Contributions to Zoology, 78 (4) – 2009
Estimating the snake species richness of the Santubong Peninsula (Borneo) in two different ways
Johan van Rooijen1,2,3
Keywords: Chao I estimator, negative exponential function, rarefaction curve, Santubong Peninsula Borneo, snakes, species richness, Weibull function
The distribution of Borneo’s species across the island is far from well-known. This is particularly true for snakes which are hard to find. Given the current rate of habitat destruction and consequent need for conservation strategies, more information is required as to the species composition and richness of specific areas of potential conservation priority. An example is the Santubong Peninsula, Sarawak, Malaysia, part of which has recently been gazetted as a National Park. In this paper, the snake species richness of the Santubong Peninsula is estimated on the basis of data obtained during 450 survey-hours. Thirty-two species were recorded. Negative exponential and Weibull functions were fitted to the rarefaction curve. The Weibull function exhibited a high goodness-of-fit, as opposed to the negative exponential function. On the basis of the fitted Weibull function, the total number of snake species was estimated to be 42. A similar estimate of 40 was obtained by applying the nonparametric Chao I estimator. Thus, less than a third of Borneo’s known 139 land snakes inhabit the Santubong Peninsula. Extrapolation of the fitted Weibull function demonstrated that direct measuring of herpetofaunal species richness of species-rich tropical ecosystems is unfeasible given the required search time. I advocate that the use of estimates is unavoidable.
Herpetological surveys of tropical ecosystems have, almost without exception, a common denominator: they result in incomplete inventories in spite of substantial investments in terms of search effort. This is particularly true for snakes which are hard to find (e.g. Lloyd et al., 1968; Inger and Colwell, 1977; Hofer and Bersier, 2001; Orlov et al., 2003). As a consequence, estimation techniques are of interest when the intended objective is to assess species richness, an elementary criterion conservationists may use when identifying priority areas. One such estimation technique consists of extrapolating the species accumulation curve. Species accumulation curves are regularly applied in herpetofaunal surveys, but they are mostly used to arrive at qualitative judgements about the exhaustiveness of the survey (e.g. Murphy et al., 1994; Zug et al., 1998; Ziegler, 2002). However, extensive and sophisticated literature exists pertaining to methods and models used to quantitatively estimate species richness, on the basis of either rarefied species accumulation curves or abundance patterns (e.g. Colwell and Coddington, 1994; Flather, 1996; Longino et al., 2002; Brose et al., 2003). In this paper, two of those techniques are used to estimate the snake species richness of the Santubong Peninsula, Borneo.
Borneo, covering a land area of approximately 743.380 km2, constitutes the largest island of the Indo-Malayan Archipelago. Its herpetofauna is relatively well studied and the accumulated information is very suitable for biogeographic studies entailing comparisons between landmasses (e.g. Inger and Voris, 2001). However, at the spatial scale which is relevant for conservation, 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 Borneo’s protected areas encompass the majority of species (Stuebing and Inger, 1999). For example, the number of snakes known to inhabit the popular Bako National Park is just 11 (Das and Charles, 1993a) and the number recorded in Lambir Hills National Park is only ten (Das and Charles, 1993b; Hazebroek and Morshidi, 2001). Actually, no efforts have been undertaken to estimate the snake species richness of such areas.
The Santubong Peninsula covers approximately 38 km2 and is situated 25 km north of Kuching, Sarawak, Malaysia. Mount Santubong (810 metres above sea level) constitutes the highest point. The mountainous part of the Santubong Peninsula is covered with mature lowland dipterocarp rainforest and hill dipterocarp rainforest. Pockets of kerangas forest exist locally within the lowland forest. The peninsula experiences a comparatively high annual rainfall (Seal, 1958; Hazebroek and Marshidi, 2001) and many streams traverse the lowland forest. Mangrove-swamp and palm-swamp 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). Recently however, it has been gazetted as a national park (Borneo Post).
Material and methods
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 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 5 km. These trails traversed hill dipterocarp forest, lowland dipterocarp forest, kerangas forest and cultivated areas and provided ample access to forest streams. The second collection method consisted of searching for road-kills along the length of the peninsula. The third entailed turning logs, fallen tree bark and similar objects in order to uncover snakes hiding underneath. Sampling effort was expressed in terms of search days where one search day was defined as roughly 4.5 search hours. Specimens were identified and photographed but were not collected, with the exception of nearly undamaged road victims. Voucher photographs were deposited in the Universiti Kebangsaan Malaysia Digital Photograph Collection (UKMDPC). Several road-killed voucher specimens were deposited in the collection of the National Museum of Natural History, Leiden, The Netherlands (RMNH). The total sampling effort equalled 98 search days which were invested during seven surveys carried out in the period 2001-2008, three in September/October and four in March/April.
A rarefaction curve was made by generating 20 random permutations of the search days and constructing an average species accumulation curve (Colwell and Coddington, 1994). Two models were fitted to the rarefaction curve. The first model corresponds with a negative exponential model (Colwell and Coddington, 1994; Flather, 1996). It can be derived as follows: A is the total number of species present in the area under investigation and Y is the total number of species found. The number of new species found per search day is assumed to be proportional to the number of as yet undiscovered species:
which can be rewritten as
with c = a constant that denotes the ease with which snakes are found. The expression can be integrated on both sides:
which leads to
This is the negative exponential.
A possible disadvantage of this model is the assumption that all species are found with the same ease (denoted with the parameter c). This may be an oversimplification as some species may occur in lower densities. Indeed, species abundance patterns generally correspond to a log-normal distribution with some species being abundant and many being rare (e.g. Lloyd et al., 1968; Coddington et al., 1996; Limpert et al., 2001; Longino et al., 2002). This also appears to apply to the snake community of the Santubong Peninsula (Van Rooijen and Van Rooijen, 2007). To account for progressively lower detection chances, the model can be refined substantially by adding two parameters (d and p) as follows:
This is known as the Weibull function.
The Weibull function has been shown to exhibit a good fit in studies of avian diversity (Flather, 1996) and diversity of small reptiles (Thompson et al., 2003). Both 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 February 1996; SPSS Inc.). The appropriateness of the models was evaluated on the basis of goodness-of-fit.
As the performance of the various richness estimators has not yet been thoroughly studied, Veith et al. (2004) recommend the simultaneous use of different estimators. Therefore, the ‘Chao I’ estimator (Chao, 1984; Coddington et al., 1996; Hofer and Bersier, 2001) was also applied in order to check for large discrepancies with the estimate based on the rarefaction curve. The estimator is based on the observed number of rare species,
where A is the total number of species, Y is the observed number of species, a is the number of observed species represented by a single specimen and b is the number of observed species represented by exactly 2 specimens.
A total number of 98 search days yielded 175 snakes, representing 32 species. In terms of eco-functional types (habitat + circadian habits), 11 species were arboreal, four (semi-)aquatic, eight (semi-)fossorial and nine terrestrial. Fourteen species were nocturnal and 18 diurnal. As such, the applied search strategy appears to have sufficiently accessed the full breadth of ecological types. Nine species each were represented by a single specimen and six by two specimens. An updated list of snake species recorded from the Santubong Peninsula is given in the Appendix. The table includes some species recorded in other studies. For illustrations of some of the recorded species see Figure 1 while for photographic records of other species see Van Rooijen and Van Rooijen (2004, 2007).
Fig. 1a-d. Illustrations of some of the recorded species. a: Boiga cynodon, subadult; b: Dendrelaphis caudolineatus, juvenile; c: Oligodon subcarinatus, adult; d: Naja sumatrana, juvenile.
Figure 2 depicts the rarefaction curve as well as the fitted negative exponential and Weibull functions. As expected, the negative exponential exhibits a lower goodness-of-fit. At first, the fitted curve is situated beneath the rarefaction curve, then slightly above and finally it drops beneath the rarefaction curve again. The suboptimal fit of the negative exponential as well as the pattern of residuals (deviations of the model from the rarefaction curve) are in agreement with results obtained with regard to avian diversity (Flather, 1996). The Weibull function on the other hand, exhibits a near-perfect fit which is also in accordance with results obtained by Flather (1996). Therefore, the fitted Weibull function may be assumed to offer the most realistic approach to the species accumulation process. The asymptote of the fitted equation equals 42. As such, a total of 42 snake species is estimated to inhabit the Santubong Peninsula on the basis of the fitted Weibull function. Application of the Chao I estimator resulted in an estimate of 40, a figure that is reasonably similar and therefore does not cast doubt on the Weibull-estimate. Taking the average of both estimates, 41 species are estimated to inhabit the Santubong Peninsula.
Inger (2003) and Voris (2006) stressed the importance of sampling methodology when estimating biodiversity. In particular, sufficient sampling coverage through both time and space is required to avoid underestimation of species richness. The estimates obtained in this study may be downwardly biased as a result of suboptimal sampling methods. For instance, although searches were carried out during both day and night, no searches were performed late at night or in the early morning (before dawn). In addition, sampling of canopy microhabitat was obviously unfeasible. Therefore, the plausibility of the obtained estimate of 41 snake species was assessed by comparing it with the species richness recorded by Lloyd et al. (1968) in a lowland rainforest of Sarawak with comparable surface area. Lloyd et al. invested thousands of man-hours. As such, their inventory can be assumed to be complete. They recorded 47 snake species, a figure that is slightly higher than, but still very similar to, the aforementioned estimate for the Santubong Peninsula. In conclusion, the richness estimate obtained in this study probably underestimates the true species richness, although not to a great extent.
Approximately 139 landsnake species are known from Borneo (Malkmus et al., 2002; Das, 2006). This is a downwardly biased figure however, as new species are still being recorded. For example, the recent descriptions of two new colubrid snakes, Dendrelaphis haasi Van Rooijen and Vogel, 2008 and Dendrelaphis kopsteini Vogel and Van Rooijen, 2007 (first record for Borneo provided by Das (2007)), have increased the number of Bornean species to 139. If the estimated total of 41 snake species is reasonably accurate, the Santubong Peninsula harbours fewer than a third of Borneo’s snakes. This finding underscores the fact that Borneo’s species are not distributed uniformly across this magnificent island. Thus, care should be taken to select conservation areas in a way that preserves the full range of species (e.g. Lombard et al., 1995).
Ideally, one wants to measure directly, instead of estimate, the snake species richness of any area of conservational interest by surveying until completion. Then the exact species composition would also be known, information that obviously has added value. However, extrapolation of the rarefaction curve on the basis of the fitted Weibull function (Fig. 2) provides a rather discouraging prospect: investing another 100 search days will yield merely six additional snake species, a return on investment that no herpetologist would consider alluring. Thus, when it comes to herpetofaunal communities in tropical ecosystems, using richness-estimates will be inevitable in the vast majority of cases.
I wish to thank three anonymous reviewers for their critical and constructive comments which contributed to the honing of the final text. Thanks to Gernot Vogel and Chan Kin Onn for reviewing an earlier version of the manuscript. Finally, I express my gratitude to my wife Myriam and my daughter Mia for putting up with my unsatiable hunger to explore the herpetological wonders of the rainforest.
Received: 28 May 2009
Accepted: 20 August 2009
Published online: 5 November 2009
Editor: J.W. Arntzen
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List of snake species recorded from the Santubong Peninsula. References: 1: Stuebing (1991, 1994); 2: Denzer (1996); 3: Van Rooijen and Van Rooijen (2004, 2007); 4: this report. Common names were taken from the TIGR Reptile Database (http://www.reptile-database.org, release 18 January 2009).