Conservation importance of limestone karst outcrops for Palaeotropical bats in a fragmented landscape
Introduction
‘Conservation landscapes’ should ideally include areas that support high biodiversity, as well as the key landscape features that sustain it (Sanderson et al., 2002). This is particularly important in the human-dominated landscapes of Southeast Asia, where forests are becoming increasingly disturbed and fragmented, and commercial pressures on remaining habitats have far reaching consequences for wildlife (Sodhi et al., 2004, Laurance, 2007). Conservation strategies designed to influence the locations and mitigation of land developments require an understanding of how features within these landscapes maintain wildlife populations; yet this information is often scarce or lacking (Meijaard and Sheil, 2007).
Among the forested landscapes in Southeast Asia, those that contain limestone karst systems are internationally recognised as areas of huge biological importance, with aesthetic qualities and groundwater value (Wong et al., 2003, Gillieson, 2005). Limestone karsts are defined as sedimentary rock outcrops that were created millions of years ago by calcium-secreting marine organisms, and have been subsequently uplifted and eroded (Clements et al., 2006). They are prominent features of much of Southeast Asia, covering over 40 million hectares, of which only 13% is nominally protected (Day and Urich, 2000). Most karsts are scattered and isolated as small limestone hills, but nonetheless many of them support high levels of species diversity and endemism of plants, vertebrates and invertebrates (Schilthuizen et al., 2005, Clements et al., 2006, Clements et al., 2008). Unfortunately, not only are karst outcrops frequently isolated by urban and agricultural areas, but they are also under immense pressure from commercial mining interests, as well as the local harvest of guano and collection of bird nests (Hutson et al., 2001, Hobbs, 2004, Clements et al., 2006). This has led to revived efforts to both include karsts in conservation planning, and by using scientifically sound criteria, promote their biological importance in land-use decisions (Clements et al., 2006, Clements et al., 2008).
Amongst the many biological attributes of karst outcrops, one that is often neglected in tropical conservation research is the importance that these landscape features have as roosting resources for bat populations. The availability of suitable roosting sites is essential in determining bat distributions (Lewis, 1995), and most bat families and genera contain species that roost in caves, which are typical features of karst areas. Cave networks can host large aggregations of bats, and although systematic documentation is lacking, karst sites often support substantial bat diversity (Arita, 1996, Niu et al., 2007, Kingston, 2009). Cave preservation is therefore important if we are to maintain viable bat populations. Indeed, cave disturbance is recognised as a major threat to bat diversity (Hutson et al., 2001, Niu et al., 2007, Racey, 2007), and may be a more important threat than landscape changes in some regions (Goodman et al., 2005). How these key roosting resources actually influence bat diversity elsewhere in a landscape, however, remains largely undocumented. Caves are a rare and unevenly distributed feature of most landscapes, and cave-roosting species appear to commute large distances to forage each night (Pavey et al., 2001, Bontadina et al., 2002, Struebig et al., 2008). Beyond these distances, however, cave-roosting species are expected to be rare, or absent (Lewis, 1995). Anecdotal reports attribute low levels of bat diversity at sites in Southeast Asia to disturbance at local caves and their surrounding habitat (Robinson and Webber, 2000, Suyanto and Struebig, 2007), yet the extent to which local assemblage structure is influenced by cave resources at distant sites has yet to be addressed. This is a simple, yet important question, because if bat assemblages are strongly shaped by the presence of a major roosting resource, then their long-term conservation over a landscape requires protection of this resource as well as the habitat in which they forage.
We assess the conservation importance of an isolated outcrop of limestone karst in Southeast Asia by quantifying its impact as a roosting resource on the assemblage structure of bats in the surrounding landscape. Our study was based in peninsular Malaysia, where more than 800 karsts have been reported, the majority isolated by agriculture and under pressure from commercial mining interests (Clements et al., 2008). We focused on the ‘narrow-space’ ensemble of insectivorous bats (sensu Schnitzler and Kalko, 2001), which are highly adapted to foraging in the forest interior, and predicted to be sensitive to land-use changes (Kingston et al., 2003). In the Palaeotropics, this ensemble is dominated by members of the families Rhinolophidae and Hipposideridae, and Vespertilionidae subfamilies Kerivoulinae and Murininae, which exhibit varying sensitivities to land-use change (Kingston, 2009). We have previously shown that tree cavity/foliage-roosting species are particularly susceptible to area-dependent effects of forest fragmentation (Struebig et al., 2008). Cave-roosting species, on the other hand, appear more resilient to fragmentation, but their long-term persistence in a landscape may be more dependent on the stability of a few major cave roost sites.
We predict that major cave systems will influence the composition and abundance of bat assemblages across a landscape, and that these trends will vary with proximity to the roosting resource. Specifically, we hypothesised that (1) the abundance of cave-roosting bat species will be higher at sites nearer to a karst outcrop, and lower further away and (2) the karst site will act as a population ‘source’, so that assemblages nearer to the site will be characterised by the same species that dominate the karst assemblage (leading to a similarity gradient). With Southeast Asian karsts facing unprecedented pressures from mining activities (Clements et al., 2006), it is vital to document their role in supporting wildlife populations, if we are to develop practical approaches to conserve biodiversity over large spatial scales (Meijaard and Sheil, 2007, Struebig et al., 2009).
Section snippets
Study landscape and sampling design
We sampled bats at the Gunung Senyum Recreation Park and other forested sites as part of a forest fragmentation study in the Krau landscape of central Pahang (3°40′N, 102°10′E) between May 2002 and June 2007 (Struebig et al., 2008). The park encompasses 1356 ha of lightly-logged lowland dipterocarp forest and comprises two limestone karst hills with at least 20 caves. The outcrops have Permian/Lower Triassic origins and are considered large and isolated from other karst areas according to
Patterns of bat assemblage composition
The karst outcrops at Gunung Senyum were characterised by a superabundance of cave-roosting bats, and absence or rarity of tree cavity/foliage-roosting bats. Of the 1962 insectivorous bats captured over the nine sites, 1080 individuals of 15 species were captured at Gunung Senyum, 99% of which are known to predominantly roost in caves (Appendix A). The karst assemblage was dominated by three cave-roosting species (Hipposideros bicolor 142 kHz, Rhinolophus affinis and Rhinolophus lepidus), which
Discussion
We found that bat populations in an isolated limestone karst outcrop had a substantial influence on assemblage structure elsewhere in our study landscape. Several lines of evidence indicated that the local structure of insectivorous bat assemblages at forested sites was in part determined by the distance to a karst outcrop. The bat assemblage at our karst site was dominated by a superabundance of three cave-roosting species, two of which were also found to dominate assemblages at sites up to 11
Acknowledgements
Thanks to the Economic Planning Unit of the Malaysian Government for granting us permission to conduct bat research in Malaysia, and the Malaysian Department of Wildlife and National Parks (DWNP), the Pahang State Forestry Department, the Federal Land Development Authority (FELDA), and numerous private landowners for allowing us access to research sites. Thanks also to Paul Banks, Christine Fletcher, Lee-Sim Lim, Nordin, Juliana Senawi, Rakhmad Sujarno Kudus, Tony Wood and Zamiza Zainal for
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2020, Basic and Applied EcologyCitation Excerpt :The different responses to landscape factors found among bat species are usually associated with different feeding habits or flight capacity: less vagile, more carnivorous species can be affected in a different way when compared with highly mobile frugivorous ones, for example (Avila-Cabadilla et al., 2012; Ducci et al., 2015; Klingbeil & Willig, 2010). Understanding how cave bat communities are related to the characteristics of caves per se and the landscape around them can facilitate a better understanding of roost selection processes, to elucidate the peculiar relationship between these animals and their roosts, as well as for the management and conservation of cave environments (e.g., Furey & Racey, 2015; Struebig et al., 2009). When selecting caves for conservation, choosing those with characteristics associated with higher bat diversity is more effective than randomly selecting them (Phelps et al., 2016).