Establishment of resident Odonata populations on the formerly waterless Cousine Island, Seychelles: an island biogeography theory (IBT) perspective

6 Pages·1998·1.3 MB·English

by  Samways, M J

Odonatologica27(2): 253-258 June 1, 1998
Establishment of residentOdonatapopulations
ontheformerlywaterless CousineIsland,
Seychelles:an islandbiogeography
theory (IBT)perspective
M.J.Samways
InvertebrateConservation Research Centre,DepartmentofZoology& Entomology,
UniversityofNatal, PrivateBag X01,Scottsville 3209,South Africa
e-mail: [email protected]
Received August6, 1997/Reviewed andAccepted October15, 1997
InMarch-April1997,Ceriagrionglabrum,Diplacodestrivialis, Orthetrum brach-
iale wrighti,Pantalaflavescens and Tramea continentaliswereresident on Cousine
Island.PriortoDecember 1993, therewerenoperennialstandsofwaterontheisland,
andwithinjustover2years, these spp.had established asbreedingresidents, having
crossedatleast 2.5km ofopensea,probablyviathe filterroute.Cousine isunlikelyto
support morethanabout4or5 continuouslybreedingresidents inrelationtoits size.
INTRODUCTION
TheSeychelles granitic
islands (4-5°S, 55-56°E)
are an isolated ancient
vestige ofpartoftheear-
lier supercontinent of
Godwanaland(BRAITH-
WAITE, 1984). They lie
about930km NofMada-
gascarand 600 km E of
Africa. They have an in-
sect fauna fairly rich in
localendemics(COGAN,
1984),withfiveofthe 19
Fig. 1.The main, central groupofgraniticislands ofthe Sey-
recordedOdonataspecies
chelles.
254 M.J. Samways
(BLACKMAN&PINHEY, 1967)occurring only in theSeychelles.
CousineIsland(Fig. 1)is asmall, naturally waterless, (26ha, 1 km long, 400m
wide)granitic islandabout5km SWofthelarger(38km2)islandofPraslin, which
hasstandsofperennialwater. Inordertoavoid desiccationstressontheintroduced
gianttortoisesandon some threatenedendemicbirdspecies, small(±1 m2to± 12
m2)pools were createdinDecember 1993.
From27 March 1997to4April 1997,1visitedthe islandandassessed theresi-
dencystatusoftheOdonata.Supplementary observationswere madeonthenearby
CousinIsland(Fig. 1), whichhas anartificially-supplemented, perennial pool of
ca30m2
.
FIELD EVIDENCE
CERIAGRIONGLABRUM(BURMEISTER)
Six malesandtwo females(both intandemandovipositing) were recorded,but
nolarvaewere found.OnCousinIsland(3-IV-1997), twoadultmalesandanadult
femalewere atthepool, andalarvawas recorded.
DIPLACODES TRIVIALIS(RAMBUR)
About4-6adultmalesandanoccasionalfemalewerepresent only atthelarger
(±12 m2) sunlit, warm water (39-40°C) pool. Larvae were not abundant, only 4
wererecoveredafterintensesampling.On CousinIsland(3-IV-1997), noadultsor
larvaewere seen.
ORTHETRUM BRACHIALEWRIGHTI(SELYS)
Noadultsofthisspecies wereseen.However, itwas recordedearlierinthesea-
son (31-1-1965) by BLACKMAN & PINHEY (1965). Early-instar larvae were
abundant(> 3-4 pernet scoop) in a shade-dappled (± 4-5 m2)pool. On Cousin
Island(3-IV-1997), thepool wasoccupied bytwo adultmalesandlarvaewerealso
present.
PANTALAFLAVESCENS(FABRICIUS)
Onlyafewadultswereseen, noneofwhichwerefemales.Onesunlit,verysmall
pool (±1 m2,±12cm deep, 35-36°C at±6h00,2-IV-1997) was occupied by larvae
solely ofthis species (21 individualsofall ages). They were alsopresentin low
numbersin smallpools thatwere shaded partofthe day. Interestingly, larvae(24
individuals,ofall ages,recovered)wererecovered froma shallow(±25 cmdeep)
pool that hadabottom temperatureof39-40°C(±16h15, 2-IV-1997). OnCousin
Odonata ofCousine Island,Seychelles 255
Island(3-IV-1997), no individuals,larvaeoradults, wererecorded.
TRAMEA CONTINENTALIS(SELYS)
This species was abundant;all theopen,sunlitpools werehawked byamaleor
a mating pair. Therewas frequent oviposition. Adults, based on wing-wear, ap-
peared tobeofallages.On CousinIsland(3-IV-1997)itwaspresentasboth larva
andadult.
DISCUSSION
COUSINE ISLANDSPECIES
BOURQUIN (1997)recorded animmigration swarmofHemianaxephippiger
(Burm.) to Cousine Islandon 6 November 1996. This species has alsobeen re-
ported fromthe larger (±68 ha)Aride Island, about25 km to the N(CARTY &
CARTY, 1996). Yet, during our visit, it was not foundto be residenton Cousine
Island.
BOURQUIN (1997)alsorecorded Tholymis tillarga (Fabr.)onCousine Island,
butno adultsor larvaewere found*.
In contrast to above, BOURQUIN (1997)did not record Ceriagrion glabrum,
whichappearsto beanewarrival, andby itsovipositing behaviour(but nolarvae
found)itmayinMarch- April 1997beresident.IthasalsobeenrecordedonAride
Island(CARTY&CARTY, 1996). C. glabrum hasawiderangeacross Africaand
its neighbouring islands (PINHEY, 1984)**.
Ofthespeciesrecordedinthisstudy, TrameacontinentalisandPantalaflavescens
arewell-knowntravellersand/ormigrantsinneighbouring Africa(PINHEY, 1985;
SAMWAYS & CALDWELL, 1989).Both species were breeding on CousineIs-
land, lateMarch- early April 1997.Interestingly, T. burmeisteriKirby, so often
seenflying with T. continentalisinSouthAfrica, was not recordedhere,norelse-
whereintheSeychelles (BLACKMAN& PINHEY, 1976).Therapid appearance
andfirmestablishmentofT. continentalisonCousineIslandsuggests thatitreadily
crosses thesea.
Pantalaflavescens wasrecordedbothasanadultandasanabundantlarva.Itwas
not seenonCousin Islandon 3 April 1997,nor has itbeen recordedfromAride
Island.
Diplacodes trivialis, likeP.flavescens and C. glabrum, hasawidegeographical
distribution,occurringacross theOrientalandAustralasianregions, butnot inAf-
* Note addedin proof: T. tillargawaspresent andovipositing onCousine Island 12 January - 6
February 1998.
** Noteaddedin proof:C. glabrumwasabsent fromCousine Island 12January- 6February 1998.
256 M.J. Samways
rica (WATSON, etal., 1991).Strangely, itwas sighted onCousineIslandbut not
onCousinIsland.IthasnotbeenrecordedonArideIslandeither(CARTY&CARTY,
1996), butit doesoccur elsewhere inthe Seychelles (BLACKMAN&PINHEY,
1967).Itappearstobeaquick islandcolonizerandwasoneofthefirstdragonflies
to colonizeKrakatau (THORNTON, 1996).
TheonlySeychelles endemicdragonflybreeding onCousineIslandisOrthetrum
brachiale wrighti recordedearlierfromAride Island(CARTY &CARTY, 1996)
,
andoccurring also onCousin Island.It too,like T. continentalis, P.flavescens, C.
glabrumandD. trivialis, appearstoreadily cross fromsomeSeychelles islandsto
others.Interestingly, itwas recordedonCousineIslandbyBLACKMAN&PINHEY
(1967) inJanuary 1965, suggesting thatitmayfairlyregularly crosstheseaorthat
itcanbreedin semi-permanent pools such as tree-holesorrock concavities.
FILTERROUTEDISPERSAL
Injust over two years, theartificialpools ofCousine Islandhaveattractedfour
residentspecies (Pantalaflavescens, Tramea continentalis, Diplacodes trivialis.
Orhetrumbrachiale wrighti). and three possibly occasional breeders (Tholymis
tillarga, Ceriagrion glabrum andHemianaxephippiger). This is arelatively high
number(Fig. 2) given the sizeoftheislandandits isolation.Itis certainly partly
accountedforbythedifferentpools beingdominatedby differentspecies, depend-
ing onpool size, shade/sunlight ratioandtemperature,ashasbeenshownon main-
land Africa (STEYTLER
& SAMWAYS, 1995;
OSBORN &SAMWAYS,
1996).
The dragonflies on
CousineIslandmayeither
have reached the island
viathesmallerneighbour-
ing islandof Cousin, or
thelarger increasingly dis-
tantislandsofPraslinand
Mahe.Ifdispersal was,for
example, Praslin-Cousin-
Fig. 2. A log. island area:Odonata speciesrichness regression
line (y =-4.84+ 4.66x,P=0.63)for the graniticislands ofthe -Cousine,thiswouldbea
Seychelles,based onrecords ofBLACKMAN & PINHEY, 1967; filter route (COX &
CARTY&CARTY, 1996andthedatainthisstudy.- [1=Cousine, MOORE, 1993).Ifit was
2=Cousin, 3- Aride,4=Cerf, 5= Fregate,6 =St. Anne, 7= aquestion ofchancearriv-
Curieuse, 8=Coetivy,9=LaDigue, 10=Silhouette, 11=Praslin,
12=Mahe. N.B. The figureforspecies richness onCousine is als (as the H. ephippiger
conservative, beingtheknown resident species. A total of7 spp. arrival may have been),
has been recorded fromthe island.] thenthiswouldhavebeen
OdonataofCousine Island,Seychelles 257
asweepstakes route (COX & MOORE, 1993).As colonizationby thisrelatively
large numberofspecies was sorapid, andthey occur throughout mostoftheSey-
chelles (BLACKMAN&PINHEY, 1967), itappears thatthey are highly vagile,
andfollowafilterroute.
SPECIESRICHNESSEQUILIBRIUM
Itis interesting to speculate howmanyOdonataspecies Cousine Islandislikely
tohold.Evidenceso farpointstotheequilibrium level(MacARTHUR&WILSON,
1967) ofabout4-5 species, withpools dominatedby Pantalaflavescens, Tramea
continentalis, Diplacodes trivialis, or Orthetrum brachiale wrightior Tholymis
tillarga, andCeriagrionglabrum beingasubdominanttoD. trivialisinparticular.
Thereis however,clearly adynamicequilibriumbetweenthesespecies onCousine
Island.
ACKNOWLEDGEMENTS
Specialthanks toMrM.F. KEELEY ofCousine Island,fortheopportunityofmakingthis study.
ThanksalsotoMrPETERHUTCHINS andMsSTELLALE MAITREformakingthe visitlogistically
possible. Financial supportinSouth AfricawasfromtheFoundationforResearch Development.Ms
PAMELASWEETkindlyprocessedthe manuscript,andMsMELINDAHATTONdrewthefigures.
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Noteadded inproof:Dr O. Bourquin capturedtwofemalespecimens ofAnas guttatus(Burm.)on
CousineIsland, January 1998. There isnoevidence yet however,that thisspeciesisbreedingthere.
Thisbringsthe totalnumberofspeciesrecorded ontheisland, butnotnecessarilybreeding, toeight.