Using biogeographical patterns of endemic land snails to improve conservation planning for limestone karsts
Introduction
Modern conservation planning involves the identification (and prioritization) of biodiversity ‘hotspots’ (Myers et al., 2000) and their variants for protection. Among tropical ecosystems, limestone karsts have been recognized as ‘hotspots’ for the large number of endemic plants and animals they support (Clements et al., 2006), and for the heightened risk they face from habitat degradation and destruction from quarrying activities (Vermeulen and Whitten, 1999). Limestone karsts (hereafter referred to as ‘karsts’) are defined here as sedimentary rock outcrops made up primarily of calcium carbonate. Karsts were formed millions of years ago by calcium-rich organisms under the sea, but were uplifted relatively recently by tectonic events.
In Malaysia, more than 800 karsts can be found scattered across the Eastern (Sabah and Sarawak) and Western (Peninsular Malaysia) regions (Lim and Kiew, 1997, Price, 2001). Karsts in this region mostly occur as sheer-sided tower outcrops riddled with caves and sinkholes. Due to their isolation from one another by non-calcareous substrata (Paton, 1961), gene flow between karsts has been restricted, with the result that certain taxa on these habitat islands exhibit high levels of endemism via allopatric (van Benthem-Jutting, 1958, Tweedie, 1961) and/or parapatric (Schilthuizen et al., 2002) modes of speciation. For example, 21% of 1216 karst-associated plant species are endemic to West Malaysia and 11% are strictly confined to karsts (Chin, 1977). Karsts are also considered evolutionary hotspots for land snail speciation (Davison, 1991, Schilthuizen, 2004, Clements et al., 2006). In Borneo, the small (0.2 km2) Sarang karst contains at least six site-endemics, while no less than 50 species are endemic to the large (15 km2) Subis karst (Vermeulen and Whitten, 1999). In karst caves, high endemism in animal communities (e.g., fishes, bats, arthropods, etc.) can also result due to evolutionary changes arising from prolonged cohabitation of species from different ecological and biogeographical origins (e.g., Culver et al., 2000, Gibert and Deharveng, 2002). Apart from preserving karsts for their biological importance, certain karsts should be conserved for the useful ecosystem services and resources (e.g., groundwater, pollination and pest control services, eco-tourism sites, non-timber forest products, guano, bird’s nest, cement, etc.) they can provide if managed sustainably (Clements et al., 2006).
Unfortunately, conservation plans for karsts have been difficult to justify due to insufficient baseline data (Dennis and Aldhous, 2004). The number and size of karsts set aside for preservation are almost always dictated by economic (e.g., commercial interests from mining companies) and political (e.g., different jurisdictions of state governments) forces (Soberón, 1992). In Malaysia, several karsts that were identified decades ago for preservation due to their biological importance have not received any form of protection to date (Davison, 1991, Kiew, 1991). By overlaying the localities of karsts sampled in this study onto a year 2000 forest cover map (Stibig and Malingreau, 2003), it is apparent that a majority of them are already situated at forest edges or within deforested landscapes (Fig. 1). Given that karst biodiversity is particularly susceptible to disturbances from human traffic (Kiew, 2001), recreational activities (McMillan and Larson, 2002, McMillan et al., 2003), crop cultivation (MacKinnon et al., 1996) and quarrying (Clements et al., 2006), there is clearly a pressing need to develop and utilize scientifically-sound criteria for karst conservation planning to prevent further population reductions (Schilthuizen et al., 2005) and extinctions (Vermeulen, 1994) of karst-endemic species.
Biogeographical information has proved useful in setting conservation priorities (Daily et al., 2003, Lourie and Vincent, 2004), but knowledge of karst species endemism and distribution patterns remains inadequate for such purposes. For example, what are the effects of karst area and isolation on species endemism? Such correlates of endemism can potentially be used to identify biologically important karsts for preservation. In addition, how do species compositions vary across karsts in different regions? Understanding such large-scale distribution patterns can also reduce bias in reserve selection because basing reserve design on species richness or endemism data alone does not necessarily result in efficient biodiversity preservation policies (Born et al., 2007).
Given their patterns of high allopatric diversity (Solem, 1984, Tattersfield, 1996), and the persistence of shells that facilitate relatively easy sampling and species identification (Emberton et al., 1999), land snails are a suitable taxon to address the abovementioned questions. Here, we use land snail data from 16 different karsts in Malaysia to identify correlates of endemism from a set of factors (i.e., karst area, isolation, surrounding soil type and geological age) hypothesized to affect species endemism using generalized linear mixed-effect models. Next, we use ordination methods to investigate how land snail species compositions varied among these 16 karsts in West Malaysia (using the same dataset), and among 27 other karsts in East Malaysia that were sampled in previous studies (e.g., Schilthuizen et al., 2002, Schilthuizen et al., 2005). This study is timely because extensive quarrying activities threaten large numbers of karst-endemic plants and animals in Malaysia and, to a larger extent, Southeast Asia (Clements et al., 2006).
Section snippets
Study area
Sixteen karsts were sampled in West Malaysia (between longitudes 100°52′E and 102°28′E, and latitudes 3°18′N and 5°40′N; Fig. 1; Appendix A) and 27 karsts in East Malaysia (between 116°10′E and 118°44′E, and between 4°38′N and 7°13′N; Fig. 1; Appendix B). The climate in Malaysia is typical of equatorial countries, with continuous warm temperatures (mean annual temperature around 27 °C) and high rainfall (annual rainfall between 1400 and 4000 mm) that vary with the arrival of the northeast
Results
Sampling from 16 karsts in West Malaysia yielded a total of 198 land snail species from 49 genera and 19 families (Appendix C), while 173 species from 64 genera and 23 families were sampled from 27 karsts in East Malaysia (Appendix D). Based on their completeness ratios (0.83–0.98; Fig. 2), sampling saturation was high for each of the 16 karsts in West Malaysia. The two model sets using the two different measures of karst isolation (total straight-line distance to the nearest karst and number
Discussion
As predicted from studies examining the total species–area (Preston, 1948, Diamond, 1969, MacArthur, 1972, Rosenzweig, 1995) and endemic species–area relationships (Hubbell, 2001), karst area appears to be a relatively important determinant of both land snail species richness and endemism (Table 1; Fig. 3). Indeed, area has been shown to be the most important factor determining land snail species richness on other island systems (Welter-Schultes and Williams, 1999), but our results also
Conclusions and conservation remarks
Based on our results, conservation planners in Malaysia should focus on preserving relatively larger outcrops (i.e., >1 km2) surrounded by yellow–grey podzolic soils because they potentially contain higher numbers of endemic land snail species. In addition, larger karsts within groups of karsts bisected by geographical barriers such as mountains should warrant even greater conservation attention. For example in West Malaysia, larger karsts on both sides of the Titiwangsa Mountain Range should be
Acknowledgements
This project was supported by research Grants from the Singapore Zoological Gardens, Universiti Malaysia Sabah and the National University of Singapore (R-154-000-222-112). CJAB was supported by a National University of Singapore visiting fellowship. The research permit (No. 1773) was obtained from the Economic Planning Unit, Prime Minister’s Office, Kuala Lumpur, Malaysia. We thank two anonymous reviewers for improving the manuscripts, as well as L. Widjesa, R. Balaji, D. Wilcove, J. Nekola,
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