The Rise and Fall of the Aldabran Giant Tortoise Population Author(s): David Bourn, Charlie Gibson, Dave Augeri, Cathleen J. Wilson, Julia Church, Simon I. Hay Source: Proceedings: Biological Sciences, Vol. 266, No. 1424 (Jun. 7, 1999), pp. 1091-1100 Published by: The Royal Society Stable URL: http://www.jstor.org/stable/51351 . Accessed: 07/09/2011 11:40 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The Royal Society is collaborating with JSTOR to digitize, preserve and extend access to Proceedings: Biological Sciences. http://www.jstor.org
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8/3/2019 Rise and Fall of Aldabran Giant Tortoise Bourn Et Al
The Rise and Fall of the Aldabran Giant Tortoise PopulationAuthor(s): David Bourn, Charlie Gibson, Dave Augeri, Cathleen J. Wilson, Julia Church, SimonI. HaySource: Proceedings: Biological Sciences, Vol. 266, No. 1424 (Jun. 7, 1999), pp. 1091-1100Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/51351 .
Accessed: 07/09/2011 11:40
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of
content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms
of scholarship. For more information about JSTOR, please contact [email protected].
The Royal Society is collaborating with JSTOR to digitize, preserve and extend access to Proceedings:
T h e r i s e a n d f a l l o f t h e Aldabran g i a n t tortoise
population
David Bourn', Charlie Gibson2, Dave Augeril, Cathleen J. Wilson', Julia Churchl
and Simon I. Hay3
'EnvironmentalResearchGroupOxfordLimited, PO Box 346, OxfordOX] 3QE, UK (bournnordergo@,compuserve.com)2BioscanLimited,StandingfordHouse,CaveStreet,OxfordOX4iBA, UK
3 TALA ResearchGroup,DepartmentofZoology, Universityof Oxford,South Parks Road, OxfordOX] 3PS, UK
At the end of the 19th century, after prolonged and extensive harvesting, indigenous giant tortoises had
been eliminated from all islands in the Indian Ocean, except Aldabra atoll, where only a few survived.
With greatly reduced levels of exploitation during the 20th century, the population recovered to a
revised estimated total of 129 000 in 1973-1974, when the first sample census was conducted. A repeat
census in 1997 revealed a highly significant reduction in numbers over the past 24 years to an esti-
mated total of 100 000. The great majority of tortoises are still found at relatively high density in
south-eastern Grande Terre, where the number of animals has declined by more than one-third. In
contrast, low-density subpopulations on Malabar and Picard have almost doubled in size, but they
represent less than 5% of the total population. Corroborative evidence for the crash in the Grande
Terre subpopulation comes from two independent observations: a significant increase in tortoise
mortality; and a significant decline in tortoise counts on long-term population monitoring transects.
These population changes are attributed to natural population regulatory mechanisms, exacerbated by
low rainfall years in the period 1980-1997, including two consecutive years of below average rainfall
in 1995-1996 and 1996-1997.
Keywords: Aldabra; giant tortoise Geocheloneigantea Schweigger; population census; monitoring;
Seychelles; Indian Ocean
1. INTRODUCTION
Scientific interest in the giant tortoises (Geochelone igantea
Schweigger) of Aldabra atoll in the western Indian Ocean
(figures 1 and 2) and concern for their survival date back
more than a century (Gunther 1877). Fears of excessive
harvesting and possible extinction of the species prompted
eminent scientists of the day, including Hooker, Owen,
Darwin, Newton and Gunther, to petition the Governor
of Mauritius and the Seychelles in April 1874 to ensure
their protection (Stoddart & Peake 1979).
By the end of the 19th century, there is little doubt that
Aldabra's tortoise population had been severely depleted
by repeated collection (figure 3a), removal and consump-
tion by man (Rothschild 1915). Wharton (1879) stated
that 'the reptiles are now very scarce' and that a party of
his sailors was only able to find one specimen'after much
trouble and search'. In 1892 Spurs, the resident lessee,
reintroduced tortoises to Ile Picard from the 'principal
section' (Grande Terre?) and claimed that there 'were
more than 1000 tortoises' on the atoll (Riseley-Griffith
1892). However, Abbott (1893), after a stay of four
months, believed this to be a 'considerable overestimate'
Voeltzkow (1895) found six tortoises near Dune d'Messe,and only three others during a four-day search of the
south-east of the atoll. Roberts (1905), after a four-month
stay, commented that most tortoises were to be found on
Grande Terre, but that he had not seen many. Fryer's
diary of a six-month sojourn on Aldabra in 1908 makes
little mention of tortoises in areas where they are now
common (Stoddart 1971). Nicoll (1908) claimed that
tortoises were restricted to the northern side of the atoll.
Fryer (1911, p. 420) reported that the tortoise 'still occurs
in fair numbers on the extreme east of the Main Island
[Grande Terre] and is scattered in small numbers in the
rest of the atoll. It is also found on Malabar and Picard
islands but is stated to have been introduced .., only two
were found on Malabar, both young specimens'. Eighteen
years later, Dupont (1929, p. 17) reported that he saw 'a
great many tortoises all over the place' and that tortoises
had 'selected the Cinq Cases area for their breeding
ground almost to the exclusion of other parts of the atoll'.
Public concern was reawakened in the mid-1960s,
when plans for development of Aldabra as a military
base became known (Gaymer 1968; Stoddart 1968a,b;
Beamish 1970). An international campaign to 'save'
Aldabra was launched and, following a strategic review
of Britain's defence policy east of Suez and devaluation
of the pound sterling, plans for military development of
the atoll were cancelled in 1967. After an initial
expeditionary phase in 1967-1969, the Royal Societyestablished a research station on the atoll in 1971, and
continued to manage a programme of studies to
investigate the terrestrial ecology of Aldabra until 1980.
Proc. R. Soc. Lond. B (1999) 266, 1091-1100
Received 1December 1998 Accepted 8 January 19991091 C)1999 The Royal Society
8/3/2019 Rise and Fall of Aldabran Giant Tortoise Bourn Et Al
Operations were financed by a ParliamentaryGrant-in-Aid, with additional support from the US NationalAcademy of Sciences, the Smithsonian Institute and the
World Wildlife Fund (now the World Wide Fund for
Nature).A key component of the researchprogramme relatedto
the ecology and dynamicsof the giant tortoisepopulation.After initial investigations by Gaymer (1968, 1973) andGrubb (1971), long-termtortoise-monitoringprogrammecommenced with a sample census in 1973-1974, and the
semi-permanent marking of some 7882 individuals withnumberedtitanium discs (Bourn & Coe 1978).A series ofinterrelated studies followed, focusing on interactions
between tortoises and vegetation (Merton et al. 1976;Hnatiuk et al. 1976);reproduction(Bourn 1977;Swingland& Coe 1978);mortality (Bourn & Coe 1979);recruitment(Swingland & Coe 1979); biomass, production and
carrying capacity (Coe et al. 1979);feeding and seasonal
movement (Hamilton & Coe 1982; Gibson & Hamilton1983); and concluding with a synthesis of populationprocesses (Gibson& Hamilton 1984).
Since 1980, Aldabra has been managed by theSeychelles Islands Foundation(SIF), as a Special Reserveunder the Seychelles National Parks and Conservancy
Act, and the tortoise population has been assessed inter-mittently using strip transects. In recognition of itsunique ecological and geological attributes,Aldabrawas
designatedas a World Heritage Site by UNESCO in 1982(Seatonet al. 1991;Amin etal. 1995).
This paper presents the findings of a repeat samplecensus of the Aldabran giant tortoise population during
July and August 1997 (early dry season) undertakenonbehalf of SIF by the Environmental Research GroupOxford Limited (ERGO 1997) as part of the SeychellesBiodiversityConservationand Marine Abatement Project
(GEF 1992).
Proc.R. Soc.Lond.B (1999)
8/3/2019 Rise and Fall of Aldabran Giant Tortoise Bourn Et Al
. ourn an o ers eassess ng a ra sg an or o se opu a on
(a) (b)
_ ~ ~~~- _h_
|_j ~~~~~~~~~~~~~~~(d)
ENK~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~V
(e) ()
A..
Figure 3. (a) Tortoise collection pens in the vicinity of Cinq Cases creels,probably dating from the early l9th century (Stoddart& Peake 1979). (b) The 1997 tortoise census team at the rock cairn with a mangrove pole and ID tag, marking the site of one of
the 1973-1974 census hectares near Cinq Case in south-east Grande Terre. (c) Carapace remains of tortoise 05654 found near
BasinFregate in 1997, marked 23 years earlier near Anse Takamaka, a straight-line distance of 6 km away. (d) Collection of
carapace remains at various stages of disintegration found in the immediate vicinity of Cinq Cases shade trees, south-cast Grande
Terre. (e) The sedge, Cyperusigularis,as spread extensively inland from Cinq Cases since the mid-1I970s.Note the numerousdeadtrees n thebackground,mostlyGuettardapeciosa.n Congregationf tortoisesn pool,south-eastGrandeTerre.
The strength of the barriers ranges from near completeisolation of subpopulationson different islands, to gradualbut slow movement between units separated only byhabitat barriers (Gibson & Hamilton (1983), based ontheir own data and that derived from Bourn & Coe(1978) and Swingland& Coe (1979)).
Bourn & Coe (1978) identified two subpopulationsinthe eastern region and along the south coast of GrandeTerre, and two other subpopulationson Ile Malabar andIle Picard.Allowances were also made for tortoises occu-pying dense Pemphis cidula crub and other suitable areas,including minor islands within the lagoon. Analysis ofrecaptured marked tortoises by Gibson & Hamilton
(1984), revealed a further subpopulationdivide within theeastern region of GrandeTerre.They identified a partialhabitat barrier formed by pool, mangrove and shade-freeridge vegetation separating a 'Takamaka'subpopulationfrom a'Cinq Cases' subpopulation.
(c) Seasonal movementSeasonal movements of tortoises between habitats in
the Cinq Cases region (figures3a-f) have been reportedsince the earliest observations on Aldabra (Fryer 1911).Vegetation and primary production studies by Gibson &Phillipson (1983a,b) provideda sound basis for reanalysisof the original tortoise census data (Bourn & Coe 1978)
Proc.R. Soc.Lond.B (1999)
8/3/2019 Rise and Fall of Aldabran Giant Tortoise Bourn Et Al
Reassessing ldabra'sgiantortoiseopulation D. Bourn and others 1099
500 -
o = 400-
300,
m 200-
+5~ 100
0
Figure 8. Total late wet season tortoise transect counts in
south-east Grand Terre.y = -0.4423x + 753.83, R2= 0.6185.
important consequences for these animals and others with
similar life-history traits. At the least extreme, time,
patience and perhaps direct intervention would be needed
to ensure their long-term survival.
Alternatively, it is known that the Grande Terre popula-
tions are very close to their carrying capacity for food
supply and, in turn, that rainfall drives both the produc-
tivity of key vegetation and reproduction and growth in
these tortoises (Gibson & Hamilton 1983, 1984). In conse-
quence, population levels, as in African elephants (Laws
et al. 1975; Phillipson 1975; Caughley 1976; Eltringham
1979) could be highly sensitive to fluctuations in annual
rainfall. Following relatively high rainfall years in the
early to mid-1970s, there were several years of drought
between then and the early 1990s (SIF, unpublished
data). If these low rainfall years alone were responsible
for the decline, there is little import for the animals'
conservation, unless there has been a permanent change
in the rainfall regime.
Feral animals have had less impact on Aldabra than
the Galapagos, where they have contributed to the
general decline of tortoises and their extinction from
some islands, through competition or predation on nests
and young animals (MacFarland et al. 1974a,b). Feral
goats have, however, been present on Aldabra at least
since the 1870s. Until the 1970s, these appeared to have
remained at relatively low levels, maintained perhaps by
human predation, but after establishment of the research
station, they increased sufficiently to have marked local
effects on vegetation cover (Hambler 1984; Newing et al.1984; Scoones et al. 1988; Burke 1988a,b). Subsequent
control measures have once again reduced goats to low
levels (Coblentz et al. 1990; Rainbolt 1995). During the
period 1987-1997 a total of some 1800 goats were killed,
with an estimated 30-50 remaining in June 1997
(M. Bergeson, personal communication).
Goats, which are mainly browsers, have two potential
effects on tortoises through food competition and through
removal of shade, which tortoises need to avoid over-
heating (Hnatiuk et al. 1976; Merton et al. 1976). Overall,
tortoises browse little and are mainly grazers, but rely on
browsing for short but critical periods during the dry
season (Gibson & Hamilton 1983). Likewise, mosthabitats used by tortoises on Aldabra have plenty of
shade, but shade is critical in others, such as the coastal
S. virginicus swards, that many tortoises rely on for short
periods of the year. Goats, therefore, may have contrib-
uted marginally to the observed decline through impactson bottlenecks in the tortoises' annual food and shaderequirements.If so, this provides further justification forthe continued control, or elimination, of feral goats overand above that of their damage to native vegetation.
The evidence presented in this paper has identifiedmajor changes in the Aldabra tortoise populations and anumber of possible causes that correlate especially withthe observed declines in the high-density populations.These changes demonstrate the value of long-term moni-toring of this last remaining set of long-lived, terrestrialreptiles living under near-natural conditions.The resultsreveal a dynamic situation, not a static one, and areinstructive for the better understanding of the ecologyand conservation of tortoisesand a wide range of animalsthat share some of their life-historytraits.
The management implications of our findings dependon what conservationobjectivesand prioritiesare consid-ered appropriate,and the financial and human resourcesavailable.Aldabra'sremote, rugged and relatively inhospi-table environmentimposes severe constraints on realisticand sustainableobjectives.The atoll is managed as aWorldHeritage Site by the SIF, under non-specific AldabraSpecial Reserve Regulations, 1981 (Seaton et al. 1991).Whether or not conservation management objectivesshould maintain the present status quo of minimal inter-ference,or take a more proactiveinterventionistapproachto the management of tortoisesubpopulations,remains anopen question.
Fieldworkor thisstudywas carriedout on behalfof the SIFandfunded by the Global EnvironmentFacility,as part of the
Seychelles BiodiversityConservationand Marine PollutionAbatement roject.Wewould iketo thankall SIFandMinistryof Tourism taff on Aldabraand on Mahe, especiallyFrancisAlcindor,GeorgeAuguist,Roselle Chapman,LindsayChong-Seng, Dave Morel,Jean-ClaudNicette,MarkNicette,WalterRoselie (deceased) nd GordonSmith, or their hardworkandenthusiasm.Many hanksalso to MalcolmCoe,Julie Hamilton,Len Mole, Jeanne Mortimer,MarianneNord, David Rogers,RichardSouthwood,David Stoddart, onToomerand WilliamWint,for theirsupportandencouragementehind the scenes.SIHis supported y a grantfromthe Departmentor Interna-tionalDevelopment DFID),LivestockProduction rogramme(ZC0012).However,he DFIDtakesno responsibilityor infor-mationprovided r viewsexpressed.
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