Evaluation of line transect method for estimating Mortonagrion hirosei Asahina abundance in a dense reed community (Zygoptera: Coenagrionidae)

2007·2.1 MB·English

by  Watanabe, M

Odonatologica36(3):275-283 SeptemberI,2007
Evaluationof linetransect methodfor
estimatingMortonagrionhiroseiAsahina
abundanceina densereed community
(Zygoptera: Coenagrionidae)
M.Watanabe¹* andS.Iwata²
1GraduateSchool ofLifeandEnvironmental Sciences,UniversityofTsukuba,
Tsukuba,Ibaraki 305-8572,Japan
[email protected]
2Graduate SchoolofEnvironmental Sciences,UniversityofTsukuba,Tsukuba,Ibaraki
305-8572,Japan
ReceivedNovember 17,2005/RevisedandAcceptedMay20, 2006
Theresults ofthe markand recapturemethod forestimatingthe number ofM.
hiroseiadultswerecomparedtothoseofcensuscountsusingthelinetransectmethod
carried outin the samehabitat,adense reed community established in brackish
water.The mark and recapture method gaveadailyestimate of about 1000and
800individuals ofeachsexatthepeakpopulationinearly Julyof2003 and 2004,
respectively. Theseresults did not agree with the estimate fromthe censuscounts,
giving600 6 atthattime in thesamehabitat. Somelimitations ofthe line transect
method werediscussed for estimates of adultsperchingin the understoryof the
dense reedcommunity.However,arelationshipwas observedwith regardtodaily
populationestimatesofthelinetransectmethodandthemarkandrecapture method,
indicatingthatthe line transect method canbe an effectivetool formonitoring
populationsoftheendangereddamselfliesinhabitingsuchadense plantcommunity.
INTRODUCTION
Estimatesofpopulation sizeandknowledge oflifehistory characteristicsare
important goals in conservation biology and such estimates are also essential
for monitoring endangered species. Thepopulation ecology of anendangered
brackish water damselfly, Mortonagrion hirosei, in a small, isolated reedcom-
munity inMieprefecture, Japan hasbeeninvestigated usingthemarkandrecap-
turemethod(e.g. WATANABE& MIMURA,2003). Although thismethodin-
*Authorforcorrespondence
276 M.Watanabe& S.Iwata
volvesmanyassumptions, therewas atremendousamount ofeffortandskillof
researcherswhomustplaceindividualnumbersonthisorganism’s vulnerablethin
wings, as wellas handletheirsmallbody whencaptured. Anaesthetizing such a
smallinsects immediatelyaftercapturingisneededto stay stillforafewminutes
to writenumbers, to measure thebody size, andto identify theage, andalso for
theinsectsto showtheirnormalbehaviourafterrelease.Inaddition,several days
of good weatherduring theflying season arerequired; unfortunately, this is the
rainy seasonin Japan. Thehigh density oftough reedstemsalso inhibits sweep-
ing aparticular areawitha net inorder tocapturesuccessfully adultsperching
in the understory ofthe community without disturbanceofotherperching in-
dividuals.Furthermore, theimpact ofrepeated recapturing mightpresentanin-
creasedrisk ofinjury to theadultinsect. Therefore, censuscounts using theline
transect method mightprovide agoodalternativemethodof obtaining apopu-
lationestimate, duetothelack ofdirectphysical contact withtheorganism.The
linetransectmethodalso requires fewerperson-days ofeffortthandoesthemark
andrecapture method,andis appreciably morecosteffective intermsofassess-
ing damselfly numbers.However, it is not soeasy to determinetherelationship
between thetinetransect methodandthemarkandrecapturemethodsinorder
toobtaintheestimatednumbersofodonatespecies.
Variouslinetransectmethodsarewidelyusedforestimatinganimalabundance;
these methods havebeen described in detailby SOUTHWOOD & HENDER-
SON (2000). Generally, anobserver travelsalong aline, recording any animals
detected.Inbrief,allanimalsare counted,andanimaldensity isestimatedasthe
numberofanimalsdividedby theareaofthestrip line, assuming a given distri-
butionpattern inthehabitat.
The line transect methodology enables estimateof the size of a variety of
many animalpopulations, butthemostpopularmethodofstudying abundance
inmany damselflyspecies remainsthemarkandrecapturemethod(BENNETT
& MILL, 1995). For example, thepopulation parametersof Calopteryxjaponi-
cainan isolatedhabitatwereassessed byWATANABE et al.(1998), using the
markandrecapturemethod.CORDOBA-AGUILAR (1994)alsoestimatedthe
population sizeandsurvivorship ofHetaerinacruentata using thisapproach and
showedthe highest dailysurvival rate among theCalopterygidae. BEUKEMA
(2002) usedthemarkandrecapture methodto assess thechanging distribution
patternof C.haemorrhoidalisalongastream. On theotherhand, BICK&BICK
(1965) usedboththe markandrecapture methodand the linetransect method
forArgiaapicalis,buttheydidnotexamineany relationships betweenresultsob-
tainedbythesetwo differentapproaches to population estimations.Few studies
haveassessed theperformance of thelinetransect methodintermsofassessing
odonatepopulations ofknownsize.
Wecompared theestimateofM. hiroseiabundanceobtainedby thelinetransect
methodandby themarkandrecapturemethod.SinceMhiroseiadultsareperch-
Evaluation oflinetransectmethod 277
ers,andare distributedacross thefloorofreedcommunities(WATANABE etal.,
2002), thecensuscountwas deemedeffectiveforassessing theabundanceofthis
species. Theresults presented here pavetheway fora discussionofapproaches
to monitoring population forconservationofthis species.
MATERIALAND METHODS
ThepopulationofMortonagrionhiroseiwasstudied inasmallreed community(ca. 10x50m)es-
tablished onanestuaryinMieprefecture, whichisknown tosupportacompletepopulation,asde-
scribedindetail inWATANABE& MIMURA(2003).Becausethericefields atthe bordersofthe
originalreedcommunitywereabandoned inautumnof2002,therhizome developmentofreedswith
denseaerialshootswasencouragedintherice fields.Consequently,inearlyJuneof2003,theareaof
thereedcommunity available toM.hirosei adults increased to840 m2,thoughtherewaslittlewater
onthefloor ofthenewadditional community,i.e.,the aqueoussection amountedto about half of
thenewarea.
Themarkandrecapturemethodwascarriedoutinthisreedcommunityon44mostlycleardaysin
late MaytoearlyAugustof2003,and 17daysinlateJunetomid Julyof2004.Adultswerecaptured
usinganinsect net;carewastaken toavoiddamagingthewings,andprocedureswereperformedus-
ingC02 anaesthesia.Then,each damselflywascarefully markedwith auniquenumber ontheun-
dersurfaceofthelefthind wingusingablackfelt-tippedpen. Thesexandageofeachmarkedindi-
vidual wererecorded. Thewingmarkingwasconsidered tohave onlyaminoreffectonflightactiv-
ity(andtheprobabilityofmating),sincemostdamselfliesbegantofly normallyafterbeingmarked,
andperchedsoonaftercomingoutfromtheanaestheticstate.Duringthemarking, 16males and 17
females in2003,and 22malesand 24females in2004 werewounded,and treatedasdeadinthecal-
culations.
Census counts ofM. hiroseiusingthelinetransect methodwerecarried outweeklyin thesame
reed communityfromJulytoAugustof2003(total:4counts),and fromMayto Julyof2004(total:
12counts).Inorder toexcludethe effectofadultdistribution inthecommunitydue todisturbance
causedbythemark andrecapture sampling,eachcensuscountinthelinetransectmethodwascon-
ductedatleastthree daysafterthemarkand recapturesampling.Wecounted allperchingadultsof
both sexes(loneindividuals andtandempairs) detectedbywalkinga89m line,with0.5m onboth
sides oftheline. This line winded largeitswayacrossthereed community.Becausethewalkingit-
selfmightdisturbtheperchingbehaviour ofadultsresultinginthedeclineinthenumbersofadults
alongthe line,thecensuscountwasonceforabout 1hourinthereed communityaroundnoon.
The probabilityofdetectingaM.hirosei adultalongtheobservation lineappearedtobelow,be-
causeperchingbehaviouriscrypticunderashade darkfloor insuch areed community. Individuals,
flyinginattempttoshifttoaperchingsitedue tobeingdisturbedbypassingresearchers,wereeasily
detected.Therefore,throughouttheflying season,individuals still perchingalongthelineweremost
likelyoverlooked.
Alladultsdetectedalongthelinewerecounted,andthesexandthesexualmaturitywererecorded.
Theratiobetweenimmatureandmatureindividuals ineachcensuscountdifferedfromthatobtained
bythemark andrecapturesampling,thatis,both ahighanda lowproportionofimmatureadults
wereobservedinJuneandJuly,respectively.Therefore,thenumber ofadultsestimated bythecensus
countsusingtheline transectmethodincluded immature andmatureindividuals for each sex.
RESULTS
Observationsmadeduringthemarkandrecapturesamplings showedthatboth
sexually immatureindividualsandmaturemaleandfemaleadultstendedtostay
278 M.Watanabe&S.Iwata
Fig. 1.Seasonal changesin theestimated dailynumber ofmale and femaleMortonagrionhirosei in
2003(opencircleand opentriangle) and2004 (solidcircle and solid triangle),usingthe Manlyand
Parrmodel. Thecurveshowsthree-pointmovingaverage.
on theunderstory ofthe reed community; moreover, the spatial distributionof
the perching sites was roughly uniform, andall individuals perched atlevels of
approximately 20 cm inheight. Therewas littledifferenceinthe observedperch-
ing height amongthe adults, located inthe original reedcommunity andin the
additionalreedcommunity lacking water.
We marked2,961 males(740 recaptured atleastonce)and2,212females (465
recaptured) throughout theflying seasonof2003, indicating thatrecapture rate
was more than25%withinseveral sampling days. Usingboth Jolly’s modeland
theManly and Parrmodel(as describedin SOUTHWOOD& HENDERSON,
TableI 2000), the dailychange inthe esti-
Totalnumber ofM.hiroseiadultscounted in 12days matednumberwassimilarbetween
oflinecensusmethodfromlate MaytoearlyAugust themodels,including theresultsfor
in 2004 for4sections from start toend ofthe line
(89m), dividedin accordancewith thereed commu- immatureaswellasmatureindivid-
nity structure uals,as both co-existedinthesame
reedcommunity. Although the SD
Section(m) Number Number Total foreachestimateinmaleswassmall-
ofmales offemales
erthanthatforfemalesusingeither
model, thesamepopulation change
0-20 35 31 66
20-40 95 108 203 was observedin bothsexes. On the
40-69 93 77 170 otherhand, 1,560 malesand 1,845
69-89 74 73 150 femalesweremarkedfromlate June
to midJulyof2004.
Total 297 289 589
Figure 1shows thatasinglepeak
Evaluation oflinetransectmethod 279
was apparentthroughout the fly-
ing seasonforbothsexes: in2003
about 1,000 to 2.000 individu-
alsof each sex calculatedin late
JunetoearlyJulyusingManlyand
Parrmodel. Most daily estimat-
ednumbers ofmales were larger
thanthoseof females.Assuming
thatthesexratiointhisreedcom-
munitywas uniform, we calculat-
edthe wholepopulation by dou-
bling the numberof males, and
the dailypopulation density was
3.5per squaremeter at the time
of the peak population. In 2004,
about 800malesand 800females
were calculatedinearlyJuly using
Manly andParrmodel, indicating
thatthe dailypopulation density
was 1.9 persquaremeter.
Because the habitatwas isolat-
ed, the immigration and emigra-
tionratesofM. hiroseiadultswas Fig, 2.Seasonal changesin thetotal density obtained
expected tobe very low through- fromcensuscountsusingthe line transectmethod for
outtheflyingseason.Thenumber male and female Mortonagrionhirosei in 2003 (open
ofimmigrantsintothepopulation circleand opentriangle)and 2004(solidcircleandsol-
id triangle).
was consideredtobe thenumber
ofnewadultsthatemerged inthehabitat.Using Jolly’s model, therewas apeak
emergenceinearly July,i.e.,approximately 400individualswereestimatedasdaily
immigrants foreach sex. Consequently, thetotalpopulation numberswereesti-
matedat8,131 malesand7,245females, indicatingthatthe totalpopulation size
was about 16,000 in2003.
Thecensuslinewasdividedinto4sections according tothestructureofthereed
community. Alongthe20mfromthestartsiteoftheline, thissectionincludeda
patch ofthenewadditionalcommunity, whichseemednot tobeasuitablehabi-
tatforbreedingofM. hiroseiduetothewater deficiencyon thereedbed.Conse-
quently, TableIshows thatthe totalnumberofadults inthesection countedin
2004waslow.Allindividualsdetectedwasperchingaloneandnosexually mature
femaleswas found. Alongthe line from20-40 m,the section was coveredby a
densereedcommunity, inwhichweobservedafewtandempairsandovipositing
females.Although thesection ofline from40-69 mincludedasmall openarea
withwater depth more than 10cm,whichwas likelyto unsuitablehabitatofthe
280 M.Watanabe& S.Iwata
larvae, 170adultswerecounted.On
the otherhand, the reed density of
the section along the line from 69-
89 m was also high but partly low
with theotheremergentplantssuch
as Scirpus spp.Ischnurasenegalensis
thatis apredator of M. hiroseiwas
observed in this section, probably
due to light environment. Howev-
er, anumberof M. hirosei utilized
thissection. Except apatch ofnew
additional reed community, there-
Fig. 3.Therelationshipbetweenthe numberofadults
detectedalonga10mline(D)andtheestimateddai- fore,adultsmight distributetheen-
lynumber ofadultspersquare meterasdetermined tirereedcommunity, andthere was
bythe ManlyandParr model(E). Circlesrepresent no differenceinthesectionselection
forfemales in2003(open)and2004 (solid),andtri- betweensexes.
angles for males in 2003 (open) and 2004(solid).
Thechanges intheestimateddai-
— [For females: logE=-0.29+0.64 logD, r2=0.56,
0.05>P>0.01; — For males; logE=-0.28+0.71 log ly number of adults according to
D,F=0.58,0.05>P>0.0I] thecensus counts ofbothsexes are
showninFigure2. Althoughfemalesshowedsomewhatmore delayedmaturation
thanmales, the sex ratio didnot clearly differfrom a 1:1 ratio, andthe popula-
tionchange was synchronous betweenthesexes.Asinglepeakwas alsoobserved
forboth sexes: about600 malesand 450 females were observed inlateJuneto
early Julyof2004, suggesting the presenceofmorethan 1000adultsdaily, are-
sult whichdidnot agreewiththe dailypopulationestimates usingthemarkand
recapturemethod.
Whenthepopulation reachedapeak inthecommunity, approximately 5 indi-
vidualsfor each sex per 10mofthelinewereobserved, indicating thatindividu-
als perched atleastevery 1 malongtheline.Eachnumberofadultsobtainedus-
ing thelinetransect methodwas compared withtheestimateddailynumbercal-
culated usingthe Manly andParrmodelatthenearest dateofthecensus count
(Fig. 3). Clearrelationships wereobserved betweenmethodsforbothsexes. The
regression linesindicatedthatthe numberofadultsdetectedby thelinetransect
method determinedthe dailypopulation density withoutreliance onany mark-
ing procedure.
DISCUSSION
The estimated daily numberof this damselfly isof great importance in the
ecology andconservationofM. hirosei, aso-called percherthatinhabitstheun-
derstoryofisolateddensereed communities.Studiesofthemark andrecapture
methodwere timedto maximisetheprobabilityofcapturing individualsduring
Evaluation oflinetransectmethod 281
theentireemergenceperiod. Theestimateddailypopulation density was found
tobe high(3.5 adultspersquaremeter in2003), and theM. hiroseipopulation
appeared to increasestartingin2001, asreported by WATANABE&MIMURA
(2003). Theestimatedtotalpopulation in 2003 had also increased to2.5 times
ofthatin2001.Theabundanceofreedstems andtheenlarged areaofthereed
community also appeared to supportthelargerpopulation.
Despite the cryptic perching behaviourofM. hirosei, aremarkably high cap-
turing intensity was achieved because the population was restrictedto a small
area. The sex ratios, based onboth the estimated daily numbers and thetotal
population numberthroughout theflying season, as basedon themark andre-
capture method, appeared to be roughly uniform. This is not always thecase
withdamselflies, particularly in the species thatadopt territorialtactics, where
maturemalesreturn to water toestablish territories,but mature femalestendto
visit such locationsonly to oviposit. Consequently, studiesofadultdamselflies
report sex ratiosthatare highly male-biased, dueto male-malecompetition and
mating systems at the oviposition sites (ANHOLT, 1997). The probability of
recapture of territorialmales is higher than that forfemales(WATANABE &
TAGUCHI, 1988),becausefemalestendtostay intheforestwithonly short vis-
itsto water sourcesforoviposition. STOKS(2001) also showedthatpopulation
sizeestimatesforLestes sponsawereabouttwiceas large formalesasforfemales,
according to themarkandrecapturemethod.However, counting theexuvia did
not showany bias inthesex ratioatemergenceinapond thatisalarvalhabitat,
andtherewerenosubstantialdifferencesin thetimingofmaleandfemaleemer-
genceinSomatochlorahineana(FOSTER &SOLUK,2004). M. hiroseifemales
stayinreedcommunitiestogetherwithmaleswithoutamaidenflightawayfrom
theemergencesite, asisalso thecasewithCalopteryxjaponica (WATANABEet
al., 1998),inwhichfemales moveonly rarelyoutoftheirnatalarea, as reflected
by the observedstabilitiesinthe sex ratioinan observedpopulation (WATAN-
ABE& MIMURA,2003).Therefore, theM. hiroseipopulation tendstobefully
isolated, andthus serves as a goodindicator forcomparisons of differentpopu-
lationsurvey methods.
Theestimates of dailypopulation size fromthe censuscounts using the line
transectmethodandfromthe Manly andParrmodelusing themarkandrecap-
ture methodwerenot ingood agreement,especially considering thatthecensus
counts werebasedonly onthe numberofadultsnoticedby researcherswalking
along theline.Although thehabitatinthis studywas entirely isolated, a consid-
erablenumberofunnoticedindividualsmusthavebeen presentalong theline at
eachcensuscount.Therefore, therearesomelimitationstocensuscountsobtained
usingthelinetransect method,as thereare withthemarkandrecapturemethod.
Oneof themost difficultproblems inthis regard is the potential for damaging
adulthabitats.Searching aregular andconfinedareaofa densereedcommunity
over aperiodofaweekcancausesignificantlocalized disturbanceofthestanding
282 M.Watanabe& S.Iwata
reeds,providing asunlitareaonthecommunity flooralongtheline,whichwould
bepreferred by Ischnurasenegalensis adultsthatare amajorpredator ofM. hiro-
sei(NISHU, 1997).Smallopenspaceinsucha densereedcommunity mightalso
allowpredatorybirdsspaceforlanding.Predationis generallythemajor causeof
mortalitythroughout theadultlifeofthisspecies;individualdamselfliesinshady
habitatsmay generally be lessexposed to predation than thoseinopenhabitats
(UEDA &IWASAKI, 1982). M.hiroseiadultsmightavoidthelineoftransection
forperching sites dueto high lightintensityanddueto high predationrisk. Fur-
thermore, becauseof cryptic perching behaviourand cryptic body colour, high
tendency toescape inresponse to thedisturbancecaused by passingresearchers;
andpoorlight intensity atperching sites (WATANABE etal., 2002), thepreci-
sion of detectionforthecensus counts usingthe line transect methodmightbe
low.Odonataliving inshadytropical forestsoftenhavevery bright points ofyel-
low orblue onthe face andtowardsthe tipofthe abdomen; these spots make
individualsshinelike lights in the gloom (BROOKS, 2002). Fourconspicuous
greenspots onthepterothorax ofsexually maturemalesofM. hiroseimightplay
the same rolein the tropical species, butdidnot increasethe probability of de-
tecting individualsundertheconditionsusedforthe linetransect method.Care
shouldbe takenwhenbasing managementdecisionson census counts usingthe
line transect method.However,whencensus counts areobtainedusing theline
transect method, onemust take care to minimizethe stressassociated withthe
captureand handling ofthe adult body while reducing sampling efforts, costs,
andnegative impactsonthehabitat.Thelinetransectmethodis ingeneral easier
to use foridentificationthanis the markand recapturemethod, which requires
highly trainedresearchers forfieldsurvey.
FuturestudiesoftheestimatedabundanceofM.hiroseishouldseek tojustify
theassumptions ofthe markandrecapturemethod, as wellas thoseoftheline
transectmethod.Inaddition, increasedemphasis shouldbeplacedonobtaining
accuratecomparisons ofthepopulationestimatesobtainedusingthelinetransect
methodandthemarkandrecapturemethod.Although therelationship between
the density ofaggressive species anddispersal remainscontroversial (CORBET,
1980),there was no dispersion of M. hiroseiwitha high density in the present
study area(WATANABE & M1MURA,2004). Therefore, protection andman-
agementof reedcommunitiesisessentialto sustainthe existing populations of
M. hiroseiadults.Repeated samplings usingthemarkandrecapturemethodcan
resultin damaged damselflybodiesandhabitat,andavoiding suchdamageiscru-
cial.Census counts obtainedby thelinetransect methodapplied to theM. hiro-
seipopulation are thought tobeabettermethodforproviding areliableindirect
meansofmonitoringdamselfly populations inadensereedcommunity over the
entireflyingseason than thoseobtainedby themarkand recapturemethod.
Evaluation oflinetransectmethod 283
ACKNOWLEDGEMENT
ThisworkwassupportedprimarilybyEspecFoundation For EarthEnvironmental Research and
Technologies(CharitableTrust).
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