Persistence and conservation of Sri Lankan rainforest snails in a landscape of fragmented forest and modified habitats
Department of Zoology, The Natural History Museum, London
Biological diversity on land is concentrated in the highly threatened forests of the tropics (Groombridge and Jenkins, 2000). Over the last decade the annual rate of tropical deforestation has been approximately 8.6 million ha and in Southeast Asia, where forest clearance is greatest, an area of 0.8-0.9% on average is lost annually (Mayaux et al., 2005). Due to this, modified habitats (i.e. habitats cleared of natural forest cover and converted to other forms of land-use, such as intensive monoculture cultivation and pasture) are becoming a dominant feature of tropical landscapes. Natural forest, which formerly extended in continuous tracts across these landscapes have become like shrinking islands, isolated and fragmented in a growing sea of agriculture. An increasing body of research over the last two decades has shown that the biota of tropical forest fragments are profoundly affected by surrounding modified habitats (also called matrix habitats) (Ewers and Didham, 2006). Data on invertebrates are sparse and although we know a great deal about the impact of modified habitats on species survival within forest fragments, our understanding of the persistence of tropical forest species in modified habitats is limited.
Here I briefly describe the preliminary findings of a study of the land-snail fauna of modified and natural forest habitats in a small and discrete geographical area. The main objective of my study was to assess the value of different modified habitats for the persistence and conservation of native snails in a fragmented lowland forest landscape in Sri Lanka.
The study area, a 10 km x 10 km landscape in Galle District, southern Sri Lanka, is a complex mosaic of fragmented lowland rainforest, cultivated habitats and forest and scrub regenerating on lands intensively cultivated in the past. The cultivated matrix consists of seasonally flooded rice paddy and dry land cultivation, the latter dominated by monoculture plantations of tea, rubber and oil palm. The land-snail species of the major forest and dry land matrix habitats were sampled using a standardized 2 m x 100 m belt-transect protocol in January and from May to July 2007. A total of 37 transects were completed across the following habitats: current monoculture (tea, rubber and oil palm) and abandoned monoculture (tea and rubber) close to forest edges (11 transects); current monoculture (tea, rubber and oil palm) and abandoned monoculture (tea and rubber) isolated from natural forest (12 transects); small rainforest fragments (11 transects) and village home gardens (3 transects). Transects close to forest edges were sampled 50 m from the closest forest edge and transects isolated from forest were sampled = 200 m from the closest forest edge. A range of environmental variables were measured (e.g. altitude, canopy density, soil pH, vegetation structure). Data on the land-use and disturbance history were collected. Analyses have yet to be completed and only preliminary findings are given below.
The preliminary data show that currently cultivated monocultures, both on forest edges and distant from natural forest, have a species-poor fauna, dominated by exotics and native species tolerant of high levels of disturbance. Characteristic snails include a species of the endemic genus Aulopoma and exotics such as Subulina octona, the veronicellid slug Semperula maculata and the semi-slug Mariaella dussumieri, a native of the Western Ghats of India. Abandoned monocultures isolated from natural forest have a fauna similar to that of currently cultivated monocultures.
Abandoned monoculture cultivation on forest edges and home gardens may harbour a small but diverse subset of essentially forest-dependent snails. Examples include the brilliantly coloured Acavus haemastoma, which belongs to an ancient group of snails thought to be more than 200 million years old and the beautiful camaenid Beddomea albizonatus, which has a brilliant green body in its living state.
By far the richest land-snail faunas were found in fragmented natural forest. The forest fauna is dominated by native species, most of them either restricted to forest or largely dependent on forest. Despite their small size and often highly disturbed state the small fragments contained a mixture of both wide-ranging snails such as Euplecta travancorica praeeminens and restricted-range exclusively forest species such as the prosobranchs Tortulosa pyramidata and Cyathopoma ceylanicum. Exotic species such as Semperula maculata occur in some of these fragments but are not a numerically dominant part of the fauna at present. These snails have most likely entered forest from surrounding modified habitats.
Clearly, modified habitats cannot be viewed as a substitute for natural forest in terms of habitat value for endemic snail species. This is supported by similar findings for a range of different taxonomic groups from other parts of the tropics. Intensively cultivated monocultures have limited value for conserving native forest land snails. Abandoned monoculture cultivation and home gardens have a potentially important role in the conservation of forest snails as refugia and habitat corridors. The data suggest that colonization of such habitats by forest snails does occur, but that this is dependent on a direct physical link with natural forest during forest regeneration on abandoned lands or the development of forest-like vegetation structure in gardens. Minimal levels of human disturbance are also likely to be important for encouraging colonization of gardens by snails.
My post at the Department of Zoology, the Natural History Museum, London, is funded by the Darwin Initiative (Project title: Developing land-snail expertise in South and Southeast Asia, Grant no: 15-018). Fieldwork was funded by the British Ecological Society, the Malacological Society of London and the Percy Sladen Memorial Fund.
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Figure 1. Sampled habitats included (a) monoculture tea plantations, (b) abandoned rubber, (c) a monoculture rubber plantations and (d) small, isolated fragments of lowland rainforest. Land molluscs recorded during the present study include (e) Tortulosa pyramidata, a snail restricted to Sri Lankan lowland rainforest, (f) the exotic slug Semperula maculata that occurs widely in monoculture plantations and home gardens, (g) Euplecta travancorica praeeminens, a rainforest species able to survive in abandoned cultivation, scrub and home gardens and (h) a species of the endemic genus Aulopoma that can survive in highly disturbed habitats such as monoculture plantations.