Terrestrial molluscs in Nee Soon swamp forest 
117 
Species diversity and richness 
The Shannon-Weiner Index (H’) is commonly used as a measure for species diversity 
between habitats (Krebs, 1989). Based on the average H’ calculated above for the 15 
sampling plots (Fig. 3), the Lower Swamp 1 had the highest species diversity (H’= 
0.53), while the Upper Swamp had the lowest species diversity (H’=0). Many of the 
non-native snails are recorded in the Outskirts, reflected in a high H’ but this does not 
reflect the true native diversity. 
The Species Richness (R) indicates the average number of species present at 
each of the survey sites (Fig. 4). Based on the table above, Lower Swamp 1 had the 
highest species richness (R=4.5) and Upper Swamp 2 had the lowest species richness 
(R=0.33). The number of non-native snails recorded in the Outskirts distorts the 
species richness value there. 
Mean population abundance 
The three most abundant species found in Nee Soon freshwater swamp forest are 
Liardetia convexoconica, Helicarion perfragilis and Hemiplecta Humphreysiana. All 
three species had a mean frequency of more than ten individuals per cycle of sampling 
(Fig. 5). However, six species of snails: Japonia ciliocinctum, Lamellaxis gracilis, 
Microcystina sp., Geotrochus lychnia, Parmarion martensi and Microparmarion 
strubelli are extremely rare in Nee Soon and had a low mean frequency of fewer than 
one per cycle of sampling. 
Discussion 
Studies on the land snails in Nee Soon freshwater swamp forest revealed a rich diversity 
despite the low abundance. Nineteen species of snails and slugs were recorded of 
which one species was documented for the first time in Singapore. Of the 19 species, 
16 are native and three are introduced. Eight are forest dependent species: Cyclophorus 
perdix aquila, Cyclotus rostellatus, Japonia ciliocinctum, Dyakia kintana, Hemiplecta 
Humphrey siana, Microparmarion strubelli, Damayantia cf. simrothi and Amphidromus 
atricallosus temasek, only observed in forested areas with a dense canopy cover. 
Introduced species are restricted to the outskirts of the Nee Soon freshwater swamp 
forest, with the exception of Achatina fulica on rare occasions. 
It is not surprising that low abundance of land snails was observed due to 
the nature of a freshwater swamp forest. As it is well known that land snails have a 
high calcium requirement for the formation of their shells and eggs, the abundance 
of snails correlates with the soil calcium content (Graveland et al., 1994). However, 
Singapore’s tropical soils are generally acidic and poor in calcium (Chia & Foong, 
1991), especially in freshwater swamp forest. Thus a shortage of calcium in Nee Soon 
soils (Nguyen et al., 2018) may be a limiting factor to the growth and reproduction 
