Limestone Karsts of Southeast Asia: Imperiled Arks of Biodiversity

摘要

The overexploitation of the world's biomes for natural products calls for the prioritization of biologically important ecosystems for conservation. Here we show that limestone karsts are “arks” of biodiversity and often contain high levels of endemism. Humans have exploited karsts for a variety of products and services, but unsustainable practices have caused population declines and extinctions among site-endemic taxa. Limestone quarrying is the primary threat to karst biodiversity in Southeast Asia, where quarrying rates exceed those in other tropical regions. Several socioeconomic, political, and scientific issues undermine the stewardship of these karsts. Mitigation of these problems will involve (a) better land-use planning to prevent karst resources from being exhausted in developing regions, (b) comprehensive assessments of a karst's economic and biological value before development, (c) improved legislation and enforcement to protect karst biodiversity, and (d) increased research and activities to promote public awareness of the importance of karsts and the threats facing them.nnHumans are extracting natural resources at un-precedented levels. About half the world's original forest cover has already been cleared for agriculture and forest products, while another 30% has subsequently become degraded or fragmented (UNFPA 2004). If the current pace of habitat loss continues, species extinctions in many areas may reach catastrophic levels (Sodhi and Brook 2006). To mitigate such a disaster, scientists are identifying areas within “bio-diversity hotspots” (regions exceptionally rich in endemic species and facing massive habitat loss; Myers et al. 2000) for priority conservation. Economically valuable ecosystems within hotspots, however, may not be adequately protected because of vested commercial interests, weak legislation, or deficient biological data. Limestone karsts are a prime example of an ecosystem in this predicament.nnLimestone karsts (hereafter referred to simply as karsts) are sedimentary rock outcrops that consist primarily of calcium carbonate. Most karsts were formed millions of years ago by calcium-secreting marine organisms (e.g., corals and brachiopods) before tectonic movements lifted them above sea level. Over the years, the softer sediments covering these karsts were removed by mechanical and chemical weathering. This process usually produces “tower” and “cockpit” karst formations in the tropics. Tower karsts are characterized by tall, precipitous (60-degree [°] to 90° gradient) cliffs riddled with caves and sinkholes (figure 1a), while cockpit karsts are generally cone-shaped and have gentle slopes (30° to 40° gradient) (MacKinnon et al. 1996).nnIn Southeast Asia, karsts cover an area of around 400,000 square kilometers (km2), with geological ages ranging from the Cambrian to the Quaternary (Day and Urich 2000). Karsts in this region, which are most extensive in Indonesia, Thailand, and Vietnam (figure 2), possess impressive geological features, such as the world's largest cave chamber (Good Luck Cave in Sarawak, Malaysia) and one of the world's longest underground rivers (St. Paul Subterranean River in Palawan, Philippines). On the highly fragmented Sunda Shelf, karsts have formed “islands within islands,” and these are known to contain high levels of endemism. Many of these outcrops, which have historically been spared from agricultural development because of their rugged terrain, may function as biodiversity reservoirs, or “arks,” that restock degraded environments during ecosystem reassembly (Schilthuizen 2004). Besides serving as natural laboratories for biogeographical, ecological, evolutionary, and taxonomic research (Ng 1991, Schilthuizen et al. 1999, 2005a), karsts also have huge potential for archaeological and paleontological discoveries (e.g., fossils of the dwarf hominid Homo floresiensis were recently excavated from a karst cave in Indonesia; Morwood et al. 2004). For these and other reasons, karsts are recognized as important ecosystems and have been included for many years in national conservation plans within the region (MacKinnon and MacKinnon 1986).nnKarsts in Southeast Asia, however, are threatened by modern destructive practices. Many outcrops are being quarried for limestone (figure 1b), an important raw material used to manufacture commercially valuable products such as cement.A cement company in Malaysia, which owned limestone quarries totaling 1.3 km2, generated about US$150 million in revenue from just one year of cement production (CIMA 2004). Quarrying is now regarded as the primary threat to the survival of karst-associated species, and it will certainly exacerbate the biodiversity crisis in Southeast Asia, a mega-diverse region that has the highest rate of natural habitat loss among the tropics (Sodhi and Brook 2006).nnIn this overview of karsts in Southeast Asia, we discuss (a) karsts' role as arks of biodiversity, (b) their importance to humanity, (c) the conservation status of karst-associated species, (d) the threats posed by anthropogenic disturbances, and (e) the challenges facing karst conservation. Karsts are severely understudied (Vermeulen and Whitten 1999, Dennis and Aldhous 2004), and we hope this article will result in further research and conservation initiatives on these vulnerable ecosystems.