Environmental Research Group Oxford Limited
Study Commissioned by the Seychelles Islands Foundation and the Global Environment Facility

Aldabra Revisited

David Bourn, Charlie Gibson, Dave Augeri, Cathleen J. Wilson, Julia Church and Simon Hay
November 1997

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 Executive Summary of Expedition Report

Aldabra Atoll in the Western Indian Ocean 

Tortoise Population Trends

The total size of the Aldabra’s giant tortoise population in 1997 is estimated to be 100,000, with 94,000 on Grande Terre; 4,000 on Ile Malabar and 2,000 on Picard.  

Since 1973/74, the overall population has declined by 20-25%. Reductions have been greatest in the south east of Grande Terre, with 30-40% fewer tortoises in the Cinq Cases and Takamaka areas. In statistically terms, this is a highly significant “crash” in numbers. 

The south coast population on Grande Terre has also declined, but the reduction of 8% is statistically not significant.  

Over the same period tortoise numbers on Malabar and Picard have increased substantially, with a highly significant doubling of population on Malabar. 

Population trends appear to be density-dependent: lowest density sub-populations on Malabar and Picard have increased; the intermediate density population on the South Coast has remained approximately steady; and the high density linked sub-populations of Cinq Cases and Takamaka have declined.  

The density of tortoise remains has also increased significantly on Grande Terre, indicating a major increase in natural mortality in recent years. 

Surviving tortoises on Grande Terre are significantly smaller than they used to be, indicating that larger animals have died off, leaving smaller individuals behind.

Vegetation Change

Species composition of inland woody vegetation in the Cinq Cases area has changed little over the past 20 years. 

The number of species in the open coastal fringe appears to have increased in parallel with a general increase in woody plant density there. 

Elsewhere, closed scrub vegetation has tended to become denser, while open vegetation has become more open.  

Local changes in the composition of ground flora were also evident, but could not be properly assessed in the dry season. Coverage by the sedge, Cyperus ligularis, appears to have increased at the expense of tortoise turf in the Cinq Cases area. 

These changes are likely to be linked with changes in both the feral goat and giant tortoise populations.

Management Implications

Aldabra’s remote, rugged and relatively inhospitable environment imposes severe constraints on its conservation and management. Financial resources and practical interventions are limited. Three management options for the future conservation a giant tortoises on Aldabra are considered together with some of their more obvious advantages and disadvantages: no intervention whatsoever; maintenance of the status quo; and adoption of a more pro-active approach.  

General Recommendations

Continued Monitoring

Monitoring of tortoises, goats and rainfall should continue, preferably on a monthly basis. Vegetation needs to be monitored much less frequently, perhaps every 5-10 years, in the wet season. Rainfall is of particular importance to record, because it drives primary production, upon which all else depends. 

Analytical capability should be enhanced through further development of a tortoise population model and easy to use spreadsheet, to provide monthly, seasonal and annual tortoise population estimates from transect data.

Remote Sensing

The potential use of remote sensing for the monitoring of vegetation and rainfall patterns should be explored. The existing vegetation map of Aldabra and vegetation transect data provide excellent ground truth information for further analysis and processing of satellite imagery, such as the 1995 SPOT image of Aldabra provided through regional co-operation and French technical assistance.

GIS Development

Geographical Information Systems (GIS) are increasingly used as an analytical and decision support tool for resource management and environmental monitoring. The Seychelles Government has recently established a phased programme to develop of national GIS capability, which is to include all islands. Given the wealth of information and background knowledge that exists about the atoll’s environment and terrestrial ecology, and the power of GIS to integrate this information, and gain real insights for understanding and management, Aldabra is a prime candidate for GIS development. 

Assured Funding for Monitoring and Scientific Research

Having established Aldabra as a World Heritage Site, justified on the basis of its unique environment, conservation importance and scientific interest; and recently having invested heavily in the rehabilitation of research and accommodation facilities, it is only logical that scientific interest and a scientific presence be maintained on the atoll.

Sustainable funding arrangements are essential, both to ensure continuity of long term monitoring programmes, and to stimulate and support further scientific studies on the atoll. Consideration should be given to the identification and dissemination of research objectives and priorities, and the establishment of an Aldabra Research Fund, to which interested parties would be invited to apply.

Technical Backstopping and Networking

Collaborative links with the wider conservation and scientific community, both nationally and internationally, are required to ensure adequate supervision and assessment of long term monitoring programmes and to advise on research objectives and priorities.

 SIF has a pivotal role to play in the co-ordination of these activities, but in order to maximise benefits, requires independent and authoritative advice on technical matters and the communication of scientific findings.

 Technical backstopping and networking arrangements have a vital role to play in the decision making process and should be strengthened.

Information Pack

An up to date guide to living and working conditions on Aldabra is required, as an aid to research planning and logistics for scientists and others wishing to visit Aldabra. The guide would provide general information about travel to and from Aldabra; what facilities and supplies are available; and what should be brought. Subjects of interest include: Accommodation; Food and Drink; Power; Field and Camping Equipment; Field Assistance; Boats and Engines; Diving Gear; Laboratory Facilities and Supplies; Library; Communications (Local and Long Distance); Medical Facilities; Emergency Evacuation Procedures; and Standard Charges.

Abstract of Royal Society Paper

 

The Rise and Fall of the
Aldabran Giant Tortoise Population

David Bourn, Charlie Gibson, Dave Augeri, Cathleen J. Wilson, Julia Church and Simon Hay

Proceedings of the Royal Society, London, Biological Sciences: 266, 1091-1100, 7 June 1999

At the end of the nineteenth century, after prolonged and extensive harvesting, indigenous giant tortoises had been eliminated from all islands in the Indian Ocean, except Aldabra atoll, where only a few survived.

With greatly reduced levels of exploitation during the twentieth century, the population recovered to a revised estimated total of 129,000 in 1973/74, when the first sample census was conducted. A repeat census in 1997 revealed a highly significant reduction in numbers over the past 24 years to an estimated total of 100,000.

The great majority of tortoises are still found at relatively high density in south-eastern Grande Terre, where numbers have declined by more than a third. In contrast, low-density sub-populations on Malabar and Picard have almost doubled in size, but they represent less than 5% of the total population.

Corroborative evidence for the crash in the Grande Terre sub-population comes from two independent observations: a significant increase in tortoise mortality; and a significant decline in tortoise counts 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 1995/6 and 1996/97.

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