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Indian Ocean Biodiversity Assessment 2000-2005

Biodiversity assessment celebrating the centenary of the

Percy Sladen Trust Expedition to the Indian Ocean

In 1905 and 1908-9 the Percy Sladen Trust Expedition visited the islands of the western Indian Ocean to investigate the biological relationships between the islands of the Seychelles, Mascarenes and Chagos groups and to locate evidence for former land connections between the islands. This expedition still forms the basis of our understanding of the region’s biogeography with the recognition that the granitic Seychelles islands are continental fragments of Gondwana, isolated from India and Madagascar 65 million years ago whilst the other islands are volcanic in origin. The continental history of the granitic islands results in a diverse and archaic fauna, with more recent immigrant taxa of African, Malagasy or cosmopolitan origin. The Mascarenes, Amirantes, Aldabra and Chagos island groups all support immigrant taxa with affinities resulting from the predominant marine currents.

Western Indian Ocean biogeography is of great interest due to the combination of the influences of dispersal and vicariance over a large geographical area and to the retention of archaic taxa recognizable as family-level endemics or as combining features of distinct families characteristic of both the Indian and Afro-Malagasy regions. Despite this ecological and evolutionary significance the islands have received relatively little attention since the Percy Sladen Trust Expedition. The smaller Mascarene islands are still virtually unexplored from a biological perspective and the Percy Sladen Trust Expedition remains the most extensive study of the Seychelles islands. Since 1909 there have been a small number of small-scale expeditions. The result of these expeditions is that collections have been made on 31 islands, of which only 12 have been studied in sufficient detail to provide meaningful data. A further 84 have not been studied with the exception of individual visits to record reptiles and birds. Collections made in the 1990s indicate the occurrence of significant faunal changes in recent years resulting from continued colonization from Madagascar and invasion by alien species.

The need to review the current biodiversity of the islands in order to determine the conservation status of the region’s biodiversity and to investigate the balance of colonization and extinction over the last 100 years prompted the development of the Indian Ocean Biodiversity Assessment. The conservation requirements of the islands are pressing due to the expansion of development on the islands, the decline of historical land management practices and the spread of alien species.

The Indian Ocean Biodiversity Assessment 2000-2005 surveyed the terrestrial and fresh-water biodiversity of all 115 Seychelles islands. This international expedition, timed to be completed for the centenary of the Percy Sladen Trust Expedition, making representative collections of all animals, plants and fungi on each island. The expedition provided the data required to locate important biodiversity sites (Key Biodiversity Areas), identify areas and taxa of concern and evaluate the current distribution of alien species and their spread over the last 100 years. Surveying the islands that remain uncollected filled in the main gaps in our knowledge of the region’s biodiversity and facilited interpretation of the biogeographical patterns. The compilation of biodiversity data for the 115 coraline and granitic islands of the Seychelles group provides a unique resource for the study of island biogeography as both the largest and the only complete data-set for all taxa and all islands in a single biogeographical region. The contrasts between oceanic and continental islands and the historical comparison available for some of the islands makes these results of exceptional significance.

The assessment resulted in the publication of a monographic series on the Seychelles fauna and a biodiversity hotspots analysis.

The bidiversity assessment resulted in the discovery of many new species such as the Seychelles palm frog Seychellophryne pipilodryas living in the high forest palm trees.