By
J. GERLACH & K.L. CANNINGIntroduction
Several species of giant tortoise inhabited the islands
of the western Indian Ocean until the middle of the last century.
The Mascarene (Mauritius, Rodriguez and Reunion) giant tortoises (genus
Cylindraspis) were driven to extinction by 1795 (Bour 1984) whilst
the Madagascar-Seychelles genus Dipsochelys still survives. The number
of species within Dipsochelys has been a source of argument for many
years with some authors suggesting that there is one highly variable
species (Arnold 1979) and other views that up to 8 species occurred
(Rothschild 1915), of which only one or possibly two survive.
This genus was found on Madagascar, the Comoros, Glorieuse, the Aldabra
group (Aldabra, Assumption, Astove and Cosmoledo), Farquhar and the
Seychelles group of islands. The only surviving wild population
is on Aldabra (the species is known variously as Dipsochelys dussurriieri,
D. elephantina or Geochelone gigantea).
The Madagascan populations (D. abrupta and D. grandidieri)
were extinct by 1,000 A.D. tortoises on the Comoros and Glorieuse
are known only from fragmentary remains that are about 1,100 and 125,000
years old respectively. The Aldabra group, Farquhar and Seychelles retained
wild tortoises until about 1830 when only captives of the species
D. hololissa, D. arnoldi and D. daudinii survived. Genetic work
completed in December 1996 has confirmed that two supposedly extinct
species (D. arnoldi and D. hololissa) survive in captivity.
Both species remain on the edge of extinction and are the subject of
conservation work by The Nature Protection Trust of Seychelles.
Evolution of Dipsochelys
The evolutionary history of these species has remained
speculative until recent revisions provided additional evidence on which
to base hypotheses. In the literature there are two alternative
views; either that the genus evolved on Madagascar and colonised all
the other islands from there (Arnold 1979) or that it evolved in Seychelles
and colonised Madagascar which then provided the source for all other
populations (Bour 1984). Both these scenarios have Madagascar as the
source for the Aldabran population due to the proximity of these islands
and the fact that Aldabra has been colonised by giant tortoises on at
least three occasions (138, 125 and 50 thousand years ago - Taylor
(et al.) 1979). It would seem probable that repeat colonisation
would require a close source population, which Madagascar would provide.
There is a major problem with this however, as all the fossil remains
from Aldabra are identifiable as D. dussumieri or a very closely
related form while the Madagascan species were both very distinctive.
Thus colonisation from Madagascar would have required the presence of
an unrecorded form close to D. dussumieri. Whilst this
is possible, it does not fit with the available evidence. Examination
of the inter-relationships of the different species and the palaeogeography
of the region suggests an alternative explanation.
The Madagascan, Seychelles and Aldabran species are all
well known, while remains from the Comoros and Glorieuse are too fragmentary
to be identifiable beyond 'Dipsochelys sp.'. The present authors have
undertaken a preliminary phylogenetic analysis of all morphological
characters in order to determine the evolutionary relationships between
the different species. This will be reported on in greater detail in
a subsequent publication. The important points in this evolutionary
scenario are that the Madagascan species form a group of their own,
the Aldabran species is closely related to a Seychelles species and
both Madagascan and Aldabran species have evolved from ancestors that
probably inhabited Seychelles. This supports the published view
(Bour 1984) that the genus originated in Seychelles and colonised Madagascar
subsequently. It differs from all previous scenarios in suggesting
that Aldabra was colonised from Seychelles and not from Madagascar.
This scenario accords with the known palaeobiogeography
and the estimated dates of extinction of the different populations but
the repeated colonisation of Aldabra from Seychelles does seem unlikely.
As all the low lying islands between the granitic Seychelles
and Aldabra would have been submerged during the periods when giant
tortoises colonised Aldabra, colonists would have had to float over
1000km of open ocean. This is possible and appears to have occurred
in many animal and plant species but as a repeat event it seems unlikely
to say the least. The less well known extinct populations may provide
an explanation here. Glorieuse is also low lying and would not
provide a source population for any colonisation events but the Comoros
would have been raised above sea level throughout this period.
Their giant tortoises were presumably also of Seychelles origin and
may have been very closely related to the first Aldabran colonists as
they lie within the same marine currents. These islands are the
closest land to Aldabra and with the direction of marine currents would
provide an even better source for potential colonists. It can
be speculated that all the islands received their first colonists from
Seychelles originally, these diverged into the three main southern species
- D. abrupta, D. grandidieri and D. dussumieri, the latter possibly
with slightly different forms on Aldabra and the Comoros. Sea
level rises subsequently eliminated the Aldabran population 138,000
years ago, re-emergence of the atoll allowed recolonisation from the
Comoros 125,000 years ago, followed by repeated inundation, extinction
and recolonisation 100,000 and 80,000 years ago. This explains
how the same form of giant tortoise could colonise Aldabra whilst appearing
to be endemic (naturally restricted) to that atoll and other lower islands
nearby. This scenario predicts that when further subfossil material
is found in the Comoros it will prove to be very close to and probably
indistinguishable from D. dussumieri.
The wider implication of this complex pattern of recolonisation
is to note the surprisingly high frequency of recolonisation events.
We know that these happened at least three times in the last 150,000
years. Each colonisation could have been achieved by a single
gravid female landing on the atoll. Assuming that there is no
significant difference in the probability of males and females being
washed out to sea and onto Aldabra this would imply that at least 6
tortoises landed on the atoll during those 150,000 years. This
represents approximately 1 every 1000 generations. This is a minimum
estimate as immigration to established populations would not be detectable.
However this does make the point that immigration was likely to have
been an important contribution to the Aldabran giant tortoise population.
Potential sources of immigrants were Seychelles, Madagascar, Comoros
and the nearby low lying islands (Assumption, Astove, Cosmoledo and
Farquhar) these last two would have been the most likely sources.
Recent history
All giant tortoise populations have declined following
contact with humans. The first to disappear were those of the
Comoros and Madagascar at about 750- 1000 A.D. The Mascarenes were colonised
in 1663 and the last of the endemic Mascarene genus Cylindraspis
is reputed to have died in around 1840. The Seychelles wild populations
persisted until much more recently and the Aldabran survives today.
This follows from the very short history of colonisation of these islands
(since 1772). This history allows us to follow the sad process
of extinction, which may provide useful insight into the behaviour of
seriously reduced and threatened tortoise populations.
In Seychelles the original size of the tortoise population
is not known but estimates made in 1787 (Fauvel 1909) indicate that
fewer than 8,000 survived in the granitic islands and that 13,000 had
been removed. This latter figure accords reasonably well with
the available figures of exports to Mauritius (summarised in Table 1.),
which indicate that at least 10,000 tortoises had been exported by the
end of 1787.
| Date
|
Estimated
export |
| 1772-1779
|
3,360 |
| 1779-1782 |
1,000 |
| 1782-1784 |
1,103 |
| 1784-1786 |
3,000 |
| 1787-1810 |
7,000 |
| Total |
C. 15,500 |
Table 1. Estimated exports from the granitic Seychelles
to Mauritius (from Fauvel 1909; Stoddart & Peake 1971)
A similar number may have been exported to India and
Africa. As these are only export figures and estimates of living
populations they do not include tortoises killed in Seychelles which
may well have accounted for over 50% of the total population. If this
rough estimate is used it would give a total population before colonisation
in 1772 of at least 68,000 tortoises in the granitic islands. After
1810 tortoises seem to have been practically extinct in Seychelles and
after 1822 Seychelles exports include animals of Aldabran origin.
Following introduction of tortoises from Aldabra the wild population
of tortoises in the granitic islands now stands at some 300, less than
0.5% of the original figure.
It is not entirely clear which of the outer islands supported
tortoises originally, but subfossil remains are known from Dennis (Stoddart
& Peake 1979) which may suggest that they were widespread. On
the Aldabra group there are reliable records from Astove, Cosmoledo
and Assumption. There are no estimates of numbers from any of
these islands. There is no reason to believe that these islands
offered significantly different tortoise habitat than does Aldabra today,
although as most consist of small islets with relatively large areas
of beach large areas may have been unsuitable for permanent tortoise
populations. On Aldabra densities can reach over 50 per hectare
(Coe & Swingland 1984); if a conservative estimate of 10 per hectare
is used the following maximum numbers would be expected:
- Cosmoledo - 4,570
- Astove - 6,61 0
- Assumption - 11,710
Although these are very approximate estimates it can
be seen that these islands would probably have supported significant
tortoise populations.
Numbers of Aldabran tortoises were reduced by a regular
export to Seychelles from 1822 and by the activity of whalers in the
region from 1823. Figures from Aldabra are very incomplete and
only small snapshots are available, these include the export of 2,400
tortoises from Seychelles in 1826 and two ships in 1842 collecting 1,200
from Aldabra. By 1892 the Aldabran population had also crashed
with only an estimated 1,000 survivors. This is reflected in export
statistics with only 13 being exported in 1890-1900. Since then
the population seems to have recovered steadily as exploitation declined
to insignificant levels. In 1925 large numbers were reported and
the present population estimate is over 150,000 (Coes & Swingland 1984).
Genetic considerations
The Aldabran population appears to have passed through
a very severe bottleneck in 1890-1900. This means that genetic
diversity on the atoll is probably extremely limited. The lack
of genetic diversity is clearly not having any significant harmful effect
on population growth, recruitment and survival and such bottlenecks
must have been experienced on several occasions in the past (following
sea-level changes as described above). However it should be noted
that the current population differs from all historical ones in its
complete isolation. In the past the relatively high immigration
rate of at least one tortoise per 1000 generations may have introduced
important genetic diversity from diverging island populations.
With the extinction of the Comoros, Assumption, Astove and Cosmoledo
populations this potentially important gene flow cannot occur and, in
the long run, the effects of inbreeding cannot be avoided. This
may suggest that action of some form will have to be taken to ensure
the long-term viability of Aldabra's large tortoise population.
The most appropriate action would be to restore those
sources of immigrants. This is impractical in the Comoros due
to extremely high levels of habitat destruction and severe human population
pressures. The other islands of the Aldabra group do not face these
problems. Assumption has suffered much habitat destruction, largely
as a result of guano mining earlier this century. A wild tortoise
population could be established, but this might remain small in the
absence of any habitat restoration measures. Astove and Cosmoledo
were both inhabited but have been abandoned. Tortoises are not
present but much suitable habitat exists for reintroductions.
Rats and cats are present but the dramatic population growth on Aldabra
has occurred in the presence of both these mammal species so their presence
does not preclude establishment of tortoise populations. Astove
is also inhabited by pigs, which could be a problem, and these should
be eradicated. Following reintroduction these islands would provide
significant tortoise populations, which could be important for gene
flow between the islands of the Aldabra group. In order to restore
some of the depleted gene pool it might be desirable to include in the
reintroduced populations some of the long-term captive Aldabran tortoises
currently kept in Seychelles.Establishment
of additional giant tortoise populations within Seychelles would serve
to restore a major part of the ecosystems of these islands. Any
tortoise populations on the uninhabited islands of Astove and Cosmoledo
should also receive some protection, possibly as an extension of the
protected status granted to Aldabra; these islands also support some
of the most important sea-bird colonies in the region and are home to
many vulnerable or endangered animal and plant species.
Conclusion
The giant tortoises
of the western Indian Ocean have had a long history of repeated island
colonisation, extinction and re- colonisation. Since humans settled
on the islands the tortoise populations have suffered severe declines
and most species are now extinct. Dramatic population recovery
on Aldabra earlier this century has been followed by several small scale
reintroductions in the granitic islands. Such reintroductions
are important to the restoration of island ecosystems and reintroductions
to Astove and Cosmoledo could be of great significance for the long-term
security of the Aldabran population.
References
Arnold, E.N.
(1979): Indian Ocean giant Tortoises: their systematics and island
adaptations. Philosophical Transactions of
the Royal Society, London B 286; 127-145
Bour, R. (1984): Les tortues terrestres geantes
des iles de l'ocean Indien occidental. Studia Geologica Salamanticensia
1; 17-76
Coe M.J. & Swingland I.R. (1984): Giant tortoises
of the Seychelles. In Stoddart, D.R. (Ed) Biogeography and
ecology of the Seychelles Islands. Dr. W. Junk, The
Hague. p.309-330
Fauvel, A.A. (1909): Unpublished documents of the
history of the Seychelles Islands anterior to 1810. Government
Printer, Mahe. 417pp.
Rothschild, W. (1915):. On the gigantic land
Tortoises of the Seychelles and Aldabra-Madagascar group with some notes
on certain forms of the Mascarene group. Novitates Zoologicae
22; 418-442
Stoddart, D.R. & Peake, J.F. 1979. Historical records
of Indian Ocean giant Tortoise populations. Philosophical Transactions
of the Royal Society, London B 286; 147-158
Taylor, J.D., Braithwaite, C.J.R., Peake J.F. & Arnold
E.N. (1979): Terrestrial faunas and habitats of Aldabra during the late
Pleistocene. Philosophical Transactions of the Royal Society,
London B 286; 47-66
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