flotu diagram of tbe algori.tbm "most corn-

tnon" for reserue selection.

advantage of MC that makes it possible to compare is-lands according to the relativc rarity of their specics.

'fhe stop level may be set to any nonnegative vallle.

For instance , stop level onc procluces a set of islandsthat contains al l species at least once. I f species occuron f 'ewcr than or ()n a number of islands equal to the se-lectcd stop level, those islands wil l be auromatical ly in-cluclcd in the near-minimum set. The MC also triives thenumeric:rl rank <lf those islands not inchlded in the near-minimLlm set (Fig. 1). I f the rarest species have the samenumber of occurrences on sever:r l is lands, the islanclwith the lclwest biodiversity score will be chosen (step 1l).

Othcr eliminirtion rules coulcl also be applied, sr-rch as thelargest island (lowcr c<tst of establishing protection) ormost rem()tc island (maximization of continuous area).

-Io investigatc nesteclness among species assemblages

we uscd the inclex N (Pattcrson & Atmar 198(r), whichme tsures how r"t ' ruch the observed pr-esencc-absence ma-trix differs from pcrf'ect nesreclness (N : O)

'I'he maxi-

mum value of ,,V (i.c., miJ'rimum nesteclness) clepcncls onthe size of the matrix ancl . thc ntrmber of specics prcscnt.

Selection of IslandsJbr Consen:ntioil lZ95

For the analysis, we diviclecl the islands into four sizeclasses: 0.Ol-0.O9 ha (28 isl:rnds), O. l-O.99 ha (83 is-lancls), l.O-9.9 h" (26 islands), and > t O.O ha (20 islancls).The analyses were also made without size classcs.

Results

Occurrence and Nestedness of Species'We

made 22,364 observations of 686 vascular plant spe-cies (hybrids were excluded) on rhe 207 islands (l-441)species/island). Most of the species were rure in ourclata; about 400 species occurred on (20 islands (< lO%of islands), and the number of species occurring only onone island was 108. Rare species were clearly concen-trated on a small grclup of islancls: I I islancls hostedmore than 10O rare species each. Common species werefew in number, with only 2O species occurring ()n m()rethan 155 islands (Fig. 2).

The island-specific biodiversity scrtre varied from 0.1to 4O.3. Fifty islands had a biodiversiry score of {l.t)(Table l). The value of the biodiversiry score depencleclboth on the rarity of species on an island ancl on the is-land's species richness. Because one unique species h:rsthe same weight as IOO relatively common species (oc-cuffing on )lOO islands), score values increased rapidlywith the prcsence of rare species.

Occurrence of the plant species showed strong nest-edness (N : 51) The value of the index N wotrlcl be 64Oif the plant species were randomly distributed amongthe islands, and maximum r\/: 1056. The spccies assem-blages of the small island size classes were incltr<ieclwithin the l;rge island size classes, and rare spccics oc-curred almost exclusively on islands falling into the larg-est size class. Without size classes, nestedness was weaker(N : 46,975), and maximum ly' : 119,640. 'I-his was clueto a great number of small islands of almost the same sizeand only slight variation of species cornposition.

Selection of illininrum Sets of Islands

"t!lost' colfi]toN" At coRIlHM

Forty-one islands were neecled in a minimum sct ()f is-Iands that included al l the plant species at leasr once(Table 2).

- fhe sct represents 2O\% of the 207 islancls

studicd but as much as 83% Q2OO t ha) of rhe roral is-lancl area.

' Ihe average size clf an island incluclecl in the

minimum set (29.3 ha) was over four t imes the tveraseislancl size among t l l 2O7 (7 t) ha).

Because the preservation of a single occurrence of Ispecies does not guarantee i ts lclng-tcrm survival, mir-r i-mlrm sets of islands rvere establ ishecl for 1-178 occtrr-rences (number of islands occupiecl) for cach spccies.Thc total island arcu inclucled in the minirrrtrnt ser ln-

( l o n s e n x i i o n l l i ( ) l o g \

V o l u n l e I J . N o 6 , I ) ( : ( ( r n l ) ( r I ( ) 9 9

Find the rarest species onevery island and mark thenumber of occurrences

Remove that island andits observations

ls /v greater inan thepredetermrned stoplimit

7296 Selection oJ Islands lbr Cintsenntion

150

100

50

0

O 20 40 60 80 100

Number of islands (%)

Figure 2. ()ccttrrence of uascular plant species on tbe

islancl-s stttcliecl. Most species are rare; otter 4OO species

-qr(ilu o/t only l0% or less ctf ctll islancl.s.

Table l. Distribution of land area, number of species, and sums ofbiodiversity scores among size classes of islands offshore tromHelsinki.

' Ranta et al.

Table 2. The 1rl islands off Helsinki included in the minimum setselected by the "most common" algorithm for reserve selection andslr other islandso selected by target species (threatened species).

Islctnd

Islctnclarea(ba)

Nurnberof plant Biodil)ersity Lctnd-usespecies sc()re tlpno

oo

d 250

o

Q)

€ zooEf,

z

Picni Leikosaari*KuminapaasiMatalaharaKoirasaari*PintiriKalkkipaasiHarmaja*KorkeakupuPi-kku

Lehclessaari

Malkasaari

Jiinissaari"TiirasaariLuoto (Klippan)

Pikku Kuivasaari*Vallisaaren

PukkisaariPihlaialuotolJunisanretIso LeikosaariPikku-MustaKotiluotoRysikariHylkysaariKalkkisaariL:insi-MustaKuivasaariReposaariItainen

PihlalasaariIte-villinkiI-entinen

PihlajasaariMelkkiI-ammassaariPikku NiinisaariMustikkamaa"HarakkaVasikkasaariKorkeas:tariKuninkaansaariKust:ranmeikkaPalosaariSeurasaariSusisaariVillinkiIsosaariIso MustasaariVartiosaari

SantahaminaVallisaari

6.8453.4o

254276

z

82 .42 12./+1 . 82 .4

3 33 .53.63.91+.O

1 0

4 . 5t l . /

4 . /

1 a5 .76.47.O7 .37 34 .78.89 0

9 19 . 1

1 0 3lo .8l l . ot t . 21 2 811r.21 4 7I z+.8

r4 .91 5 01 5 . 3lB .31 9 12 t . u22.02 4 q

a t 73 8 910.2

1 . 1 6 9 7o.52 592.52 81+3 .21 r )63.60 93o.29 7a1 .84 l l 5o.53 54

1.60 l4 l3.44 r413.90 1603.r4 167r.80 r39z.Oa 185

1.76 1603."4 I 852.20 t575.26 r924.oo 1783.36 r85

10.04 2254.oo 2233 13 1818.50 24(J

l 1 3 3 2 4 42.lo 206

pripubrespubpubpubmilpub

pnpubmilpnpubmil

milpubpubpripub (sf)pubmilpubpripubmilpri

pubmil

pubmilpubpri * pubpubrespfipubmilpub (sf )pripubpub (sf)pri + pubmilpub (sf)pri + prrbmilmit

Size clcts,s(Lrct)

Nurnber Totrll etrea Numberof islancls (ba) of species

Sum ofbiocliuersity

.scores

19.OO 26740.50 298

8.70 24a3 1 . 8 8 2 5 737.50 301

8.80 30717.91+ 29i25 .30 26 i37 .20 32233.50 275

2.O0 19540.60 31033.50 3 r O

l3/+ 50 3757J.Ji 3t524.40 27089.00 352

i90 20 411)/ o . / u l r )

0 0 l - 0 0 90. I -0 .99

1 0 - 9 9> I0 . t )Total

2a83/ t )

20201

l . u38.6

239 4r 172 .4r152 .2

t62J23555648686

8 7o / . )

259 6aiO.z(ru(r.0

creascs rapidly to almost IOO% as thc numbcr o[ prc-cletermined species occurrcnces incrcases (Fig. 3). Forinstancc, a mrinimum set of islands consisting of morethan 20 occr,rpiccl islands for each species inclttcled prac-tically the e ntirc islxn(l arca but only apl)roximately 150

( . o n s c r v e t r o n l J i o l o g v

V o l u m c l J . N o ( ) . I ) ( ( c r n b c r 1 9 9 9

" These six otber isknttls ttre rnarked u,itb lrt aslerisk." [.and-use t')tPe: pu.b, pL,blic rrred: res, ,tdtt.,r'e reserre; pri. prit'dte

rtrea; sf, Sttcrnteti l inna sea.fortress; trt i[, nri l l tct4'or co.tst gltord ored

is lands. This implies that the islands not included in this

minimum set were small and had few plr lnt 5pccics.

On thc othcr hand, the nttmber of occurrcnccs of

nrany plant species remains belorv the sclccted nunlbcr

Ranta el al.

19 22 27 30 33 38 41 44

Selection stop l imit

of occupied islands because they do not occur on somany islancls. Conse<1uently, the increasc in the numberof islands selected does not have a positive effect on thefuture persistence of very rare species in that a speciesoccurring on a certain number of islands will not occuron more islands regardless of the number of islands se-lected for protection.

SELEC'TION BY TARGET SPECIES

To examine the useftllness of the most coftunon algo-rithm (MC) we compared sets of islancts selected by itand sets selectecl by other merhods. A simple method ofselecting sites for protection is to use tarl;et species. Wemade the selection using two kinds of pre-defined targetspecies: ( l) uniclue species (108 species with only oneoccurrence) and (2) narionally or provincially threat-ened species (l1r spccies; Rassi et al. l99Z).

Unique species were fbund on j7 islands. -fhe five

most unique spccies-r ich islands had nearly hatf (53) ofthc 108 species, and they were also among the most spe-cics-r ich islands. Because islands with a unique specieswerc al l t()matical ly included in the minimum set of , i l ,

only fotrr of the mininlum set did not harbor unique spc-cies.

' fhus, select ion using MC with stop level I and se-

Iecting the islancls harbrtring unique species gavc nearlythe same set ol. islands

Selecting islands based on the occurrence of threat-encd species produccd a somewhat cl i f ferent set. The l4threatcned specics occurred on 24 islancls, lg of whichrverc the samc zrs in the minimum set procltrcecl by MC.Threatened species lvcre fotrnd mostly on Iarge , species-r ich islancls. The combinarion of the two target speciesgroups as a sclcct ion cri terion proclucecl a set of 1r3 is-lancls with 6 cl i f fcrent islancls than were in thc minimumset procluced bv NI(. ( ' l 'able 2).

.leleclion of lslands jtr Cotnen'ation llgj

Figure 3. Cumulatiue increase inarea and number of islands at selec-tion stop limitsfrom I to 178 (mini-mum sets uitb I - 178 occu.rrences).

BIODI}'NRSIfi SCORN, ISLAND SIZE, OR SPECIf,S NUMBER

The sum of the biodiversity scores of the minimum setof zrl islands selected using MC was 466.2, (-B% of thevalue of the whole archipelago. Comparing the MC-pr<>duced set of 4l with a set of the same number of islandswith the highest biodiversity score revealed that 32 is-lancls are the same in each group. Thus, selectior.r usingthe biodiversify score produces a set fairly similar to thatusing the algorithm.

Other criteria for selection might include islancl sizeand species richness. A comparison between the set of4l islands procluced by MC and the 4l largest islands re-vealed that 25 islands are in both groups. A similar com-parison between the MC set and the 41 rnost species-rich islands revealed that 3l are the same in each set. Inboth comparisons, pracrically all of the 2O highest-rankecl islands are also included in the minimum set pro-duced by MC.

The observation that island size, species richness, andbiodiversity score produced a set of islancls similar tothat produced by NIC indicates the high correlation be-tween these variables. Species number and island sizeccrrrelated posit ively (r, : 0.88, n - 2O7,p < O.OOI), asdid island size and biodiversiry score (r. : O.87, n:207, p < 0.001). Fufthermore, there was a posit ive cor-relation befween species nnmber and bioclivcrsity scorc(r, : 0.97, n - 2O7, p < O.OOI). Ihcse correlat ions turdicate that the conserwation val l te of an island increaseswi th i t s s ize .

Discussion

[,cological Features Important for Conservation Planning

()trr study clcmonstratcs that biodivcrsity score, islanclsize, ancl species rich-rcss reflect the conselvation valuc

(-onscar ' l l ion f l i i ) largy

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'lo0

90

\o- 8 0o

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t 0

L298 ,\election of lsktnds for C:onsen'ation

of the islancls studied fairly well. But these parametersdo not provide information about species compositionand are therefore not usefirl critefia fbr conservation ev'.rl-uation by themselves. The use of species identities in con-servation planning is thus prcferable.

Furthermorc, the strong nestedness among vascularplants in the Helsinki archipelago implies that species-richislands also harbor the rarer species. Nestedness amongspecies assemblages is a common feature on islands @at-terson 1987, 1990; IJlake 1991; Cutler 1991, 1991+: Sim-trerloff & Martin 1991; Vright & Reeves 1992; Atmar &Patterson 1993; Cook 1995; Kadmon 1995), but itscauses are often unclear (Yiming et al. 1998). In the Hel-sinki archipelago, nestedness of species is to a large ex-tent explained by the geological history of the islands.As islarid size increases constantly due to land uplift, pio.neer commllnities invade the islands as soon as suitablehabitat appears, and the same cofiununities are present:rlong the shores of the larger islands. Large islands alsocontain species that occur in habitats that do not existon smaller islands. Colonization from the mainland or

,'---\,1- / \_-/-� ' t ,, '

\r,--_

.' ' ' t '-. )

Helsinki

Rattta et al.

other, larger islands could therefore be thc nrain fhctorthat has generatecl this nestedness (Patterson 199O; Cook& Quinn 1995).

Strategy fbr Conservation Selection

The "most common" algorithm used here was testedwith the Finnish Bird Atlas data (250,000 observations,3,8O0 grid cells of l0 X l0 km). The algorithm for.rndabout the same number of grid cells as the simple greedyalgorithm I (fype 1) presented by Csuti et al. (1997), butthe set was different (Tanskanen 7996a, 1996b). Usingplant data from the Flelsinki archipelago, the prog,ressiverarify algorithm 7 (Csuti et ltl. 1997) produced a ser ofexactly the same number of islands as did MC (41 is-lands), only one island being different. The sea-kale(Crambe maritima) occurred only on these two islands;consequently, one of them had to be inclucled in the set.

The minimum set of 1r1 islands that included all thespecies at least once serves as a roug;h guide to where

I:igure 4. Tbe group ctf islctttcls se-

lecterl for conserucr.tion (in blctck) irt-

clucles tbe 41 islancls of tLte mini-fitum set ancl 6 otber isla.ncls tttith

tbreatenecl species. Tbe nurnbered is-lcrnds (l-i) tuitb clense urban settle-

,nent u.tere not ittcludecl itt tLte stucly

areo.

0 r 2 3 4 5--fr-

( . . ) n s c o i r t i ( ) l r l } r ) 1 1 ) ! l i

V , ) l L r r l c I i , N o 6 . I ) ( ' ( ( m f r c r l 1 ) ( . ) r )

]ldttta et ttl.

the essential plant diversity exists in the Flelsinki archi-

pelago. The selection methocl needs several ref ine-

ments, however, to procluce results that can be applied

in conseryation planning. The first consideration is the

ncecl for f i l ter ing of species. Iror example, the occur-

rence of peanut (Aracbis hypogae(t) is cleady irrelevant

for conservation purposes because it is an accidental in-

trocluction and has no viable populations on Baltic is-

lands. Peanut and similar species should be f i l tered otrt

bef-ore the analYsis.

Another essential ref inemcnt is thc inclusion of desir-

able target species. Because the minimum set may in-

clude only one occurrence per species, i t does not nec-

essarily include all the islands on which target species(e.g., threatened species) occur. A solut ion may be a

post-selection of the islands harboring target species but

not included in the minimum set. A procedure for the

selection of islands or other sites for conservation could

thus proceed as follows. First, irrelevant species are re-

moved from the database. Second, the selection algo-

rithm selects a set of islancls with the recluired number

of occurrences per species. Third, occurrences of target

species are ensured by post-selection.

Following this procedure, we selected a network of

4l islands produced by MC on which each species oc-

curred at least once. Thcreafter, an adclitional six islands

harbrlring endangered species not selected by the algo-

rithm were incluclecl in the proposed network of prr>

tectcd islancls (Fig. 4). Several of the islands lbund by

the algorithm are protected fbr reasons other than their

ecokrgical value (e.g., the World Heritage site of Suomen-

linna Sea Fortress). Species found on these already pro-

tectecl islands could be used as a starting point in seek-

ing islands that add ncw species to the combination.

This proceclure follows a pragmatic selection strategy,

combining reserve selection algorithms with other re-

serve-design criteria (Beclward et al. 1992).

In an archipelago off a large city with healy recre-

ational use such as Helsin-lii, a network of protection that

covers one-fifth of the islands, and as much as four-fifths

of the total area, woulcl hardly be realistic. For maintain-

ing biodiversiry, however, there is no need to protect en-

tire islancls or to exclllde other rypes of land use from the

islands selected, if essential habitat patches are pre-

served. 1'hese patches may cover only a small fraction of

the island area. For example, a habitat-based plant map-

ping was done on the islancl of Isosaari (73 ha). -l'he

is-'and

rvas inch-rcled in the minimum network (355 spe-

ics, 7 of them trniclue ancl 3 threatenecl), but the habitat

rtclrcs support ing thcsc species reprcsented only about

1 oi the t()ta[ area of the islancl. Lf a comprehensive hat>

map is not available , thc list of habitat specialists with'ow sizc range coulcl be usecl for the identil-ication of

nost important habitats f irr prcltect ion.

r study emphasises the need tbr comprehensive,

prel i ty ecokrgical clatab:rses for conservati()n evalu-

,\electiorr of Ishnds.f'or Consen aliln 1299

ation. Unfortunately, these are relatively rare, especiallyfor larger areas. Furthermore, as mentioned above, thereare strong arguments that each habitat patch shoulcl alsobe analyzed separately (Deshaye and Morisset l9tJ8;Worthen 1996). Experienced professionals are needed tocarry out the fieldwork, which may take several f-ield sea-sons. This requires a sllbstantial investment in researchwork, but the total cost of comprehensive mapping of ataxonomic group (e.g., vascular plants) in a city like Hel-sinki (and area ltlT kmz) is modest compared with in-vestments needed in urban infrastructure planning ingeneral. Consequently, the fundamental issue is rccog,ni-tion of the importance of the incorporation of conserva-tion biology into the pl:rnning process ancl the accep-tance of the associated costs.

Acknowledgments

This stucly was flnancially supported by the Ciry of FIel-sinki Environment Centre and the Finnish BiocliversityResearch Programme (grant 39715). We :rre thanklirl toL. Helynranta for help in the field and to J. Kotze for con-structive coffrments on the manuscript.

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