| Back | Patterns of Land Snail Distribution in the Western Ghats Aravind N. A., Rajshekhar K. P. and N. A. Madhaystha |
Molluscs are the second largest invertebrate group in abundance next only to insects (Abbott 1989; Bouchet 1992). The estimated species richness in this group varies from 80,000 to 135,000 species (van Bruggen 1995). Among these 31,000– 100,000 are marine, 14,000–35,000 terrestrial and about 5,000 freshwater species (Abbott 1989; Seddon 2000). Much of this diversity occurs in the tropical world. Despite this great diversity, hardly any studies have been carried out in the topical world. Much of the studies worldwide have focused on the freshwater and land molluscan ecology in the temperate regions of the world. However, diversity of land molluscs in the tropics, did not receive much attention till recently. Tropical rainforests are known for their rich land snail diversity and majority of them are found in the leaf litters and the soil (Emberton 1996) and their biomass is of great ecological significance. Most tropical molluscs remains to be described, partly because of under-exploration, and partly due to their minute sizes (Stanisic 1990; DeWinter 1992, 1995; Tattersfield 1994; Cowie et al , 1995; Emberton 1995a,b, 1996).
Molluscs exhibit a great deal of adaptation to specialized ecological niches, making this group more vulnerable to modifications in the environment (Bouchet 1996). Consequently molluscs have suffered a severe decline in diversity and abundance due to human induced alteration of habitats, pollution, siltation, deforestation, poor agricultural practices, the destruction of riparian zones and invasion by exotic species (Biggins et al , 1995). Hence, conservation efforts are urgently needed to maintain and recover these unique components of terrestrial and aquatic biodiversity. Land snails are an important component of the leaf-litter/soil biota and are responsible for nutrient recycling along with other soil invertebrates. Land-use change is projected to have the largest global impact on biodiversity by the year 2100, especially in the tropics (Chapin et al , 2000). Introduction of exotic species has led to extinction of native flora and fauna. High rates of extinction due to human activities have occured in bivalves of continental rivers and land snails of Islands (Pimm et al , 1995). Non-marine molluscs, as a group, comprise the largest number of recorded extinctions (284 species) in the last 300 years (Groombridge 1993). This is far more than bird and mammal extinctions in the same period. Hence, there is an urgent need to document the cryptic taxa such as land snails which is undergoing rapid extinction. Although there are some localized studies and reviews in the Western Ghats (Tonapi 1971; Sathymaurthi 1960; Naggs 1997; Seddon 2000), a comprehensive document of the diversity, distribution and endemicity of land snails is lacking. Here, we review basic pattern of diversity, distribution and endemicity of land snails of Western Ghats based on our observation over the past decade.
Studies on Land Snails of the Western Ghats
Majority of the studies on land snails of the Western Ghats were during 19 th century by British naturalists, namely William Benson, Beddome, Blanfold and Godwin-Austen. Benson was regarded as a pioneer in the Indian sub-continent on malacology (Naggs 1997). But the earliest publication on Indian malacology is by Woodward with a review of land molluscs, way back in 1856 (Mitra et al , 2004). Later several British and Indian malacologists contributed considerably to the taxonomy on terrestrial snails of India. They are Annandale and Rao (1933a,b), Benson (1832, 1836, 1851, 1852, 1854, 1857, 1859a,b,c,d, 1860a,b, 1861a,b, 1863a,b,c and 1864), Beddome (1875, 1891 and 1906), Blanfold (1862, 1865, 1870, 1880 and 1899), Blanfold and Blanfold (1860 and 1861), Godwin-Austen (1876, 1879, 1882, 1893, 1895, 1914, and 1917), Gude (1906, 1914, 1915 and 1921), Hanley and Theobald (1876), Hutton (1834), Nevill (1878 and 1881), Pilsbry (1909 and 1910), Preston (1908 and 1915), Rao (1924), Theobald (1860, 1875, 1876 and 1878) and Tryon (1869). Annandale and Rao have contributed on the biology and distribution of both freshwater and terrestrial molluscs during early and mid 20 th century.
Recently a couple of compilation works on land snails have been published by the Zoological Survey of India. One publication has reviewed endemic land mollusc of India which also includes Western Ghats (Ramakrishna and Mitra 2002) and another gives a brief description of selected land snails occurring in India. Recently Madhyastha et al . (2004) reviewed the status of land snails of the Western Ghats and Mavinkuruve et al . (2004b) put a case forward for use of mollusc in conservation planning. Same authors made a provisional checklist of Karnataka state (Mavinkuruve et al . 2004a). An attempt to address the diversity, endemism and geographical distribution patterns land snails of the Western Ghats has been made by Aravind et al . (2005) that showed a south-north gradient richness on terrestrial snails with the southern Western Ghats harbouring more endemic species. Apart from these studies, several papers appeared/appears on the inventory of land snail diversity in several protected areas such as Rajiv Gandhi (Nagarahole) national park (Ganesh et al . 2002), Kudremukh national park (Madhyastha et al . In press), Sharavathi wildlife sanctuary (Mavinkuruve et al . 2005).
Diversity and Endemism of land snails in the Western Ghats
Till date 1487 species of land snails belonging to 32 families and 140 genera have been reported in India (Ramakrishna and Mitra, 2002; Table 1). Of which, 269 species belonging to 24 families and 57 genera (Table 2) are known to occur in the Western Ghats eco-region. Like some of the lower vertebrates such as amphibians and reptiles, land snails also shows very high percentage of endemicity. 204 species out of 269 species are endemic, which is almost 76% of the land snail fauna of Western Ghats. Western Ghats harbours 92% and 41% of the family and genera respectively occur in India (Ramakrishna and Mitra 2002; Figure 1). Of the 57 genera, genus Glessula is species rich with 54 species of which 41 species are endemic, followed by Ariophanta, Cyathopoma and Euplecta with 17, 16 and 16 species respectively (Table 3). Significant genera of such as Cyclophorus , Indrella , Diplommantina and Alycaeus are completely endemic to the Western Ghats. Some interesting land snails include Indrella ampulla which is a mono-specific genera is endemic to the Western Ghats (Figure 1). All the species in the genus Cyathopoma are endemic to the Western Ghats. A number of species of land snails are known only from the type locality. For example Lithotis rupicola , amphibious snail which was described in 1863 from Khandala waterfalls near Pune, is known only from type locality. There could be several reasons for this, one could be the species itself be rare due to several climatic and microclimatic reasons, anthropogenic pressures and could be due to lack of proper surveys as this species probably seen only during monsoon season (Naggs 1997).
Table 1: Molluscan diversity in India
Forms |
# Families |
# Genera |
# Species |
% |
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Source: Ramakrishna and Mitra 2002 |
Table 2: Taxonomic break up and comparison of land snail diversity in India and in the Western Ghats.
No. in India |
No. in WG |
% |
|
Families |
26 |
24 |
92.31 |
Genera |
140 |
57 |
40.71 |
Species |
1488 |
269 |
18.10 |
Source: Aravind et al. 2005 |
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Figure 1: Number of species, endemic species and endemic species in Western Ghats.
Table 3: Number of species and endemics in each genus of land snails of Western Ghats.
Genus |
Total species |
Endemics |
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Distribution pattern of land snails in the Western Ghats
We have divided the Western Ghats into three regions, viz., South (08 0 N to 12 0 N) central (12 0 N to 16 0 N) and north (16 0 N to 21 0 N) based on earlier classification (Dahanukar et al . 2004) and overlaid the digitized molluscs data. Our study shows that southern region is species rich with 210 species followed by central and north with 85 and 62 species respectively (Figure 2). This pattern of high species richness in southern Western Ghats is also seen in other groups such as plants, birds, mammals and butterflies except for amphibians in which central Western Ghats is richest. Based on the above analysis, three regions of high species richness and endemism have been identified, they are as follows:
Most of the endemic species occur in the southern Western Ghats. 56 % of the 210 species recorded from the southern Western Ghats, were endemic, whereas central Western Ghats and northern Western Ghats has 16 % and 26 % of endemic species respectively. Species richness shows a south-north gradient with the richness decreasing towards the north. This could be attributed to high rainfall, shorter dry periods and climatic variability in the southern part of Western Ghats. Much of the endemic land snail distribution, like that of birds and plants, is found at mid to high altitudes.
Several species of the Western Ghats are known only from the type locality. The significant difference in the species richness between south and central and south and north could be due to lack of extensive studies on this lesser known taxa in central and northern region. The distribution of land snails in the plains and coastal region are less known. Significant proportion of the study was mostly concentrated at hilly regions especially in the southern Western Ghats. The genus Craspedotropis has disjunct distribution found in Western Ghats, Eastern Ghats and North-East India.
Figure 2: Number of species in southern, central and northern region of Western Ghats.
Altitudinal and Latitudinal variation in species distributionAltitude and latitude plays a significant role in the distribution of organisms. The effect of altitude and latitude on the distribution of land snails of Western Ghats was assessed (Aravind et al . 2005). The analysis shows that there is no significant correlation in the species richness as the altitude increases ( r= 0.41, p>0.05 ). However, low to mid altitudinal ranges harbours more species than high altitude ranges (Aravind et al . 2005). Latitude range from 10 to 12 0 N harbours nearly 65 per cent of the land snails and then the richness decreases significantly towards north for all species ( r = -0.74, P<0.05) endemic species ( r=0.75, P<0.05) and non-endemic species ( r=-0.63, P<0.05 ). The number of families also decreases significantly from south to north ( r= -0.74, P<0.01 ) (Aravind et al . 2005). The Sorenson's qualitative similarity index was computed to assess the per cent similarity between 8 to 22 0 N latitudes. It is clear that there is very high degree of dissimilarity ( =1-similarity ) between the regions (Table 4). Only regions 8-10 and 10-12; 14-16, 16-18 and 18-20 0 N have moderate levels of similarity (40 to 50 per cent).
Table 4: Sorenson's similarity index between different latitude.
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Source: Aravind et al . 2005 |
The land snails are very diverse and ecologically important group as they cycle nutrients form the soil. Western Ghats land snail fauna is unique with 76% of them being endemic, highest compared to several other taxa (Table 5). Several genera are found only in the Western Ghats and some are known only form the type locality. Experts feel that there could be 10% more land snails to be discovered in the Western Ghats hotspot. Figure 3 shows the species discovery since the time of Linnaeus till date as per the species accumulation curve.
The south-north latitudinal gradient in the distribution of land snails and endemicity is in accordance with the several studies world wide that the southern latitude is richest and in the Western Ghats for fishes, amphibians, birds and endemic plants.
Table 5: Comparison of endemic species and per cent endemicity of land snails with that of plants, invertebrates and vertebrates of Western Ghats.
Group |
Taxa |
Total species in Western Ghats |
Number of endemic species |
% endemics |
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(Table adapted and modified from Subramanian and Sivaramakrishnan 2002; D aniels, 2003; Babu and Nayar, 2004; Dahanukar et al., 2004; Gururaja, 2004) |
Figure 3: Discovery pattern of land snails from the Western Ghats
Acknowledgements
The work reported here is partially supported by International Foundation for Science (IFS), Sweden to Aravind N. A. and Ministry of Environment and Forest (MoEF), Govt. of India to N. A. Madhyastha. We thank Dr. T. Ganesh, Dr. Soubadra and Dr. Gururaja K. V. for their valuable comments and suggestions on the manuscript. We also like to thank Mr. Bipin Charles for providing map of the Western Ghats.
Aravind N. A.
Ashoka Trust for Research in Ecology and the Environment (ATREE),
#659, 5 th A main, Hebbal, Bangalore 560024.
Phone: +91-80-23530069, 23638771
Fax: +91-80-23530070
Email: aravind@atree.org
Rajshekhar K. P.
Department of Applied Zoology,
Mangalore University, Mangalagangothri-574119,
Dakshina Kannada District, India.
N. A. Madhaystha
Co-ordinator, Malacology Centre,
Poorna Prajna College, Udupi-576101, India.
