Contributions to Zoology, 78 (4) 141-147 (2009) Estimating the snake species richness of the Santubong Peninsula (Borneo) in two different ways Johan van Rooijen1, 2, 3 1 Zoological Museum Amsterdam, Mauritskade 61, 1092 AD Amsterdam, The Netherlands 2 Tulpentuin 313, 2272 EH Voorburg, The Netherlands 3 E-mail: [email protected] Key words: Chao I estimator, negative exponential function, rarefaction curve, Santubong Peninsula Borneo, snakes, species richness, Weibull function Abstract stantial investments in terms of search effort. This is particularly true for snakes which are hard to find (e.g. The distribution of Borneo’s species across the island is far Lloyd et al., 1968; Inger and Colwell, 1977; Hofer and from well-known. This is particularly true for snakes which are hard to find. Given the current rate of habitat destruction and Bersier, 2001; Orlov et al., 2003). As a consequence, consequent need for conservation strategies, more information estimation techniques are of interest when the intend- is required as to the species composition and richness of spe- ed objective is to assess species richness, an elemen- cific areas of potential conservation priority. An example is the tary criterion conservationists may use when identify- Santubong Peninsula, Sarawak, Malaysia, part of which has re- ing priority areas. One such estimation technique con- cently been gazetted as a National Park. In this paper, the snake species richness of the Santubong Peninsula is estimated on the sists of extrapolating the species accumulation curve. basis of data obtained during 450 survey-hours. Thirty-two spe- Species accumulation curves are regularly applied in cies were recorded. Negative exponential and Weibull func- herpetofaunal surveys, but they are mostly used to ar- tions were fitted to the rarefaction curve. The Weibull function rive at qualitative judgements about the exhaustive- exhibited a high goodness-of-fit, as opposed to the negative ex- ness of the survey (e.g. Murphy et al., 1994; Zug et al., ponential function. On the basis of the fitted Weibull function, the total number of snake species was estimated to be 42. A 1998; Ziegler, 2002). However, extensive and sophis- similar estimate of 40 was obtained by applying the nonpara- ticated literature exists pertaining to methods and metric Chao I estimator. Thus, less than a third of Borneo’s models used to quantitatively estimate species rich- known 139 land snakes inhabit the Santubong Peninsula. Ex- ness, on the basis of either rarefied species accumula- trapolation of the fitted Weibull function demonstrated that di- tion curves or abundance patterns (e.g. Colwell and rect measuring of herpetofaunal species richness of species-rich tropical ecosystems is unfeasible given the required search Coddington, 1994; Flather, 1996; Longino et al., 2002; time. I advocate that the use of estimates is unavoidable. Brose et al., 2003). In this paper, two of those tech- niques are used to estimate the snake species richness of the Santubong Peninsula, Borneo. Contents Borneo, covering a land area of approximately 743.380 km2, constitutes the largest island of the Indo- Introduction .................................................................................... 141 Malayan Archipelago. Its herpetofauna is relatively Material and methods ................................................................... 142 well studied and the accumulated information is very Results ............................................................................................. 143 suitable for biogeographic studies entailing compari- Discussion ...................................................................................... 144 Acknowledgements ...................................................................... 145 sons between landmasses (e.g. Inger and Voris, 2001). References ...................................................................................... 145 However, at the spatial scale which is relevant for con- Appendix ........................................................................................ 147 servation, much needs to be learned. As for snakes, the distribution of many species is poorly known (e.g. Stuebing and Inger, 1998) and it is uncertain whether Introduction Borneo’s protected areas encompass the majority of species (Stuebing and Inger, 1999). For example, the Herpetological surveys of tropical ecosystems have, number of snakes known to inhabit the popular Bako almost without exception, a common denominator: National Park is just 11 (Das and Charles, 1993a) and they result in incomplete inventories in spite of sub- the number recorded in Lambir Hills National Park is 142 van Rooijen – Snakes of the Santubong Peninsula only ten (Das and Charles, 1993b; Hazebroek and Mor- veys carried out in the period 2001-2008, three in shidi, 2001). Actually, no efforts have been undertaken September/October and four in March/April. to estimate the snake species richness of such areas. A rarefaction curve was made by generating 20 ran- The Santubong Peninsula covers approximately 38 dom permutations of the search days and constructing km2 and is situated 25 km north of Kuching, Sarawak, an average species accumulation curve (Colwell and Malaysia. Mount Santubong (810 metres above sea Coddington, 1994). Two models were fitted to the rar- level) constitutes the highest point. The mountainous efaction curve. The first model corresponds with a part of the Santubong Peninsula is covered with ma- negative exponential model (Colwell and Coddington, ture lowland dipterocarp rainforest and hill diptero- 1994; Flather, 1996). It can be derived as follows: A is carp rainforest. Pockets of kerangas forest exist locally the total number of species present in the area under within the lowland forest. The peninsula experiences a investigation and Y is the total number of species comparatively high annual rainfall (Seal, 1958; Haze- found. The number of new species found per search broek and Marshidi, 2001) and many streams traverse day is assumed to be proportional to the number of as the lowland forest. Mangrove-swamp and palm-swamp yet undiscovered species: areas are situated in the southern areas and along the Sarawak River, which borders a part of the peninsula. Mount Santubong has been a proposed national park for some years (Hazebroek and Morshidi, 2001). Re- which can be rewritten as cently however, it has been gazetted as a national park (Borneo Post). with c = a constant that denotes the ease with which Material and methods snakes are found. The expression can be integrated on both sides: Three collection methods were applied. The first, and predominantly applied, method corresponds to visual encounter survey (VES), a simple method which has been shown to be effective for surveying rainforest which leads to herpetofauna (Doan, 2003). VES was carried out both during day and night (e.g. Coddington et al., 1996; Doan, 2003). Existing trails were used in an altitudinal range of 0-500 m and with a total length of This is the negative exponential. 5 km. These trails traversed hill dipterocarp forest, lowland dipterocarp forest, kerangas forest and culti- A possible disadvantage of this model is the assump- vated areas and provided ample access to forest tion that all species are found with the same ease (de- streams. The second collection method consisted of noted with the parameter c). This may be an oversim- searching for road-kills along the length of the penin- plification as some species may occur in lower densi- sula. The third entailed turning logs, fallen tree bark ties. Indeed, species abundance patterns generally cor- and similar objects in order to uncover snakes hiding respond to a log-normal distribution with some species underneath. Sampling effort was expressed in terms being abundant and many being rare (e.g. Lloyd et al., of search days where one search day was defined as 1968; Coddington et al., 1996; Limpert et al., 2001; roughly 4.5 search hours. Specimens were identified Longino et al., 2002). This also appears to apply to the and photographed but were not collected, with the ex- snake community of the Santubong Peninsula (Van ception of nearly undamaged road victims. Voucher Rooijen and Van Rooijen, 2007). To account for pro- photographs were deposited in the Universiti Ke- gressively lower detection chances, the model can be bangsaan Malaysia Digital Photograph Collection refined substantially by adding two parameters (d and (UKMDPC). Several road-killed voucher specimens p) as follows: were deposited in the collection of the National Mu- seum of Natural History, Leiden, The Netherlands (RMNH). The total sampling effort equalled 98 search days which were invested during seven sur- This is known as the Weibull function. Contributions to Zoology, 78 (4) – 2009 143 The Weibull function has been shown to exhibit a good species represented by a single specimen and b is the fit in studies of avian diversity (Flather, 1996) and di- number of observed species represented by exactly 2 versity of small reptiles (Thompson et al., 2003). Both specimens. the negative exponential and the Weibull model were fitted to the data using nonlinear regression analysis (e.g. Norusis and SPSS, 1993) with SPSS (release 14 Results February 1996; SPSS Inc.). The appropriateness of the models was evaluated on the basis of goodness-of-fit. A total number of 98 search days yielded 175 snakes, As the performance of the various richness estima- representing 32 species. In terms