J. GERLACH & K.L. CANNING
The Nature Protection Trust of Seychelles, 53 River Lane, Cambridge CB5 8HP.
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.
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 tor toise 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.
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 - 6,610
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).
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.
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.
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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
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Testudo Volume Four Number Three 1996