Dragonflies of Nee Soon swamp forest 
143 
that are associated with an open canopy. It is commonly associated with fast flowing 
water, but hardly found in shaded forest streams with high canopy cover. When they 
described a new species of freshwater snapping shrimp Potamalpheops amnicus, Yeo 
& Ng (1997: 171, 172) commented that “Interestingly, there is a high likelihood that 
these shrimps originated from unintentional introductions from Peninsular Malaysia. 
This is because the Sedili drainage in Kota Tinggi, Johore, is one of the water sources 
for Singapore with water from there being transported via pipeline to the Upper Pierce 
Reservoir. The Sedili drainage is where Potamalpheops amnicus is found in far greater 
abundance than in Singapore. It is therefore entirely possible for the shrimps to enter 
Singapore via the pipeline and become established in certain areas suitable to their 
habitat preference like the present stretch of stream.” Prodasineura humeralis has long 
been a common and widespread species of forest streams in Peninsular Malaysia (Orr, 
2005). There is, therefore, a likelihood of larvae being incidentally transported from 
Malaysia via pipeline into Singapore waters. Both its rapid dispersal history, and the 
current distribution patterns mainly in open forest streams at the outskirts of the nature 
reserves, mirror those of many aquatic introduced species in Singapore (Yeo & Chia, 
2010). Its potential impact upon other native damselfly species needs to be closely 
monitored. 
Community structures and distribution patterns 
Odonates of Nee Soon freshwater swamp forest can be grouped into three distinct 
community assemblages: 1) headwater, 2) mainstream channel, 3) outskirts and/or lower 
stream communities. Each of these community assemblages is linked to a distinctive 
suite of riparian vegetation, hydrological and physicochemical characteristics. 
Community assembly theory is founded on the premise that the relative 
importance of local environmental processes and dispersal shapes the compositional 
structure of metacommunities. Four general models describe interesting combinations 
of these factors and are frequently used to interpret observed communities: 1) neutral 
model, 2) patch dynamics, 3) mass-effect, and 4) species sorting (cf. De Marco et 
al., 2015). The species sorting model predicts that assemblages are dominated by the 
environmental filtering of species that are readily able to disperse to suitable sites. 
De Marco et al. (2015) propose an eco-physiological hypothesis for the mechanism 
underlying the organisation of species-sorting odonate metacommunities based on the 
interplay of thermoregulation, body size and the degree of sunlight availability in small- 
to-medium tropical streams. They considered that narrow streams are more affected by 
riparian vegetation than are wide streams, which reduces light input but also generates 
a more stable thermal environment. They considered these characteristics favour small 
species that are less dependent on exposure to direct sunlight for thermoregulation, 
and limit the colonisation of larger, heliothermic species. To quote directly: “The 
taxonomic distinction between small streams (dominated by Zygoptera) and larger 
rivers (by Anisoptera) is an incidental consequence of differences in body size and 
associated thermal properties between these two groups” (De Marco et al., 2015). 
The species composition and distribution patterns of odonates observed in Nee Soon 
support such a hypothesis. The dominance of small Zygoptera in forested headwaters 
