Nordic Society Oikos
Metapopulation Persistence of an Endangered Butterfly in a Fragmented Landscape
Author(s): Ilkka Hanski, Timo Pakkala, Mikko Kuussaari and Guangchun Lei
Reviewed work(s):
Source: Oikos, Vol. 72, No. 1 (Feb., 1995), pp. 21-28
Published by: Wiley on behalf of Nordic Society Oikos
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OIKOS 72: 21-28. Copenhagen1995
in a
ofan endangered
Metapopulation
persistence
butterfly
fragmented
landscape
Ilkka Hanski,TimoPakkala,MikkoKuussaariand GuangchunLei
Hanski,I., Pakkala,T.,Kuussaari,
M. andLei,G. 1995.Metapopulation
of
persistence
- Oikos72: 21-28.
an endangered
in a fragmented
butterfly
landscape.
We describean extensive
metapopulation
studyon theGlanvillefritillary
Melitaea
of1502discrete
cinxia,ina network
habitat
patches,
theentire
comprising
distribution
of thisbutterfly
speciesin Finland.A thorough
surveyof theeasilydetected
larval
groups
revealed
a localpopulation
in536patches
(drymeadows).
Wedemonstrate
that
thissystem
satisfies
thefournecessary
conditions
fora speciestopersist
in a balance
between
stochastic
localextinctions
andrecolonizations.
Patterns
ofpatchoccupancy
severalqualitative
and quantitative
support
modelpredictions.
Withdecreasing
regionaldensity
andaverageareaofhabitat
patches,
thebutterfly
occursina diminishing
fraction
ofsuitablehabitat.
To ourknowledge,
thisis thefirst
conclusive
demonstraofmanynonspecific
tion,basedona comparison
ofdeclining
metapopulations,
habitat
and henceof increasing
occupancy
threat
to survivalcausedby increasing
habitat
fragmentation.
L Hanski,T Pakkala,M. Kuussaariand G. Lei, Dept of Zoology,P.O. Box 17,
FIN-00014 Univ.ofHelsinki,
Finland.
A metapopulation
of manyextinction-prone
local pop- Withtheseresults,supplemented
withtheresultsof a
ulationsmaypersist
in a fragmented
landscapein a bal- moreintensive
studyconducted
in a 50-patchnetwork
ance betweenstochastic
extinctions
andrecolonizationsduringthreeyears,we establish
beyondanyreasonable
(Levins 1969,Hanski1991a). The possibility
of such doubtthatM. cinxiapersistsin Finlandin a balance
metapopulation-level
persistence
has receivedmuchat- between
stochastic
localextinctions
andrecolonizations.
tention
in population
biology(GilpinandHanski1991, We also demonstrate
thattheoccurrence
ofthisbutterfly
McCauley1993) and in conservation
biology(Western initsnaturally
fragmented
environment
is consistent
with
and Pearl 1989,Falk and Holsinger1991,Fiedlerand twoqualitative
predictions
and one quantitative
predicJain1992),buttheempirical
evidencehasremained
con- tionstemming
frommetapopulation
models.
troversial
(Taylor1991,Harrison1994,Thomas1994).
Wehavelittleknowledge
ofhowfrequently
speciespersistinfragmented
landscapes
duetometapopulation
processes,in contrast
to long-term
survivalof local popMaterial and methods
ulations.
Our purposein thispaperis to describethe most Melitaeacinxiawentextinct
on mainland
Finlandin the
extensive
metapopulation
investigation
conducted
so far, late1970s,anditnowoccursinFinland
onlyonthemain
on theGlanvillefritillary
Melitaea cinxia. We havesur- Alandislandanda fewnearby,
largeislands(Marttila
et
veyedtheentireFinnishdistribution
of thisendangered al. 1990).Melitaeacinxiahas twohostplantson the
butterfly
specieswithinan area of 50 by 70 kM2,in a Alandislands,Plantago lanceolataand Veronicaspicata,
network
of 1502discrete
habitat
patches(drymeadows). of whichtheformer
is widespread
butthelatteroccurs
Accepted28 July1994
Copyright
(C) OIKOS 1995
ISSN 0030-1299
- all rights
Printed
in Denmark
reserved
21
OIKOS 72:1 (1995)
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All use subject to JSTOR Terms and Conditions
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Fig 1 Ama o te tuy re,te lad slnd, it c 100 m2oflad itinanara f 5 b 7 k2.Ocupedan ept
er otsuveed bt he d otgeerll hvesutal
pachs reshwnbyblc ad haeddosrepetvey.Smllisans
w hv
ubeto 5 ptceswic
haedrein n henoter prto te ai ilndinlueste
haitt orMcnxa.Th
since1991(Hanskiet al. 1994).Thegridsize is 10x 10 km2.~~C
intensively
studiedmore
in thenorthern
habitat
mostly
partofthemainisland.Bothhost butadditionally
manysmaller
patches(5-25 M2)
whendiscovered.
plantsgrowon drymeadowsofvariouskinds,whichare werealso recorded
The totalsurvey
Themead- timewas2660h. Beforethefieldworkthestudents
thepotential
habitat
patchesforthebutterfly.
were
well delimited
fromthesurroundingtrainedto identify
and describethe suitablekindsof
ows are generally
andexperimental
knowl- meadows.Severalattributes
environment.
Ourobservational
of themeadowswererehabitat
(Kuussaari
et corded,butthepresent
edgeofthebutterfly's
requirements
paperonlyuses information
on
allowsustoreliably theirareasandspatiallocations.
al. 1993,Hanskietal. 1994,unpubl.)
forbreeding
by
identify
themeadowswhicharesuitable
andfollowing
Duringthesurvey,
thedescription
ofthe
M. cinxia.
habitat
thepresence
andthesizeofa possiblelocal
patch,
of M. population
Since 1991,we havestudieda metapopulation
was established
by counting
thenumber
of
inthenorthern
network
cinxialivingina 50-patch
partof larvalwebsin thepatch.Thoughthelarvalgroupsare
themainAlandisland.In thepresent
to relatively
paper,we refer
easyto detectbecausethelarvaespina consomepublished
(Hanskiet al. 1994) and someunpub- spicuousweb(ThomasandSimcox1982),notall groups
in werediscovered
lished resultsobtainedfromthis metapopulation
duringa relatively
rapidsurvey.We
1991-93.
estimate
that35% ofthegroups
wereactually
discovered,
In latesummer
with20 students
the basedon a control
1993,we surveyed
censusbyall students
of a carefully
Alandislandswithinan areaof 50 by 70 km2 (Fig. 1). mapped2-km2
area.All meadows>0.1 ha andrecorded
The aimofthesurvey
was to locateall meadowswhich as empty
(nolarvae)inthemainsurvey
wereadditionally
aresuitableforM. cinxia andwhichare>25 m2in area, re-surveyed
after1-2 weeks.In there-survey,
17% of
22
OIKOS 72:1 (1995)
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All use subject to JSTOR Terms and Conditions
cof
05
c 0.4 n 03
0
0i8i
2
-
(
tg@,00000
Cd
MN,0
@'
1 2 4 8 16 32
numberof larvalgroups
a fragmented
landscapeis due to metapopulation
dynamics, as opposed to local dynamics,one shouldestablish
the following:(1) thatthe habitatpatchessupportlocal
breedingpopulations;(2) thatno single populationis
large enough to ensurelong-termsurvival;(3) thatthe
patchesarenottoo isolatedto preventrecolonization;
and
(4) thatlocal dynamicsare sufficiently
to
asynchronous
make simultaneousextinctionof all local populations
unlikely.Fulfillment
of conditions(1) to (3) eliminates
the possibilityof 'patchypopulations'(Harrison1994)
withouta distinctmetapopulation
and theposstructure,
sibilitiesof mainland-island(Hanski 1991a, Schoener
1991, Harrison1994) and non-equilibrium
metapopulations(Harrison1994), respectively.
distribution
of the sizes of the536 local
Fig. 2. Frequency
1993.PopofM. cinxiainFinlandinlatesummer
populations
ulationsize is expressedin unitsof larvalgroups,withan
shownherearethe Condition1: Populationstructure
averageof39 larvaepergroup.Thefigures
oflarvalgroupsdetected.
We estimate
thatca 35% of The drymeadowswhichare suitableforbreedingby M.
numbers
in thesurvey
theexisting
groupswerediscovered
(see text).
cinxiaoccuras discrete,smallhabitatpatches.The mean,
medianand maximumpatchareas on theAland are 0.13,
0.03 and 6.80 ha (n = 1502). We have previouslyestithesemeadowswerefoundto be occupied,usuallyby matedthatroughly80% of butterflies
spendtheirentire
from
thesefigures
that life-timein the natal patch (Hanski et al. 1994). We
justonelarvalgroup.Weconclude
results
reflect
thepres- conclude thatthe discretehabitatpatchessupportlocal
thesurvey
reasonably
accurately
enceofthebutterfly
on themeadows.
breedingpopulations,thoughthereis substantialmigraA totalof 1502meadowswererecorded,
ofwhich536 tionamongnearbypopulationsand patches(Hanskiet al.
werefoundto be occupied(Fig. 1). Most local pop- 1994). In thisrespectM. cinxia appearsto be similarto
ulationswereverysmall,withonly1 or2 larvalgroups several otherspecies of fritillaries(Ehrlich 1984, Baof larvaeper guetteand Neve 1994, Warren1994).
discovered
(Fig. 2). The averagenumber
groupwas 39 (ina sampleof381 groupsfrom78 meadwithour
ows), whichis a low figurein comparison
results
previous
(ca 70 larvaepergroupin 1991,n= 71).
4
The smallnumberof larvaeper group,and thesmall
number
ofgroupsperpopulation,
bothreflect
thegeneral
declinein thedensity
of M. cinxiain 1992-93,which CO
3
occurred
dueto twosuccessiveunfavourable
summers.
0)~~~~
We analysedthesurvey
results
theentire
bydividing
in
studyareainto2 x 2 km2squares,whichcorrespond
2 _
size to themovement
rangeofthespecies(Hanskiet al. (D
1994;nextsection).Our conclusions
belowaboutpatternsofpatchoccupancy
in thesesquaresareconserva- _o
1_
thatemergeareobserved
in spiteof
tive,as anypatterns
thevariableinfluence
exerted
potentially
bypopulations
locatedoutsidebutinthevicinity
ofthefocalsquare.We o0~
have also analysedthe resultsby dividingthe 1502 CD 0 _
patchesinto126 morenaturally
delineated,
semi-inde- 0
Theresults
ofthetwoforms
pendent
patchnetworks.
of
analysisweresimilar,and we reportonlytheformer
inthispaper.Analyses
results
basedonthesquareshave
-1
O
Gu
Q
-t
the advantage that the size of the region (4 km2) is
constant.
Four conditions
formetapopulation-level
persistence
To demonstrate
thatthepersistence
ofa specieslivingin
2 2 2
5
@D
0
@
1
G
2
3
4
log populationsize in 1991
Fig.3. Comparison
between
localpopulation
sizesinJune1991
andin August1993.The 1991results
refer
to thenumbers
of
adultbutterflies
(estimates
froman intensive
mark-recapture
study;Hanskiet al. 1994),whereasthe 1993resultsreferto
numbers
oflarvae(estimates
froma complete
censusoflarval
groups).Population
sizes(N+1) werelog-transformed.
Thereis
no significant
correlation
between
thepopulation
sizeestimates
inthetwoyears.Multiple
datapointsareindicated
bynumbers.
23
OIKOS 72:1 (1995)
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All use subject to JSTOR Terms and Conditions
in 1993.n1 is the
on extinction
ratebetween1991and 1993andon patchoccupancy
Table1. Effects
ofpatchareaanddensity
in 1991,ofwhichthepercentage
inthe50-patch
network
by1993.Occupancy
number
ofoccupiedpatches
givenby% wentextinct
ina square.In the
ofsquaresandP is themeanfraction
ofoccupiedpatches
wascalculated
forthe4-km2
squares.n2is thenumber
wasmeasured
bythe
case ofoccupancy,
patchareais theaveragepatchareainthesquare.In thecaseofextinctions,
patchdensity
ofpatchareaanddensity
on theprobability
of
aroundthefocalpatch.The effects
ofpatchesin a 4-km2
number
squarecentred
= constant+area+density.
Botheffects
weresignificant
extinction
weretestedwiththelogisticregression
model,logit(extinction)
= 44.76,df=39,p = 0.24). The effects
of averagepatchareaandpatch
(p = 0.016 and0.024;no significant
interaction;
deviance
classesshowninthetable.Both
density
on occupancy
(P) weretestedwithANOVAsonranks,
usingthe4 patchareaanddensity
= 4.21,p = 0.006;no significant
interaction).
effects
werehighlysignificant
(area:F3,251= 5.69,p=0.001; density:
F3,25,
Area
(ha)
<0.01
0.01-0.10
0.10-1.0
>1.0
Patcharea
Extinctions
Occupancy
n1
%
n2
P
4
15
20
3
50
73
30
0
23
138
88
6
0.24
0.24
0.40
0.56
Condition2: Risk of extinctionof the largestlocal
populations
Number
of
patches
(per4 kM2)
1
2-3
4-7
>7
Patchdensity
Extinctions
Occupancy
nj
%
n2
1
1
7
33
100
100
71
36
61
70
58
66
P
0.21
0.32
0.25
0.41
Condition4: Asynchrony
Theintensive
study
ofthemetapopulation
inthe50-patch
In theautumn1993,thelargestlocalpopulation
had69 network
hasrevealedlargely
localdynamasynchronous
larvalgroups(an accuratecount),and only68 popula- ics in 1991-93(Fig. 3), in spiteof a generaldeclinein
tionshad>20 groups(estimated
on theassumption
that densityduringthisperiod,due to two successiveun35% of existing
in thesurvey,
groupsweredetected
as favourable
summers.
described
in theprevious
section).Basedon ourunpub- A degreeofasynchrony
amonglocalpopulations
ofM.
lishedresults
on larvalmortality,
we estimate
thatthere cinxiais maintained
byatleasttwofactors.
thereis
First,
willbe ca 620butterflies
inthelargest
localpopulation
in anobviousinteraction
between
theeffects
ofweather
and
1994 (numberof larvalgroups[69] timesnumberof habitat
qualityinlocaldynamics.
Thus,whiletheexceplarvaepergroup[50] timesoverwinter
survival[0.32] tionallydryand warmsummer
of 1992 causedmuch
timessurvival
frompost-diapause
larvaeto adultbutter- larvalmortality
onthedriest
meadowsonrockyoutcrops,
flies[0.56]).Suchpopulations
arenotsafefromextinc- populations
moister
meadowssurvived
occupying
better
tion.We haveobserved
theextinction
ofa population
of (unpubl.).In 1993,Junewas verywindyand greatly
ca 650 butterflies
inonly2 years(Fig.3). Resultsforthe restricted
theflight
activity
ofbutterflies
on openmeadrelatedBay checkerspot
butterfly
Euphydryas
edithain ows, but windhad less effecton the moresheltered
Californiasuggestan equallyhighriskof extinctionhabitatpatches.Theseobservations
are consistent
with
(Harrison
et al. 1991,Foley1994).We concludethatall themoreextensive
bodyofdatafortheBaycheckerspot
localpopulations
in thepresent
system
areso smallthat butterfly
Euphydryas
edithain California
(Ehrlichand
theyhavea significant
riskofextinction,
hencethelong- Murphy
1987,Weisset al. 1993).
termpersistence
of thespeciescannotbe understood
at
Second,Melitaeacinxiais attacked
bytwospecialist
thelevelof isolatedlocalpopulations.
in ourstudyarea,Cotesiamelitaearum
parasitoids
(Wilkinson)andHyposoter
horticola
(Grav.).Theseparasitoidsimposesubstantial
mortality
onthebutterfly,
butthe
Condition3: Recolonization
levelofparasitism
variesfromonepopulation
toanother
We conducted
an intensive
mark-recapture
studyof M. (froma fewpercentto >50%), becausetheparasitoids
cinxiain the50-patchnetwork
in 1991 (Hanskiet al. themselves
exhibit
distinct
metapopulation
dynamics
and
1994).In thisstudy,1731 butterflies
weremarkedand areoftenabsentfromhostpopulations
(G. Lei, unpubl.).
released.Weobtained
741 recaptures,
ofwhich9% were Specialist
parasitoids
maygenerally
havea strong
impact
froma new patch.The mean,medianand maximum on fritillaries
(Porter1983).
distances
movedbythemigrants
betweenpatcheswere
In summary,
we haveshownthatM. cinxiainFinland
590,330 and3050 m,with20% ofthemigrants
having satisfies
thefournecessary
conditions
formetapopulamoved>1 kmbutonly3% havingmoved>2 km.Among tion-level
persistence.
We therefore
attribute
its longthetotalof 1502 patches,themeannearest-neighbour
termsurvival
on theAlandislandsto a balancebetween
distanceis 240 m (median128 m,maximum
3870 in). stochastic
localextinctions
andrecolonizations.
The sigwe mayconcludethatthepatchesarenottoo nificance
Therefore,
ofmetapopulation
dynamics
in theoccurrence
isolatedto prevent
recolonization
(Fig. 1) and thatthis ofM. cinxiaon theAlandis further
highlighted
bysupmetapopulation
does notexemplify
thenon-equilibrium
typediscussedbyHarrison
et al. (1991).
24
OIKOS 72:1 (1995)
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All use subject to JSTOR Terms and Conditions
1.0
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0
o
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00
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1.0
1.5
ofpopulations
lognumber
theaveragepatcharea(log-transformed)
inthe4-km2
and(b)
squares(a) against
size(log-transformed)
population
Fig.4. Average
size was measured
in thesquare,omitting
of localpopulations
squareswith<12 patches.Population
bythe
againstthenumber
ofpopulation
of averagepatcharea,t= 1.35,NS; effect
model:effect
of larvalgroups.Resultsof a multiple
regression
number
number,t= 4.4 1,p = 0.0001; R2= 0.38, n = 30. Analogousresultsforall squares,regressionweightedwiththenumberof patchesper
square: patcharea, t= 2.62, p = 0.01; populationnumber,t= 6.34, p < 0.0001; R2 = 0.25, n = 136. - We noteherethatwhendata for
ofpatches
affected
bypatchareabutalsobythefraction
localpopulation
sizewas significantly
individual
patcheswereanalysed,
withthenumber
ofpatchesintherespective
modelweighted
regression
square.In a multiple
square,
occupied,
P, intherespective
forpatcharea(log-transformed)
andP were9.22 (p<0.0001) and6.87 (p<0.0001),
coefficients
thet-valuesof theregression
(R2= 0.17).
respectively
frommetapopulationmeadowdeclinewithincreasing
distance
from
portto threepredictions
thesource
stemming
population
modelsandto whichwe shallnowturn.
(Hanskiet al. 1994),hencecolonization
rate
mustalso do so. Second,theextinction
rateincreased
withdecreasing
patchdensity
(Table 1), apparently
because local population
Effectsof patcharea and densityon patch
size tendsto be smallerin the
moreisolatedhabitat
patches(Hanskietal. 1994;Fig.4).
occupancy
Theseresultssupport
therescueeffect
(BrownandKoA fundamentally
of all metapop- dric-Brown
important
prediction
1977),whichhereoperatesat themetapopulation
models(Levins1969,1970,Hanski199Ia) is that ulationlevel (Hanski 1991a, Hanskiand Gyllenberg
thefraction
of occupiedpatchesat a stochastic
steady 1993,HanskiandZhang1993):withdecreasing
P, the
withregional
state,
denoted
byP, increases
patchdensity, existing
populations
are smaller(Fig. 4) and are hence
becauseincreasing
recoloniza- expectedto havea higher
patchdensityfacilitates
riskofextinction.
tion;and thatP increaseswithaveragepatcharea,because largerpatchestendto support
largerlocal pop- Testof modelpredictions
ulationswitha smallerriskof extinction.
We shallfirst The effects
of averagepatchareaandpatchdensity
on
demonstrate
thatthepremises
ofthesepredictions
aremet patchoccupancy(P) were analysedusingthe 4-km2
by thepresentsystem,
thentestthepredictions
them- squaresas replicates.Both effectswere statistically
selves.
highlysignificant
(Table1).
Assumptions
In theintensively
studied
50-patch
we recorded Two othermodelpredictions
network,
22 extinctions
and 6 colonizations
between1991 and The core-satellitehypothesis
1993.Extinctions
exceededcolonizations
apparently
be- Metapopulation
modelsincorporating
therescueeffect
causeofthegeneral
declineindensity
thisperiod. predict
during
that,
fora rangeofparameter
values,thedistribuThe smallerpatchestendedto havesmallerpopulations tionof the patchoccupancyfrequency
P is bimodal
of M. cinxia(legendto Fig. 4), and as expected,the (Hanski1982a, 1991a,Hanskiand Gyllenberg
1993).
extinctionrate was higherin the smallerpatches Previoustestsof thebimodalcore-satellite
distribution
(Table1).
have used data frommultispecies
assemblages,
which
Withonly6 colonizations
between1991and1993,we createsproblems
ofinterpretation,
andtheprevious
studcouldnotconfirm
theexpecteddecreasein colonization ies mostly
deal withinappropriate
spatialscales(Hanski
ratewithdecreasing
patchdensity
(increasing
isolation). 1982a,b, GotelliandSimberloff
1987,GastonandLawButtheeffect
ofisolation
oncolonization
rateis strongly ton 1989). The spatialscale shouldcorrespond
to the
impliedbytwootherresults.
Firstandmostobvious,the movement
rangeof thespecies.Thomas(1994) has renumbersof migrantsoriginating
froma particular cently
reported
bimodaldistributions
ofP valuesin two
25
OIKOS 72:1 (1995)
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All use subject to JSTOR Terms and Conditions
at an appropriatespatialscale, but
species of butterflies,
thenumberof patchnetworks(and henceP values) in his
studywas verysmall.
7
The bimodal core-satellitedistribution
of P values is
generatedby the rescue effect,which we have demon6
stratedto operate in M. cinxia (Table 1). Additional
supportfor the rescue effectis providedby a positive
co)
I
(D
relationshipbetweenaverage size of local populations
and thenumberof local populationsin the4-km2squares
04
(Fig. 4). Of thefourrecognizedexplanationsof positive
0
abundance-distribution
relationships (Hanski 1991a,
Hanski et al. 1993, Lawton 1993, Nee et al. 1991), of
whichFig. 4 is an example,all otherexplanationsapart
frommetapopulation
dynamicswithrescue effect(Hanski 1991a) can be excludedin thiscase forthefollowing
reasons.First,thehypothesisof ecological specialization
(Brown 1984) is not applicablebecause our studydeals
withonlyone species. Second,thehypothesisof varying
0
1i
averagecarrying
capacityofpatches(Nee et al. 1991,see
also Hanski 1991b) amongthesquarescan be excluded,
fraction
of patches occupied,P
because therewas only a weak or no effectof average
patcharea on averagepopulationsize (Fig. 4). And third,
Fig. 5. The frequency
distribution
of thepatchoccupancy
frerelationdeviations
quencyP inthe4-km2
squares.Standard
(lines)were the hypothesisthatthe abundance-distribution
obtained
thecalculations
100timesforgridsdis- ship is a samplingartefact(Wright1991) can be exbyrepeating
placedsystematically
at200-mintervals
inbothmaindirections. cluded,because ourresultsare based on nearlycomplete
Whilecalculating
theP values,smallpatches<500 m2were populationcounts.In summary,the assumptionsof the
becausetheiroccupancy
is erratic
omitted,
(Table 1) and be- core-satellite
hypothesis(Hanski 1982a) are met.
causethesmallpopulations
havelittleinfluence
on thedynamWe testedthecore-satellite
hypothesiswiththeP valics ofthemetapopulation.
Squareswith<5 patches(?500 M2)
werealsoomitted
becausetherespective
P valuesareunreliable ues calculatedforthe4-km2squares,whichcorrespondin
andarelimited
to a fewpossiblevalues.
size to the spatial scale of movementsof the butterfly
(previoussection).As predicted,thedistribution
of theP
values in the4-km2squaresis distinctly
bimodal(Fig. 5).
8
10
I
O'
1-
0.8
co
-00
o
9co
0.4
)
0.8
-_
04
0
0.2
0.0
-
b
a
02
0.2
0.4
0.6
predicted
P
0.8
1.0
0.0
o.2
0.4
OBe
0.8
1.0
predicted
P
Fig.6. Comparison
between
thepredicted
andobserved
patchoccupancy
P inthe4 x 4 km2squaresinthenorthern
frequencies
part
ofthemainAlandisland.(a) Predictions
basedona logistic
regression
model,inwhichpatchoccupancy
is predicted
bypatcharea
only.Thereis norelationship
between
thepredicted
andobserved
P values(R2= 0). (b) Prediction
basedonthedynamic
incidence
function
model(Hanski1994a).Thereis a highlysignificant
relationship
between
thepredicted
andobserved
values(p= 0.001,
R2=0.56). In bothcases,thehorizontal
linesgive thestandard
deviation
for10 independent
predictions.
Bothmodelswere
parameterized
withdataobtained
from
the50-patch
in 1991,roughly
network
corresponding
tooneofthe4 x 4 km2squares,
inthe
northern
partoftheAlandisland(Fig. 1; thissquarewas omitted
fromthetest).Theparameter
valuesoftheincidence
function
modelarereported
inHanski(1994a).Eachpatchwas assignedan additional
5% probability
ofcolonization
pergeneration,
when
tomodellong-range
empty,
migration
fromoutsidethemetapopulation
(thisis thought
tobe a realistic
value,butwe do nothave
actualdatato estimate
thevalueofthisparameter).
26
OIKOS 72:1 (1995)
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All use subject to JSTOR Terms and Conditions
A quantitativeincidencefunctionmodel
survivalis possibleonlyin networksof habitat
long-term
Plantago lanceolata and Veronicaspicata, whereasonly
theirtraditional
land use practices.
The limitedsuccess of thequantitative
incidencefunctionmodel in predicting
patchoccupancyis bothencouragingand sobering.We are clearly a long way from
being able to predictthe consequencesof habitatfragmentationforthe persistenceof species in quantitative
terms.One particularly
worrying
aspectis thatthemodel
predictionsassume a stochasticsteadystate,which the
real metapopulations
in changinglandscapesmay never
have timeto reach (Hanski 1994c, 1995). Qualitatively
correctpredictionsmay be thebest we can achieve,but
even such predictionscan be valuable forconservation
biology,where the questionoftenis to select the best
optionout of two or morealternatives.
a newapproach patches, in which the local dynamicsare sufficiently
described
Hanski(1994a,b) hasrecently
witha general- asynchronousto make simultaneousextinctionof all lotomodelling
ofmetapopulation
dynamics
whichallowsonetomakequan- cal populationsunlikely.
izedincidence
function,
in particular This sortof situationappearsnotto be exceptionalin
aboutpatchoccupancy
titative
predictions
Warren(1992) has recentlyanalysedtherate
The modelis describedin detailby butterflies.
patchnetworks.
on naturereservesin the
estimates
for of extinctionsof rarebutterflies
parameter
Hanski(1994a),whoalsoreports
fromthe intensively
studied50- UK between1960 and 1982. Even thoughmanyof the
M. cinxia,estimated
in thenorthern
partof themainAland reserveshave been managedfor some particulartarget
patchnetwork
extinctions
have been rampant:all the 16 popbutterfly,
islandin 1991.
modeltopredict
theP ulationsof Lycaena dispar, Maculinea arion, Carterofunction
We usedtheincidence
valuesin 4 x 4 km2squares,insteadof the 2 x 2 km2 cephalus palaemon and Mellicta athalia on naturereof patchespersquare serves went extinct,and in Hesperia comma and Lysquares,to havea largernumber
morereliableP values.Furthermore,
to in- sandra bellargusaboutone quarterof thepopulationson
andthereby
effect
ofpatchessur- reserveswentextinct.Thomasand Harrison(1992), Thocludethemetapopulation
dynamic
the4 x 4 km2 squares,themodelwas actually mas and Jones(1993), Thomas (1994) and Hanski and
rounding
theresult Thomas (1994) reviewotherdata on Britishbutterflies
in6 x 6 km2squares,
iterated
forpatches
though
of isolated populafortheinner4 x 4 km2squareonly.Model indicatingthe extinction-proneness
was recorded
withall patchesoccupied,
andthe tions,butalso thepersistenceofbutterflies
in networks
of
iterations
werestarted
resultswereobtainedfromgeneration
100, whenthe favourablehabitatpatches.The clearmessagefromthese
had practically
reacheda stochastic studiesis thatlocal populationsof butterflies
are notsafe
metapopulation
fromextinction,
not even on managedreserves,and not
steadystate.
withsubstan- even forthemodestperiodof 20 years.We suspectthat
function
modelpredicted
Theincidence
tialsuccesstheP valuesinthenorthern
partofthemain similarconclusionswill also applyto manyotherspecialfrom ist,rareand endangeredspecies, of whichmuch less is
network
Land island(Fig.6), aroundthe50-patch
In con- knownthanabout butterflies.
whichtheparameter
valueshadbeenestimated.
We foundforMelitaea cinxiathatthefraction
model,which
trast,a non-dynamic
logisticregression
of occupredictsoccupancyon thebasis of patchareas only, pied habitatpatchesdeclineswithdecreasingpatcharea
predictions
(Fig.6).
generated
entirely
unsuccessful
and with decreasingpatch density,in agreementwith
The good newsthusis thatthemodelsuccessfullymodel predictions(Levins 1969, Hanski 1991a, b). To
in thepartof thestudyarea ourknowledge,thisis thefirststudybased on a comparipredicted
patchoccupancy
in son of many nonspecificmetapopulationsto confirm
fromwhichtheparameter
valueshadbeenestimated
1991.The bad newsis thatthemodelfailedto predict thesepredictions.
The extinction
ofM. cinxiafrommainin thesouthern
partsof theAlandis- land Finlandin the 1970s (Marttilaet al. 1990) is most
patchoccupancy
lands.This failuremaybe due to someenvironmentallikelyrelatedto thedecreasingdensityof suitablemeaddifferences
betweenthedifferent
partsof theAlandis- ows on mainlandFinlandduringthepastdecades (Palkas
lands.For instance,
thehabitatpatchesin thenorthern1993). Fortunately,
drymeadowsare stillcommonon the
partof the mainislandhave bothlarvalhostplants, Aland islands, where farmershave largelymaintained
P. lanceolataoccurselsewhere.
Alternatively,
patchocsteady
cupancymaynotbe veryclose to a stochastic
in different
partsof
state,andpossiblymetapopulations
theAlandislandshavebeenperturbed
at different
times
andto different
A morethorough
directions.
analysisof
thesequestions
is inpreparation
(Hanski,Moilanen,
Pakkala andKuussaariunpubl.),
butto resolvetheseissues
without
dataforseveral
conclusively
maybe impossible
years.
Discussion
Our resultsdemonstrate
beyondany reasonabledoubt
thatthelong-term
ofMelitaeacinxiain Finpersistence
landis basedon a balancebetweenstochastic
local extinctions
andrecolonizations.
Alllocalpopulations
areso
smallthattheyhavea substantial
riskofextinction,
and
- Wethank
Acknowledgements
S. Harrison,
T. Kawecki,
S. Nee,
J.Lawton,
E. Ranta,C. ThomasandP.Turchin
forcomments
on
an earlierversion
ofthismanuscript.
We acknowledge
thecontributions
madeby thestudents
whoparticipated
in thefield
work:S. Airaksinen,
P.Ala-Opas,J.-P.Backman,
P. Hellstedt,
J.
E. Huitu,
N. Kangas,M. Koivula,S. Kokkonen,
Holopainen,
M.
27
OIKOS 72:1 (1995)
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All use subject to JSTOR Terms and Conditions
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