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FACTORS INFLUENCING REPTILE COMMUNITY COMPOSITION IN THE FORESTS OF BUTON ISLAND

Introduction

The effects of tropical deforestation are typically considered in terms of habitat loss or gross habitat change, often associated with deforestation of large areas, as land is clear-felled for timber, or converted to agricultural or grazing use (Fearnside 1986, 1989; Fearnside and Ferraz 1995), or with partial deforestation that results in fragmentation (Klein 1989; Stouffer and Bierregaard 1995). Comparatively minor habitat modifications occur throughout the world’s forests, and many potential effects on animal populations have not been explored (Vitt et al. 1998).

On Buton Island, south-east Sulawesi, forest structure varies considerably. A variety of factors undoubtedly contribute to determining this structural variation, such as elevation, geology, aspect, and rainfall. However, anthropogenic disturbances associated with timber harvesting, rattan collection and other effects of human encroachment are now having a massive impact on the extent of forest and the structure of remaining forest. These impacts are undoubtedly adversely affecting the biodiversity of this region. Different species and communities will be affected in different ways, depending upon their habitat and ecological requirements.

Reptiles are important elements of forest ecosystems, both as secondary consumers and prey items for larger predators (Vitt et al. 1998), yet even basic information regarding their use of forest habitat is poorly known (Brown and Nelson 1993). In tropical forests of South-east Asia the micro-habitat requirements of most reptile species have not been examined, and factors that influence community composition are poorly understood. Sulawesi, has a rich and diverse reptile assemblage (Iskandar and Tjan 1996), but basic information on the ecology and habitat requirements of these species is limited.

How landscape and structural characteristics of forest habitats influence reptile community composition in Sulawesi is poorly-known. Studies in other tropical ecosystems have found that structural changes to rainforest associated with selective timber harvesting can have significant adverse impacts on reptile community composition (Vitt et al. 1998). Preliminary studies on Buton Island have found strong habitat partitioning between two common lizards species (Sphenomorphus textum and Mabuya rudis). These species exhibited significant variation in relative abundance along forest structural habitat gradients and levels of forest disturbance (Gillespie et al. in prep.).

A greater understanding is required of the factors that influence reptile abundance and species composition in heavily utilized forests of South-east Asia. Such knowledge will contribute to understanding of the longer-term anthropogenic impacts on these reptile communities, and how management may be best directed to ameliorate these effects.

• Document the species composition of reptile communities in forests in central Buton Island.

• Assess landscape and structural attributes of forest habitats that may influence relative abundance of species and community composition.

Study areas have been established at two forest sites in Central Buton Island. These sites differ with respect to elevation and geology, and level of disturbance. In each study area, fourteen pitfall trap-lines were established at sites with a broad range of local landscape and habitat attributes, such as ridge-top, mid-slope, gully, riparian. Pitfall trap-lines comprising five 60 L buckets were embedded in the ground, spaced 4 - 5 m apart, with a 1 mm gauge wire mesh drift fence 30 cm high and 20 m long, passing over the centre of each bucket. Each pitfall trap-line was checked every morning. The traps were operated from 19th June to 23^rd August 2001. All animals captured were measured and weighed, given a temporary mark, and released on the site.

Diurnal and nocturnal censuses for reptiles were also conducted in the vicinity of each trap site. Censuses were area-based over approximately 0.25 H.

Detailed measurements were made of habitat structure at each sampling site. Quadrats 5 m² were established at each end of each pitfall trap-line. The following measurements were taken for each quadrat:

• Slope gradient and aspect

• Distance to nearest ridge, gully and permanent water course

• Litter depth was measured at nine points; % cover of exposed rock was estimated

• The length and mean diameter of each log within, or entering, the quadrat

• The diameter at 1.5 m height was measured for the stem of each plant greater than 4 m high

• Vegetation density was estimated across each quadrat by one person standing on one side of the quadrat and counting the number of gratuations visible on a 1.5 m pole held horizontally by another person standing on the opposite side of the quadrat

• A photograph was taken of the canopy, which will be used to calculate % canopy cover

• From the quadrat, the distance to, and height of, the nearest tree greater than 50 cm at chest height was measured.

Twenty species of reptile and six species of frog were captured in pitfall traps. Two additional species of snake were detected at trap sites during censuses. Presence/absence data from pitfall traps and censuses will be pooled and ordination techniques will be used to examine relationships between forest structural habitat characteristics and herpetorfanal species composition. Relationships between habitat structural variables and the relative abundance of the more commonly-trapped species will be analysed by linear regression.

A report on the findings of this survey will be produced, entitled ‘The influence of forest structure on herpetofauna community composition on Buton Island, Sulawesi Tengarra’ by Dr Graeme Gillespie, Arthur Rylah Institute, Melbourne by June 2003. This report will provide a preliminary assessment of the effects of forest disturbance on the herpetofaunal assemblage, and a basis for assessing appropriate sample sizes and sampling effort for more comprehensive investigation the of these effects.

References

Bosch, H. A. J. (1985). Snakes of Sulawesi: Checklist, key and additional biogeographical remarks. Zoologische Verhandelingen. Uitgegeven door Het Rijksmueum van Natuurlijke Historie Te Leiden. No. 217.

Brown, G. W. and Nelson, J. L. (1993). Influence of successional stage of Eucalyptus regnans (mountain ash) on habitat use by reptiles in the Central Highlands, Victoria. Australian Journal of Ecology, 18: 405-417.

Fearnside, P. M. (1986). Human Carrying Capacity of the Brazilian Rainforest. Columbia University Press, New York.

Fearnside, P. M. (1989). Forest management in Amazonia: the need for new criteriain evaluating developmental options. Forest Ecology and Management, 27: 61-79.

Fearnside, P. M. and Ferraz, J. (1995). A conservation gap analysis of Brazil’sAmazonian vegetation. Conservation Biology, 9: 1134-1147.

Iskandar, D.T and Tjan, K. N. (1996). The amphibians and reptiles of Sulawesi, with notes on the distribution and chromosomal number of frogs. In: D.J. Kitchener and A. Suyanto (eds.), Proceedings of the First International Conference on Eastern Indonesian-Australian Vertebrate Fauna, Manado, Indonesia. Pp. 39-46.

Gillespie, G. R. (2000). Herpetofauna Biodiversity Survey of the Labundo Bundo region of Buton Island, Sulawesi Tengarra, Indonesia, July-September, 2000. Unpublished report.

Manthey, U. and Grossmann, W. (1997). Amphibien and Reptilien Sudostasiens. Natur & Tier-Verlag, Berlin.

Vitt, L. J., Avila-Pires, C. S., Caldwell, J. P. and Olivera, V. R. L. (1998). The impact of individual tree harvesting on thermal environments of lizards in Amazonian rainforest. Conservation Biology, 12: 654-664.