The vegetation of Aldabra Atoll: preliminary analysis and explanation of the vegetation map

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Abstract
This paper describes the construction of a vegetation map of Aldabra atoll in the Indian Ocean, at a scale of 1:25000, and discusses the island's vegetation in terms of its structure and relationships with other vegetation types. The map was based on a classification of 487 releves, in each of which the numbers of each species of woody plant and the percentage cover of different species of ground layer vegetation were recorded. Classification was done within a hierarchy of criteria, of which the relative abundance of common woody plants was taken to be the most important, followed by the relative abundances of ground layer species, the other woody plant species, and total woody plant and ground layer vegetation cover. Twenty-six vegetation types were mapped at a 1:25000 scale. The vegetation varied from open ground, with sparse vegetation on bare rock, through a range of scrub forest types, to structured vegetation 10-15 m tall. Changes in vegetation type were spatially correlated with topographical changes, with shelter from the prevailing southeast trade winds, and with the amount of grazing by giant tortoises. Patterns of environmental and biotic change resulted in vegetation heterogeneity at scales ranging from decimetres to kilometres. The nature of these mosaic patterns needs further investigation, but they probably have a strong effect on the vegetation and on the giant tortoise population. Aldabra island has a remarkably species-rich flora for an atoll. This may be due to its being a raised atoll, to its large land area per se, to the land area providing scope for the development of an unusually large freshwater lens, and to the shape and structure of the atoll providing a range of wind-shelter characteristics. The vegetation types are similar to those found on raised reef and other coralline deposits on the East African coast. In earlier times these vegetation types may have been typical of raised reef deposits throughout the world: lack of quantitative recording and especially the extent of human interference on other raised atolls mean that Aldabra's vegetation is now an isolated example of this set of vegetation types. The effects of a large herbivorous reptile help to set the vegetation structure and make Aldabra unique.

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