Spatial distribution and species composition of larval Odonata in the artificial reed community established as a habitat for Mortonagrion hirosei Asahina (Zygoptera: Coenagrionidae)

2009·2.8 MB·English

by  Iwata, S

Odonatologica38(4):307-319 December I,2009
Spatial distribution andspecies composition
of larval Odonataintheartificial reed
communityestablishedas ahabitatfor
Mortonagrionhirosei Asahina
(Zygoptera: Coenagrionidae)
S.Iwata¹and M.Watanab²e*
'GraduateSchoolofEnvironmental Sciences,UniversityofTsukuba,Tsukuba,
Ibaraki 305-8572,Japan;— [email protected]
"Graduate SchoolofLifeandEnvironmental Sciences,UniversityofTsukuba,
Tsukuba,Ibaraki 305-8572,Japan;—[email protected]
ReceivedAugust13,2008/ReviewedandAcceptedFebruary 18,2009
Yearlychangesintheodonatelarvalcommunityweresurveyedfor4yraftersetting
upanartificialreed communityadjacenttotheoriginalhabitatofMortonagrionhiro-
sei,anendangeredbrackish watersp. OnlyM. hiroseilarvaewerefoundintheorigi-
nalhabitatduringthesurveyperiod.Inthefirstyroftheestablished habitat,Ischnura
senegalensiswasthedominant sp. inthe larvalcommunity.Althoughtheabundance
ofM.hiroseilarvaeincreased yearbyyear,becomingthemostabundant sp.afterthe
secondyear,the speciescompositionofthelarval communityoftheestablished hab-
itat wasdifferent betweenthe WestandEast because ofenvironmental factors,such
assalineconcentration andreed shoot density.M.hiroseilarvaehadexpandedtheir
distribution tothe entireareaoftheestablished habitat in 2005,while thedistribu-
tionofI.senegalensishad beenrestricted toseveralpatchyareasin accordancewith
adecreaseintheirpopulation.Therewasanegativerelationshipbetweenthenumber
ofM.hirosei andI.senegalensislarvae. Theprey-predatorrelationshipandcompeti-
tiverelationshipbetweenthe2spp.shouldhaveaffectedthepopulationdynamicsand
distribution ofM.hirosei. The odonatelarval communityand habitatenvironment
that isoptimalforM.hiroseiconservation arediscussed fromtheviewpointofboth
biotic andabiotic factors.
INTRODUCTION
Dragonflies havebeenusedasbioindicatorsbecauseoftheirspecies composition
andspeciesrichness beinginfluenceddirectlyby theenvironmentalconditionof
*CorrespondingAuthor:[email protected]
308 S.Iwata&M.Watanabe
wetlands(WATSON, etal., 1982;CHOVANEC, 1998).Acertainareainhabitedby
manyodonatespecies tendstoberegarded as“asuitablehabitatfordragonflies”
becauseof high species richness anddiversity(STEWART& SAMWAYS, 1998;
CHOVANEC, 1997).CHWALA&WARINGER(1996) clarifiedthatspeciesrich-
ness increases atponds with abundantaquatic andsemi aquaticplants usedby
dragonflies for oviposition ratherthanatponds shadedby trees withoutaquatic
vegetation. On the otherhand, HIGASHI & WATANABE(1998) foundhigh
species richnessinartificialponds locatedattheupperstreamofrice paddy fields
whichhadbothopenandclosedenvironments, becauseodonateadultsthatpre-
fereachenvironmentcan coexist.Thus, bothlarvaeandadultsofdragonflies are
dependentonthewaterenvironment,aswellas theaquaticandaerialvegetations.
Mortonagrion hiroseihas been designated as oneof the endangered odonate
species (IUCN, 2004). Theadults inhabitthe understoryof densereedcommu-
nitiesestablishedin brackish water. In Japan, about30 local populations have
beendiscovered, mainly onHonshuIsland, andallofthemarelocatedinestuar-
ies(MATSUDA etal.,2002). Inrecent years,various developmentprojectssuch
as shoreprotection, reclamationworks andriver improvements, havedisturbed
muchofthenaturalbioticenvironmentofestuarines, includingreedcommuni-
ties,andthusthehabitatsofM. h/rose/(SOMEYA, 1998)aredecIining(HIROSE,
1985; HARA, 2000). Intensiveconservation projectsareessentialforpreventing
thelocal extinctionofthis species.
In 1998,an isolatedsmallhabitatof M.hiroseiwasdiscoveredin theestuarine
near Miyagawa River, Mie Prefecture, Japan, in an area where a construction
project forasewagetreatment plant was going tobeundertaken(WATANABE
& MIMURA, 2004). Inordertopreserve the localpopulation ofM. hirosei, a
reedcommunity was artificiallyestablishedon the abandonedrice paddy fields
adjacent to the original habitatby transplanting reedrhizomes in January2003
(WATANABE& MATSU’URA,2006).
Beforethetransplantation, theabandonedrice paddy fieldshadbeenkeptas a
shallowopenwater environment, withIschnurasenegalensis, I.asiatica, M. selen-
ionandseveralanisopteran species inhabiting (WATANABE & MATSU’URA,
2006). Becausetheseodonatespecies are predators or competitors forM. hiro-
seiin bothlarvalandadultstages(HIROSE & KOSUGE, 1973;NISHU, 1997;
INOUE & TANI, 1999),allowing theirinvasiontotheestablishedhabitatmight
affectthelocalpopulation dynamicsofM. hirosei.POLLARD(1991) statedthat
species-specific odonatelarvaldistributionsare correlatedwithwater quality. In
orderto preventodonatespecies except for M. hiroseifromentering to thees-
tablishedhabitat,thewaterenvironmenthadtobemanaged andthesalinewater
supplied at aconcentrationaboutthe sameas thatof the brackish water inthe
originalhabitat(MATSU’URA& WATANABE,2004).Thesalineconcentration
ofbrackish water has beenkeptat 5 to 15%oinordertoexcludeI. asiaticaand
M. selenionfromtheestablishedhabitatbecausethey hadlittletoleranceagainst
Distribution oflarval Odonatainareed community 309
15%osalinity (IWATA & WATANABE, 2004). However, I.senegalensis seemed
to have a certainsalinetoleranceandto beableto inhabitabrackish water en-
vironment.Therehavebeenmany reports showing thatseveral Ischnuraspecies
haveas highasalinetoleranceforbrackishwateras M. hirosei(FOX&CHAM,
1994). KEFFORD(2006) reported thatI.heterostictaacceleratestheirlarval de-
velopmentmoreunderacertainconcentrationofsalinewaterthaninfreshwater.
IWATA& WATANABE(2004) also showedthatboth M. hiroseiandI. senega-
lensishavealmostthesamesalinetoleranceofupto 15%o. Therefore,excluding
I.senegalensis fromtheestablishedreedcommunity iscriticalformaintainingthe
localpopulation ofM. hirosei.
Becauseoftheworld widedeclineofwetlandareas, theconstructionofarti-
ficialaquatic habitatshavebeenconductedas compensation forecologically de-
graded regions (MOORE, 1991;CHOVANEC & RAAB, 1997). BANG (2001)
reported theodonatespecies composition ina constructedwetlandandtherela-
tionship betweenodonatecommunitiesandenvironmentalfactors whichdeter-
minetheirspecies richness. Elowever, therehavebeenfewstudiesfocusing onthe
conservationof asingleodonatespecies. Although thereare severalprojects for
the habitatconservationof M. hiroseiin Japan (YAMANE et al.,2004), plan-
ningandmonitoring basedon quantitative datahavenot yetbeen reported.
Inthepresentstudyweclarifiedthechanges ofspecies composition inthelarval
dragonfly community including M. hiroseiin the artificiallyestablishedhabitat
from2003to2006.Theresults will representthe dispersionprocess ofM. hirosei
fromtheoriginal habitat,andsuggesttheinterspecificrelationsamongthecoex-
istingodonatespecies intheestablishedhabitat.Anevaluationoftheestablished
habitatforM. hiroseiisalso discussed.
MATERIALANDMETHODS
STUDY SITE - TheoriginalhabitatofM.hiroseiwasareedcommunitylocatedinIsecity,Mie
prefecture, Japan(34°29’N,136°42’E),The habitatwas430m2andcoveredwith densereedshootson
shallow brackishwater(WATANABEetal.,2002)thesalinityofwhichwas6.7%oonaverage from
April2003toMarch2005.
Theestablished habitat,acreated reed community(2065m2) wasadjacenttothesouthernpart of
theoriginalhabitat. Thewatersalinitywasartificially maintainedbythe3infallssupplyingthebrack-
ish water. However,thesaline concentration and reed densitywerenotuniform inthe area, prob-
ably due torainfall and differencesofmicro-topography.We dividedthe established habitatinto 3
subareas fromwest toeast,accordingto differentenvironmental factors,such asreed density,wa-
terdepth,and salineconcentration. Thewesternsubarea(A)was725m2andcomprised3infallsfor
brackish watersupply.Theeastern subarea(C),which was 570m2, had severalsmallareasdried up.
The middle subarea (B)was770m2, andhad anintermediate environmental conditions between A
and C.MATSU’URA &WATANABE(2004)reportedsaline concentration wasthe highestatthe
western partoftheestablished habitatandthelowest attheeasternpartonaveragefromApril2003
toMarch2005,thoughtherewasnoconsiderabledifference after2006.The meansaline concentra-
tionin2006were4.0%o,4.7%o,and4.7%oatthewestern,themiddle andtheeastern partoftheestab-
lished,respectively (unpublished),while itwas2.1%oattheoriginalhabitat.
310 S.Iwata&M.Watanabe
In2003,reed densityattheoriginalhabitat wasabout 20shoots/625cm2(25cmx25cmquadrate),
andthe meannaturalheightofreedwas 130cmduringtheflyingseasonofM.hirosei. Ontheother
hand,thereed densityattheestablished habitatwasabout 17shoots/625cm2,wherethemeanheight
was40cmshorterthanthatin theoriginalhabitat,indicatingthattheestablished habitat ofthefirst
yearwasopen. In2004and 2005,theestablishedreed communityhaddevelopedand theshootden-
sityhad reached about40shoots/625cm2inwholearea,while itwas25shoots/625cm2intheorigi-
nalhabitat.Since partialmowingwasconducted intheautumnof2005bothattheoriginaland the
established habitat,thereed densitywasabout 19shoots/625cm2in both habitats. However,there
wereafewsmall openpatcheswith lowreed densityinsidetheoriginalhabitatin 2006.
INSECTS — Generally, odonate species have aso-called maiden flightimmediately aftertheir
emergenceleavingthewaterandspendingseveraldaysin forestsorgrasslandsuntiltheyaresexually
matured (CORBET, 1999).However,M. hirosei do notshowthe maiden flightactivity and stay in-
sidethereed communitywheretheyemerge.Theyusuallyspendalmostalloftheirlifetimeperching
onreed stemsorleavesat 20cm abovethe watersurface (WATANABE& M1MURA,2004).They
flytoshiftto anotherperchingsite5timesanhour andeachflightis lessthan30cm,indicatingthat
theirhome range ofactivityis verylimited. I.senegalensisadultspreyonM. hirosei adultsalongthe
edgeofthereedcommunity(NISHU, 1997),andthe otherodonatespecies,suchasOrthetrum albi-
stylumcould be thepredatorfor M.hirosei. Thoughthere have been fewreportontheactivitiesof
M.hiroseilarvae,mostlarvaetend toperchonsubmergedlitternearthewatersurface.
FIELD SAMPLING - We setup 5 quadrates(0.25mx0.25 m)in theoriginal habitat and 10
quadratsin each subarea,asshown in Figure 1.We sampledodonatelarvaein theoriginaland es-
tablished habitatsinNovember2003,MayandNovember2004,Mayand November2005,andMay
2006(6samplingdates).
Since theonsetoftheflightseasonofM.hiroseiislateMay,samplinginearlyMayfacilitated the
assessment ofthetotaladultpopulationthatemerged.Inaddition,becauseyoung larvaeofthespe-
cies thathibernated attheegg stagecould be sampledin May,theentireodonate larvalcommunity
inthe established habitatwascomparablewith thatin theoriginalhabitat. Thedifficultyofcollect-
inglarvaeduetotheirsmallbody sizemightalsoaffectthedatafortheNovember samplings.Thus,
weusedthesamplingdatain November fortheanalysisoflarval distributionespecially forthespe-
ciesthathibernated atthe larval stage.
Fig. 1.Mapofthestudyareaincludingtheoriginalhabitat andtheartificialestablished habitat.Each
dotshowsaquadrateforsamplingOdonatalarvae.Three opensquaresindicatetheinfallsthatsup-
plybrackishwatertotheestablished habitat. A,BandC showthewestern, middle and eastern side
oftheestablished habitat,respectively. Stripedrectanglesindicateresidences.
Distribution oflarval Odonatainareed community 311
RESULTS
Intheoriginal habitat,onlyM. hiroseilarvaewerefoundinthespringsampling
for3 years(Fig. 2).In theestablishedhabitat, incontrast, I.senegalensis was the
most abundantspecies foreach subarea in2004. In subareaA, theproportion
of M. hiroseipopulation in thelarvalcommunity was less than 10%,indicating
a few M. hiroseiadultsenteredandlaideggs fromthe original habitatin2003.
In2005, thenumberof M. hiroseilarvaedrastically increased andthenM. hiro-
seirepresented themost abundantspecies. In2006, only M. hiroseilarvaewere
collectedinsubareaA, indicatingthatthespecies compositionbecamethesame
as inthe original habitat.In subarea B, although thenumberof M. hiroseilar-
vae occupied 40%ofthelarval community in 2004,/. senegalensis was themost
abundantspecies. Thenthenumberof M.hiroseilarvaeincreased,andbecamethe
mostabundantspecies in2005, whilesmallI. senegalensis population remained.
Thespecies composition ofthelarvalcommunity in2006was similarto thatin
2005, withthecoexistenceofbothI. senegalensis and Sympetrum spp.Only one
M. hiroseilarva wascollected in subareaC in2004, indicating thatless M. hiro-
seifemalesenteredto thissubarea foroviposition compared to the othersubar-
eas. Consequently, the species composition in 2004was mainlydominatedby I.
senegalensis andI. asiatica despitethatI. asiatica prefer to fresh water environ-
ment. In2005,thepopulationof M. hiroseilarvaeincreasedto60%inthe larval
community, whichwas the lowestamong thesubareas. In2006, the numberof
M.hiroseilarvaeincreasedandoccupied 90%ofthelarvalcommunity, although
severallarvaeofSympetrum spp. were collected.
Fig.2.YearlychangesinspeciescompositionofOdonatalarvaeinthespringcommunityintheestab-
lished habitatand theoriginalhabitat. The numerals indicatethemeannumber oflarvaecollected.
312 S.Iwata&M.Watanabe
Thediversity index (Shannon-Weaver H’) of each subareaand the original
habitatwere calculated.Becauseasingle species ofM.hiroseiwas foundduring
3 yearsinthe original habitat, theindex ineach yearwas 0.The diversity index
insubareaAwas 0.51 and0.46in 2004and 2005,respectively, anddecreased to
0 in2006.On theotherhand, theindicesofB andCin2004werearound 1 and
fluctuatedduring 3 years.
BecauseM. hiroseiis aunivolutinespecies, thelarvaeinearly May belong to
the same generation as thosein thelast November. In the original habitat, the
mean densityoflarvaecollectedateach quadratewas0.6inNovember2004and
1.6inMay 2005 (Tab. I). Such discrepancy was also foundbetween November
2005(2.0)and May2006(8.6). Themean density oflarvaesampled at subareas
B and C was significantly lower than that in the original habitat in November
2003. Theautumn sampling in 2003mightindicatethatfemaledispersal inthe
established habitatfromtheoriginal habitat in thefirst yearwas stillrestricted.
Thesimilardistributionfoundin the autumn sampling in subareas AandC in
May2004 suggested thatthedistributionofM. hiroseiwas stillpartialinthees-
tablishedhabitat.After2004,there were no significant differencesin the mean
density of M. hiroseilarvaecollectedbetween theoriginal andestablishedhabi-
tatsexcept for subareaA inMay 2006, suggesting thatthe larvaehad inhabited
withalmostthe same density.
Themeandensity ofI. senegalensis larvaedecreasedwithyear(Tab. II).In the
original habitat, I.senegalensis was notcollectedexceptforNovember2004,prob-
ably as a consequenceof partial mowing. Inthe establishedhabitat, the mean
density of the larvae was significantly larger than inthe original habitat until
November2004.However, thesedifferencesdisappearedafterMay 2005, whilea
TableI
The numberofM.hiroseilarvaesampledperquadrateinthe establishedhabitatcomparedwiththat
intheoriginalhabitat(±S.E.).A,B andC meanthe western,middle,and eastern side oftheestab-
lishedhabitat,respectively
Established habitat
Samplingdate Originalhabitat
A B C
2003 November 1.3±0.4 (4) 0.2±0.2 ( 4) 0.210.2* (8) 0.010.0* (4)
2004 May 6.0±0.8 (5) 0.610.5**(10) 4.8±3.0 (10) 0.110.1**(10)
November 0.6±0.4 (5) 1.4±0.6 (10) 1.511.1 (10) 0.510.3 (10)
2005 May 1.6±0.5 (5) 2.811.4 (10) 5.613.4 (10) 2.310.9 (10)
November 2.0±0.7 (5) 4.3±1.7 (10) 7.713.4 (10) 7.013.0 (10)
2006 May 8.610.8 (5) 3.5il.0**(10) 5.712.8 (10) 8.510,8 (10)
():Number ofquadratssampled
* and**showthenumberoflarvaethatwasdifferentfromthat intheoriginalhabitatwiththeprob-
abilityofless than0.05 and0.01,respectively,usingthe Mann-WhitneyU-test.
Distribution oflarval Odonatainareed community 313
smallnumberofI. senegalensis larvaewerecollectedinthe establishedhabitat.
Changes inthedistributionofbothM. hiroseiandI.senegalensis larvaeinthe
establishedhabitatwereexaminedusing them*mmethod, wheremis themean
numberof individualsand m* is the mean crowding (LLOYD, 1967). IWAO
(1968)showedthatm*lm representstherelativeaggregation pattern. Whenm*lm
is lessthan 1,thedistributionis uniform.Whenm*!m isunity ormorethan 1,it
meansrandomoraggregated distribution, respectively.
AsshowninFigure3,m'*/mforM. hiroseiinMay2004washigh,indicatingin-
tensive concentrateddistribution, whilem*/mforI.senegalensis was 1.81,which
mightindicate arandomdistribution.In fact, M. hiroseihad beenfound inonly
afewquadrates andI.senegalensis haddistributedwidely intheestablishedhab-
itat.Afterward,M. hiroseifemalemightdisperse andoviposit aroundtheentire
areaoftheestablishedhabitat,resulting inlowm*/mofthelarvae,whilethatof
I.senegalensis larvaehadbeenincreasing. InMay 2006, them*lm forM. hirosei
and7senegalensis was 2.0and6.6,respectively. Itwas revealedthatthedistribu-
tionof M.hiroseihadbecomerandom, while7.senegalensis, incontrast,showed
intensiveconcentrateddistribution.Inparticular, m*/mofM. hiroseisharply de-
creasedfromMay to Novemberof2004. Becausem*lmin Novemberreflected
theoviposition areabythefemales, thefemalesinthesummerof2004musthave
dispersed andoviposited over theentireareaofthe establishedhabitat.
AsshowninFigure4,therewasanegative relationship betweenthenumberof
M. hiroseiand7. senegalensis larvaein subarea A(Spearman p-test,p = 0.007).
Similar tendencieswereobservedinsubareas BandC,though therewere nosig-
nificantcorrelations.The distribution and population size ofI. senegalensis in
theestablishedhabitatmightaffectthoseof M. hirosei.
TableII
Thenumber ofI. senegalensislarvaesampledper quadrateintheestablished habitatcomparedwith
thatintheoriginalhabitat(±S.E.).A,Band Cmeanthewestern,middle,andeastern sideofthees-
tablished habitat,respectively
Established habitat
Samplingdate Originalhabitat
A B C
2003November 0.0±0.0 (4) 9.215.6* (4) 11.314.0*(8) 0.311.1* (4)
2004May 0.010.0 (5) 4.712.4*(10) 7.012.2*(10) 7.911.6*(10)
November 0.2±0.2 (5) 0.810.4 (10) 4.8il.7*(10) 2.010.8 (10)
2005 May 0.0±0.0 (5) 0.310.3 (10) 0,110.1 (10) 0.310.2 (10)
November 0.010.0 (5) 0.010.0 (10) 1.110.7 (10) 0.610.3 (10)
2006 May 0.010.0 (5) 0.210.2 (10) 0.610.5 (10) 0.210.1 (10)
():Numberofquadratessampled
*shows thenumber oflarvaethat wasdifferent fromthat intheoriginalhabitatwith theprobability
oflessthan0.05,usingthe Mann-WhitneyU-test.
314 S.Iwata&M.Watanabe
DISCUSSION
Theoriginal habitatof M. hiroseiisa brackish water environmentseldomin-
habitedby odonatespecies except fora few species that are salinetolerant. In
addition, densereedshoots in theoriginal habitatprevent odonateadultsfrom
entering the reed community (MATSU’URA & WATANABE,2004). Conse-
quently, theodonatelarval community was simple, as itwas dominatedby M.
hirosei. Inorderto conserve this species in anartificiallyestablishedreedcom-
munity, therefore, the diversityofthe larvalcommunity mustbe kept as low as
possible, andasingle species wouldbeideal.
Becauseodonatelarvaeare generalistsfromtheviewpoint oftheirforagingbe-
havior, inwhichthey feedon conspecific and heterospecific larvaeas theirprey,
intra-guildpredation mightseverelyaffecttherespective larvalpopulation dynam-
icsandthenthelarvalcommunity. Whenseveralodonatespecies coexisttogether,
large larvaewillgenerallykillandeatsmalllarvae(W1SSINGER, 1987;JOHAN-
SON, 1992;ANHOLT, 1994).BAKER(1986)reported thatwhenEnallagmaebri-
umandI. verticaliscoexist, small-sizedlarvaeinbothspecies are fedbylarge-sized
larvaeirrespectiveofthespecies. WISSINGER(1988) alsostatedthatinafishless
pond, themortalityofeachodonatespecies inthelarvalcommunity ismoreinflu-
encedbyintra-guildpredation thanbyotherfactors,suchas foodlimitationorhi-
bernation.Inthepresentstudy, themean depthofthebrackishwaterintheestab-
lishedhabitatwas about5cm,thoughseveral patchy areasdriedup, thenthehab-
itat seemed to beun-
suitableforfishesthat
affectlarvalmortality.
Therefore, inorderto
maintainthe popula-
tion of M. hirosei it
,
is necessary to pre-
vent thepredationnot
only by fish but also
by otherodonates.
Odonate larvae
generally adapt to in-
terspecific competi-
tion in a communi-
ty and avoid distur-
bance by their pred-
ators when coexist-
Fig. 3.Correlation ofm *lmvalues between M.hiroseiandI. senega-
ing (McPEEK, 1990;
lensis. Asolidlinerepresentstheregressionwhichissignificantly posi-
tive(Spearmanp-testp =-0.900,p =0.037).Each brokenlineshows LIMA&DILL, 1990;
the PoissonlineforM.hirosei and/. senegalensis,respectively. PADEFFKE&SUH-
Distribution oflarvalOdonatainareed community 315
LING, 2003). Sympetrumfonscombii larvaereducetheirlocomotory activity in
thepresenceoflargerlarvaeofOrthetrumcancellatum(SUHLING &LEPKO-
JUS,2001). JOHANSON(1993) showedasimilarresultforCoenagrion hastula-
tum,whichperformsthebehaviorofrotating andfreezingratherthanswimming
whentheirpredatoradvancestowardsthem.Incontrast,whenEnallagma larvae
inhabitlakes withanisopteran predators, they showbehavioraladaptations for
increasedswimming activityinordertoavoidpredation (McPEEK & SCEIROT,
1996). However, M. hirosei seemingly has not developed anti-predator activity
duringtheirlifehistorybecausethe larvaeappeared not to experienceany coex-
istencewithodonatelarvae.
Although thespecies composition andspeciesrichness intheestablishedhabi-
tatbecamesimilarto the original habitat asa whole, some differencesremained
among the subareas.Thewestern areaof theestablished habitatwitha higher
salineconcentrationdueto theinfallsandthehighest reedshoot density among
the subareasappeared tobe asuitableenvironmentfor M. hiroseiadults(MO-
RIMOTO etal., 2006). Theodonatelarvaeexcept M. hiroseidisappeared from
subarea A in2006. On the other hand, subareas B andCallowedafewspecies
withouthigh salinetoleranceto survive, dueto thelowsalineconcentrationdur-
ing 4years. Because bothsubareas were located far fromthe infalls, the saline
concentration was easily affectedby rainfallanddecreased.
WhiletheexclusionoftheodonatelarvaeexceptforM. hiroseifromtheestab-
lishedhabitatwas incompletefor4yearsafter theartificial reedcommunity was
setup,M. hiroseihadexpanded its distributionallovertheareaandincreasedin
population, andwas dominantintheodonatecommunity. In2006, afewspecies
remainedin the establishedhabitat but seemed to havea littleeffecton the M.
hiroseipopulation.Theoriginal habitatincludednoodonatelarvaeexceptforM.
hirosei.Thisresultwas consistentwith otherreports pertainingtothehabitatsof
Fig.4.Relationshipbetween thenumber ofM.hirosei andI.senegalensisineach samplingtime for
theestablishedhabitat(A-C).Circlesandsquaresshow thespring andautumncommunities,respec-
tively. Solid lines forArepresent theregressionwith aprobabilitylessthan0.001 (p=-0.899)bythe
Spearmanp-test.p forBand Care-0.687 (p=0.087)and-0.657(j>=0.078),respectively.
316 S. Iwata&M.Watanabe
M. hiroseishowing thatonly M. hiroseilarvaeare collectedfromthe reedcom-
munity bed (HIROSE& KOSUGE, 1979; HARA, 1997; NISHU, 1997). From
theviewpoint ofthelong-term population maintenanceofM. hiroseiinthees-
tablishedhabitat, theabsoluteexclusionofotherspecies, as intheoriginal habi-
tat,mightbenecessary.
A negative relationship between thenumberof M. hiroseiandI. senegalensis
was found, probably due to theprey-predator relationship or competitive rela-
tionship betweenthem. IntheestablishedhabitatwhereI. senegalensis occupied
alargeproportion ofthe larvalcommunity, aprey-predator relationship as well
as anintra-guild predation shouldhaveoccurred frequently andaffectedthede-
creaseofnumbersofM. hirosei. Even though I.senegalensis has asimilarextent
ofsalinetoleranceto M.hirosei(IWATA& WATANABE,2004), thedistribution
ofbothspecies wasnegatively correlated.BecauseI.senegalensis adultsgenerally
inhabit in an openenvironment, anincreaseofreedshoot density inthe estab-
lishedhabitatmakestheadultsdifficultto enter thedensereedcommunity. They
are speedy fliersandcovera long distancepersingleflight (N1SHU, 1997),thus
theadultdistributionwas restricted torelatively openareaswithlowreedshoot
density remaining patchilyintheestablishedhabitat.Consequently, thelarvaeof
I. senegaensis showed highpatchy distribution,especially in2006.
Inconclusion, it is clearthat artificially maintaining a densereedcommunity
andkeeping abrackish water environmentresultedinanincreaseintheM. hiro-
seipopulation. Thispopulation became themostabundantspecies in thelarval
community ofthe establishedhabitatby excluding otherodonatespecies. Low
species richness at the establishedhabitat should be suitable for the particular
species thatcan adapt to a severe abioticenvironment.
ACKNOWLEDGEMENTS
We wouldlike tothank the local governmentofMie prefecturefortheir hospitalityatthe study
site andforgivingusresearchpermission.Tamano Consultants Co.,Ltdalso assistedus.Wewould
also like tothankS. MATSU’URA, K.SUSA, Y.TERAMOTO,Y.TAKAHASHI,Y.TAJIMA,
M.MORIMOTO,H. IWASAK1,andH. TOMINAGAfortheir assistance inthefield.This study
waspartiallysupportedby the ProNatura Foundation and AEON Environmental Foundation.
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