The size, structure and distribution of the giant tortoise population of Aldabra

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Abstract
The results of a two year field study of the endemic giant tortoise population (Geochelone gigantea) on the western Indian Ocean atoll of Aldabra are described. The work entailed an extensive sample census from which the size of the population was estimated to be 150000 individuals. A large scale marking programme was also carried out. 6882 tortoises (4.6% of the total population) have now been individually marked and measured. Tortoises are unevenly distributed over the atoll with local densities ranging from 0 to 217 per hectare. Various environmental factors are considered in relation to their distribution. A method of age estimation, based on growth ring counts, is described and the characteristics of populations in high and low density areas are compared. Analysis of preliminary recapture information shows that the majority of tortoises are relatively sedentary in their habits, although some are capable of long distance movements; and that individual growth rate is density dependent. The remains of 643 natural tortoise mortalities were examined. Life tables of survivorship and mortality were calculated for the tortoise population in the southeast of the atoll. Crude estimates of mortality and recruitment indicate that the tortoise population in that area is on the decline. Tortoise biomass estimates are far in excess of those for other natural tropical ecosystems with similar rainfall. The vegetation in the southeast is being greatly modified by tortoise activity and there is every indication that the tortoise population there exceeds ‘the stable carrying capacity’.

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