The rise and fall of the Aldabran giant tortoise population

At the end of the 19th century, after prolonged and extensive harvesting, i ndigenous giant tortoises had been eliminated from all islands in the India n Ocean, except Aldabra atoll, where only a few survived. With greatly redu ced levels of exploitation during the 20th century, the population recovere d to a revised estimated total of 129 000 in 1973-1974, when the first samp le census was conducted. A repeat census in 1997 revealed a highly signific ant reduction in numbers over the past 24 years to an estimated total of 10 0 000. The great majority of tortoises are still found at relatively high d ensity in south-eastern Grande Terre, where the number of animals has decli ned by more than one-third. In contrast, low-density subpopulations on Mala bar and Picard have almost doubled in size, but they represent less than 5% of the total population. Corroborative evidence for the crash in the Grand e Terre subpopulation comes from two independent observations: a significan t increase in tortoise mortality; and a significant decline in tortoise cou nts 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.