Literaturdatenbank
Literaturdatenbank |
|
 |
|
Gerlach, J. , Re-establishment of giant tortoises in the seychelles islands. Paper presented at Turtle Survival Alliance 2007 Annual Meeting.
Added by: Admin (13 Dec 2008 22:23:40 UTC) |
| Resource type: Proceedings Article BibTeX citation key: Gerlach2007 View all bibliographic details  |
Categories: General Keywords: Aldabrachelys, Aldabrachelys arnoldi, Aldabrachelys dussumieri, Aldabrachelys hololissa, Fortpflanzung = reproduction, Haltung = husbandry, Schildkröten = turtles + tortoises, Testudinidae Creators: Gerlach Collection: Turtle Survival Alliance 2007 Annual Meeting |
Views: 5/768
Views index: 13% Popularity index: 3.25% |
| Abstract |
|
Testudinidae Over the past 50 years there have been several introductions of Aldabra giant tortoises (Dipsochelys dussumieri) to granitic islands in Seychelles, islands from which distinct, endemic forms had been earlier extirpated. The comparative success of these various introduction efforts is reviewed. In December 2006, in an initial effort to repatriate native Seychelles tortoises, the first captive-breed group of Arnold’s giant tortoises (Dipsochelys arnoldi) was released on Silhouette Island. The initial ecological impact of this reintroduction and future translocation plans are described. Historically giant tortoises occurred on most of the islands in the Western Indian Ocean. By 1840 the only wild populations to survive were on Aldabra atoll and Cerf Island in the granitic Seychelles. Beginning in the late 1700s Cerf Island was used as a depot for tortoises brought from other islands and the population there represented a mixture of tortoises of uncertain origin. Aldabra tortoises were also released on the island in the 20th century and today dominate the small population. Throughout the second half of the 20th century there have been attempts to establish additional wild tortoise populations of Aldabra tortoises on other islands in Seychelles. These have been largely initiated by private landowners motivated by the possibility of establishing populations that could be exploited for the international tortoise trade. In 1979 Curieuse Island was selected as the site for an experiment to establish an additional wild population of Aldabra tortoises. The island was chosen as it was government property (and included in a National Park) and easily accessible. The aim of the experiment was to create a giant tortoise population that would reduce any potential pressure for tourism on Aldabra itself, and to provide tortoises for both the domestic trade and for export. At this time it was assumed that only the Aldabra tortoise species remained and no attempt was made to check for potential survivors of original granitic Seychelles forms. A total of 276 tortoises were introduced to Curieuse from 1978-2000. Due to a combination of poaching (adults for food and young for the pet trade), rat predation (on hatchlings) and other factors, this population had declined to 112 tortoises in 2006. There has been some habitat monitoring associated with the Curiesue tortoise introductions, and although largely superficial and qualitative, the observations do provide some interesting indications of vegetation response. Generally, in areas of high tortoise density, the favored food plant (Ipomoea pes-caprae) has been replaced by short grass communities. Over the past 5 years the main tortoise nesting area has changed from open grassland with scattered shrubs to a shrub-thicket of the invasive plant Chrysobalanus icaco. This may have profound implications for breeding in this immediate area, and possibly for recruitment to the population as a whole. As the Aldabra tortoise is primarily a grazer it has little direct impact on this invasive shrub community. The tortoises do however consume the fallen fruit and disperse seeds of C. icaco, exacerbating the habitat deterioration problem. All these past releases involved Aldabra tortoises or a mixture of tortoises, dominated by Aldabran stock. The granitic Seychelles lie outside of the natural range of Aldabra tortoises and thus, they must be technically considered alien species introductions. Whilst they may provide an analogue to some of the extirpated native tortoises of the region, there are notable differences in the feeding behavior of captive Aldabra and granitic Seychelles tortoises, which suggest that their impacts on the environment may not be identical. Recent consolidation and captive breeding of a small number of surviving Seychelles giant tortoises has allowed the possibility of reintroduction of the original native tortoise species to the granitic islands. The first step in re-establishing these species in the wild occurred in December 2006 with the release of 5 Arnold’s tortoises Dipsochelys arnoldi on Silhouette Island. This will be followed by the release of an additional 138 captive-bred juveniles in future years. Tortoises were released in an isolated part of the island, in what is considered optimum habitat: a mixture of native coastal woodland, beach-crest vegetation and abandoned plantations and gardens. This allows the tortoises free access to a wide range of plant species; include those that they have previously consumed in captivity. The movements of the released tortoises are being monitored and their diet recorded. Immediately on release their movements were moderately wide-ranging, but have since narrowed to small home ranges (some overlapping, some isolated). Diet is similarly diverse, based on home range selection: one male settling into an area with food strongly preferred in captivity (the introduced breadfruit Artocarpus altilis), the two females feeding in natural coastal vegetation, and two males remaining in abandoned gardens, selectively feeding on the ornamental Wedelia trilobata. Potential re-introduction of the other Seychelles species D. hololissa will take longer. Currently the surviving population of this species stands at only 6 adults and 15 juveniles. Discussions are ongoing with conservation managers of other islands, and it is hoped that several wild populations of this species can be established in the future. As the third largest of the granitic islands, Silhouette Island will remain the main focus of re-establishment of the endemic granitic island tortoises.
Added by: Admin |