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Land-snail Diversity in Sri Lankan Rainforest RemnantsLand-snail Diversity in Sri Lankan Rainforest RemnantsLand-snail Diversity in Sri Lankan Rainforest Remnants.

A Survey of the Land-snail Fauna of the Adam's Peak and Kanneliya-Dediyagala-Nakiyadeniya Forest Complexes, South-western Sri Lanka, June - November 1998

Sri Lanka possesses a rich land-snail fauna and an astonishing 83% of the 247 species known from the island are endemic. Much of this diversity and endemism is concentrated in the rainforests of South-western Sri Lanka.

These forests are restricted for the most part to small isolated fragments, representing about 2000 km2 in total extent and just 3% of the land area of Sri Lanka. The forests of the Adam's Peak range (the Peak Wilderness) and the Kanneliya-Dediyagala-Nakiyadeniya (KDN) complex are amongst the largest and least disturbed of these remnant forests and are likely to contain a significant share of the country's endemic land snails.

I was part of a team of two British and three Sri Lankan nationals, who carried out an intensive six-month survey (3 June-12 November 1998) of the land-snail fauna of the Peak Wilderness and KDN forest complexes.

The main objectives of our study were:

  • to assess the importance of the Peak Wilderness and KDN forest groups for the conservation of Sri Lankan land snails
  • to investigate the land-snail fauna of a range of different natural forest communities.
  • to examine the influence of forest degradation on land-snail diversity
  • We surveyed a range of habitats including steamy, leech-infested lowland evergreen rainforest and misty, dense lower and upper montane rainforest. A total of sixty six individual localities were studied in the Peak Wilderness and KDN areas. Records were kept of all land snails encountered in each locality and representative samples were collected. Detailed information was gathered on the altitudinal range and microhabitat preferences of individual taxa.

    A total of 91 species belonging to 13 families and 29 genera were recorded. This total includes 79 of the species endemic to Sri Lanka and all five of the endemic genera. Notable were the three species of the endemic genus Acavus, large arboreal snails with brilliantly coloured shells, which are abundant in both forest and home gardens. The closely related Oligospira, in contrast is muted in colour and was found only in forest, where it lives in leaf litter. Amongst large prosobranch snails the cryptically coloured, discoid- or turbinate-shelled Theobaldius were particularly noteworthy. Several species of Theobaldius were found in leaf litter in lowland, lower montane and upper montane rainforest. Like many Sri Lankan land-snails Theobaldius is restricted to rainforest habitats. Also recorded were twenty nine species of micro-snails, that is, land-snail species which in their adult state possess a shell with a maximum dimension less than 5mm. Examples include species of the cyclophorid Cyathopoma with their minute, intricately sculpted shells and Thysanota eumita, which bears a shell ringed with many fine, rigid hairs. Interestingly the study yielded twenty three species of land-mollusc new to science, including six new species of Cyathopoma and a new Ratnadvipia, an endemic genus named after the ancient Sanskrit epithet for Sri Lanka, meaning "island of gems".

    Ratnadvipia irradians is a characteristic snail of Sri Lankan lowland forest, where it is frequently encountered in leaf litter on the forest floor.

    Our results show that the Peak Wilderness complex is by far the richer of the two forest areas, containing many more species and endemic species than the KDN system. While a relatively large share (66%) of the KDN fauna occurs in the Peak Wilderness, only a small proportion (26%) of the Peak Wilderness fauna is common to both complexes. This pattern is at least partly related to differences in habitat diversity. The KDN system is an area of tropical lowland rainforest (30-458m). In contrast, the landscapes of the Peak Wilderness complex are highly diversified and stratified, with an altitudinal range of 30-2243m and three major types of forest (lowland rainforest, lower and upper montane rainforest).

    Within the Peak Wilderness, it is the lowland rainforest formation which harbours the largest number of species and endemic species of land-snails, followed in turn by upper montane rainforest and lower montane rainforest. Our findings suggest that there are two major land-snail communities in the Peak Wilderness complex: a lowland rainforest fauna, characteristic of forest at elevations up to about 1070m, above which it is replaced by a montane (lower montane and upper montane) rainforest fauna. The three species of Acavus are, for example, characteristic lowland rainforest species. The ariophantid species Euplecta isabellina, with its fragile creamy-gold shell, and the handsome Euplecta gardeneri, carrying a chocolate-brown shell banded with white, are leaf-litter snails of montane forest.

    The survey collection representing 1126 specimens (499 preserved snails and 627 shells and shell fragments) is currently deposited at the Natural History Museum, London, where it is being labelled and catalogued. We hope to establish a national reference collection of land-snails for Sri Lanka by combining material from this collection and that of a previous survey carried out in 1995. This effort will form part of a three-year Darwin Initiative project, commencing in October 1999, between the Natural History Museum, London and the National Science Foundation, Sri Lanka. Duplicate collections will be retained at the Natural History Museum, London.

    This study has shown that at least 20-30% of Sri Lanka's land-snail species are contained in the KDN and Peak Wilderness complexes, which together account for a mere 4.8% of the total area of the island under closed canopy natural forest. While these results are only preliminary, they underline the importance of the these two forests, for the conservation of Sri Lankan land-snails.

    We gratefully acknowledge the financial support provided by The Malacological Society of London towards the cost of the survey.

    Dinazarde Raheem

    Caption: Ratnadvipia irradians is a characteristic snail of Sri Lankan lowland rainforest, where it is frequently encountered in leaf litter on the forest-floor.

     


     

     

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