Conservation Challenges and Action for the Critically Endangered Singapore Freshwater Crab Johora Singaporensis
Total Page:16
File Type:pdf, Size:1020Kb
Conservation challenges and action for the Critically Endangered Singapore freshwater crab Johora singaporensis D ANIEL J.J. NG ,DARREN C.J. YEO,N.SIVASOTHI and P ETER K.L. NG Abstract The Singapore freshwater crab Johora singapor- little is known about the ecology of tropical stream ensis is known only from three streams in Singapore and is decapods, with most studies focused on their zoogeography among the top 100 most threatened species in the world. It is and taxonomy (Dobson et al., 2007). the only member of the genus Johora, endemic to the Malay Primary freshwater crabs of the superfamily Potamoidea Peninsula, to be categorized as Critically Endangered on the are arguably the least well studied freshwater decapods. IUCN Red List. Extensive surveys conducted over 1 year to These crabs are adapted to freshwater environments and determine the extent of the species’ distribution revealed are independent of the marine environment for their entire that the population has declined significantly at its type life cycle (Yeo et al., 2008a). Many primary freshwater locality in Bukit Timah Nature Reserve, Singapore Island’s crabs have a restricted geographical distribution, show a oldest protected forest area. The species remains extant in high degree of endemism, and display direct development, two unprotected streams in Bukit Batok and Bukit Gombak, producing small broods of large, yolky eggs, which hatch which are located outside the nature reserve. Acidification of directly into juvenile crabs (Hartnoll, 1988; Ng, 1988; Anger, streams as a result of acid precipitation may be a factor in the 1995; Yeo et al., 2008a; Cumberlidge et al., 2009). decline of the species. If so, this will be the first documented Anthropogenic habitat disturbance is threatening many instance of a species being affected negatively by anthro- species of freshwater crabs, with potamids and pseudothel- pogenic acidification of a tropical freshwater system. The phusids facing the highest risk of extinction (Cumberlidge occurrence of this threatened species in a protected area has et al., 2009). Species under threat are often semi-terrestrial, not, therefore, guaranteed its survival. There is a need for with restricted distributions (Cumberlidge et al., 2009). long-term and continuous monitoring of species of high Stenotopic species that are endemic to islands are vulnerable conservation value, together with other measures that focus to human-induced disturbances such as development and on habitat protection. Conservation efforts by the National pollution (Bahir et al., 2005; Ng & Yeo, 2007; Yeo et al., Parks Board of Singapore in collaboration with other 2008a; Cumberlidge et al., 2009). Conserving these species government agencies are ongoing in an effort to ensure the is challenging, as they are often found in unprotected areas survival of this important species. (Cumberlidge et al., 2009) and therefore their decline may go unnoticed because of lack of monitoring (Ng & Yeo, Keywords Acid precipitation, Bukit Timah Nature Reserve, 2007). freshwater decapod crustacean, Potamidae, Singapore, The freshwater crab fauna of Singapore consists of stream acidification six species in three families: Geosesarma peraccae and Geosesarma nemesis (Sesarmidae), Parathelphusa maculata, Parathelphusa reticulata and Irmengardia johnsoni Introduction (Gecarcinucidae), and Johora singaporensis (Potamidae). The latter three are endemic (Ng, 1990, 1997). With the ecapod crustaceans perform an important role in exception of P. maculata, the species are mostly confined to ecosystem function in tropical streams, serving as D forest streams (Ng, 1997). detritivores and omnivorous consumers (Mantel & The endemic Singapore freshwater crab J. singaporensis Dudgeon, 2004; De Souza & Moulton, 2005) and as an Ng, 1986 is one of only three freshwater invertebrates in the important food source for predators (Hinojosa-Garro & top 100 most threatened species (Baillie & Butcher, 2012). Zambrano, 2004; Nyström et al., 2006). However, relatively Although most species of Johora are confined to relatively fast-flowing streams in montane regions (Yeo et al., 2007), DANIEL J.J. NG (Corresponding author), DARREN C.J. YEO and N. SIVASOTHI J. singaporensis is found at relatively low altitudes, although Department of Biological Sciences, National University of Singapore, 14 Science restricted to hilly parts (Ng, 1988). First discovered at Jungle Drive 4, Singapore 117543, Republic of Singapore 1986 E-mail [email protected] Fall Valley stream in Bukit Timah Nature Reserve in , the species was only known from this type locality until PETER K.L. NG Department of Biological Sciences, Raffles Museum of Biodiversity Research, National University of Singapore, Republic of Singapore subsequent surveys of other parts of the island revealed two 1990 Received 30 November 2012. Revision requested 8 January 2013. other populations in the early s. Historically, the type Accepted 5 April 2013. First published online 29 April 2014. locality supported the largest population, although numbers Oryx, 2015, 49(2), 345–351 © 2014 Fauna & Flora International doi:10.1017/S0030605313000707 Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.58, on 02 Oct 2021 at 08:25:59, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605313000707 346 D. J. J. Ng et al. FIG. 1 The study area, with built-up areas and major roads (exact localities of the Singapore freshwater crab Johora singaporensis populations have been omitted at the request of the National Parks Board). The rectangle on the inset indicates the location of the main map in Singapore. there appeared to have fallen (Ng, 1995, 1997), with the last Singapore has few hill streams. These are usually shallow confirmed sighting in the early 1990s. Prior to this study (, 1 m), fast flowing, well shaded by the forest canopy but J. singaporensis had only been reported from the two other sparse in riparian vegetation, and less disturbed than locations: a stream within a small patch of disturbed lowland streams. The substratum consists mainly of rocks secondary forest close to Bukit Batok Nature Park, and a and/or sand, with accumulations of leaf litter (Ng, 1991, 1994; stream at Ministry of Defence land at Bukit Gombak Yeo & Lim, 2011). (Ng, 1990, 1995, 1997; Yeo et al., 2008b). Bukit Timah Nature Based on Ng (1990, 1995, 1997), museum records in the Reserve is one of only four gazetted nature reserves with the Raffles Museum of Biodiversity Research at the National highest level of protection in Singapore; the other two University of Singapore, our own unpublished data known locations of the species are not within a formal (PKLN and DCJY), and discussions with carcinologists nature reserve and are therefore unprotected. The ecological who have worked and sampled in freshwater environments information available for this species is based on ad-hoc in Singapore (C.D. Schubart, S.H. Tan & N. Tohru), the observations and no systematic ecological study has been known distribution of J. singaporensis comprised three sites: carried out (Ng, 1988; Ng & Yeo, 2013). The species was a stream at Jungle Fall Valley in Bukit Timah Nature Reserve initially categorized as Endangered on the IUCN Red List (S1), a stream in Bukit Gombak (S2) and a stream in Bukit (Yeo et al., 2008b), based on its restricted range, but Batok (S3). The exact locations of these sites are not shown following a reappraisal it was categorized as Critically (Fig. 1), to prevent poaching. Endangered (Cumberlidge et al., 2009). J. singaporensis is part of a basal lineage of Johora (Yeo et al., 2007) and represents the southern limit for the distribution of the Methods genus (Ng, 1988), which makes its conservation an even We surveyed all three sites during October 2007–February greater priority. Our objectives were to determine the status 2008 to determine the status of their J. singaporensis of J. singaporensis through a survey of its known distribution populations. Care was taken to minimize disturbance to the range, including historical collection sites, and to assess the species. challenges for its conservation. We conducted preliminary surveys (1 day and 1 night), including sieving coarse woody debris and leaf litter, to Study area determine whether the species was present at each of the three sites. We detected J. singaporensis at S2 and S3 but 2 Singapore (c. 700 km ) is an island in South-east Asia, at the not at S1. southern tip of the Malay Peninsula. It is a highly urbanized Based on the preliminary surveys we set different lowland island, with only remnants of primary forest objectives for each site. At S1 the objective was to conduct remaining (Corlett, 1997). The temperature is relatively a thorough investigation to determine if J. singaporensis was consistent, with an annual mean of 27 °C, mean annual present. On eight occasions two experienced field workers relative humidity is 84.2%, and rain falls throughout the searched the entire stream by hand during 19.30–21.30. They year (2,342 mm annual total). wore headlamps and combed through coarse debris, turning Oryx, 2015, 49(2), 345–351 © 2014 Fauna & Flora International doi:10.1017/S0030605313000707 Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.58, on 02 Oct 2021 at 08:25:59, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605313000707 Singapore freshwater crab 347 TABLE 1 Results of the surveys for the Singapore freshwater crab Johora Singaporensis, from three sites in Singapore (Fig. 1), with mean number of individuals detected per survey, density, and water pH. One-way ANOVA was used to compare numbers detected per survey, density and pH between sites. The Student–Newman–Keuls test was used to assess the difference in pH between sites. Mean no. of Mean density ± SE − Site individuals ± SE1 (m 2)1 pH ± SE2 Bukit Timah Nature Reserve (S1) 0 0 4.5 ± 0.2 Bukit Gombak (S2) 9 ± 13± 0.5 6.6 ± 0.2 Bukit Batok (S3) 27 ± 210± 0.8 6.4 ± 0.2 1 Significant difference between S2 and S3 2 Significant difference between S1 and S2 and between S1 and S3 over small rocks.