Evaluation of aquatic insect diversity in natural water-filled tree holes and their artificial analogues, in a tropical forest of Western Ghats

Background and questions:

Insects of small aquatic habitats found in plants, called 'phytotelmata' (plant-held waters), have attracted low attention from naturalists for the greater part of the last century. For biological investigations, the relatively small accumulations of water occurring in epiphytes, and tree holes offer several methodological advantages over lakes, streams, and other comparatively large systems. They are: 1) phytotelmata are discrete and can be treated as individual units for sampling and faunal surveys, 2) these habitats are often abundant where they occur, permitting sample sizes appropriate for statistical analyses, 3) the macro fauna of phytotelmata is often specialized and of manageable diversity and abundance-this is especially true of the aquatic insect inhabitants, and 4) water-filled tree holes are among the most tractable of small aquatic systems, in part because they are relatively persistent, and can be mimicked with plastic cups, bamboo sections, or other inexpensive materials. Despite these unique features of these water holes in the trees and their specialized inhabitants, the extent to which processes affecting their biodiversity and community structure can be generalized to larger systems remains to be examined.

This study proposes to evaluate the variation in the structural and compositional diversity of the aquatic insect community at two spatial scales, viz., 1) vertical distribution of the aquatic insect community in natural tree holes, and 2) the community structure at different tree species in a tropical rainforest. The major objectives are: to enumerate the potential differences in the species assemblages in the water filled tree holes and its artificial analogues; to test the hypothesis that tree holes with higher quantity of water and leaf litter support higher richness and densities of common resident insects than tree holes with lower amounts of these resources; to determine the vertical stratification of the species composition in the trees and finally to  evaluate the aquatic insect community differences at the spatial scales at different locations in a tropical rainforest to assess/ predict the general ecosystem health and properties of forest stand dynamics.

A view of the New Amarambalam Reserve Forest

Camp site at the Upper Nilgiris

Natural Tree holes: For quantifying the number of tree hole phytotelmata in the study area we have carried out initial survey in treck path covering 30 km by scanning 20 meter coverage in breadth of the path. In the survey we have marked and collected data of tree hole characteristics of 137 tree holes (Table-1). Most of the tree holes are formed by branch and trunk breakage as rot holes (Figure-01) and remaining are formed by morphological formation in tree root buttress as pans (Figure-02). They occur in height level from ground to 4 meter of the tree and vary in size from 2.5-3 cm in diameter to 30-40 cm diameter. Around 32% of the marked tree holes are having water in it and sampled for aquatic insects.

Table-01: Marked Tree Holes in Study Area

During this period we have standardized the sampling of aquatic habitat in tree holes. Initial stage the sampling is composed of transfering all the materials in tree hole comprised of water and detritus to a tray and then searching for organisms and preserving it. This mode of sampling is very time consuming and destructive to the habitat. So we have changed the sampling regime based on the volume of Tree hole phytotelmata and large tree holes having water holding capacity greater than 2 litre are sub sampled. The volume of water in these holes is ranged from 50 ml to 15 litre and comprise of litter and soil debris. The recorded families of aquatic insects from these holes are Culcidae, Syraphidae, Tipulidae, of Diptera and Hydrophilidae of Coleoptera, and Leptoceridae of Trichoptera and Veliidae of Hemiptera. Frogs and lizards of Philautus sp. and Cnemaspis sp. respectively are sited from the tree holes of Silent Valley National Park.

Initial literature survey shows that complete absence of Order Trichoptera from Tree hole aquatic habitat from other studies carried out in different parts of the world.  The insect belonging to the Leptoceridae, which is making their casing with plant material is a logically obvious inhabitant in tree hole phytotelmata due to presence of litter mass in tree hole. Tree hole aquatic insects are evolutionarily preadapted to live in this unique habitat. The occurrence of Trichoptera is another noticeable observation, as per the available reports no other studies conducted whole over the world, reported the trichoptera.

The occurrence of canopy frogs, lizards, snakes and nesting birds shows tree hole's importance as habitat for higher invertebrates and vertebrates. Ongoing work is proposed to quantitatively survey the occurrence and characteristics of tree holes which can be used by other larger organisms which gives insight about the conservation and management option to evaluate this unique habitat.

Artificial Analogies of Natural Tree holes: Tree hole phytotelmata as discrete habitat acts as one of the important tool in community ecology. It is used for study species dispersal, food web dynamics, species diversity and ecosystem functionality. Plastic containers are extensively used as an artificial analogy for tree hole to circumvent physical variability of natural tree hole for varies experiments. In the present study along with plastic containers (Figure-03) we experimented with other materials such as Bamboo internodes (Figure-04) and Tyre tubes (Figure-05) as tree holes analogies (Table-02).

Table-02: Details of Artificial analogies established in study area.

To test the hypothesis that tree holes with higher quantity of water and leaf litter support higher richness and densities of common resident insects, we have used artificial analogies having standardized litter source and different volume level. To study the aquatic insects we have also employed 20 tyre tubes having filtered water and standardized litter source positioned in varied distance from the species pool. To assess whether the plant species diversity influence on the aquatic insects colonizing tree hole we established analogies comprised of standardized litter source from five dominant tree species viz Mesua ferrea, Palaquium ellipticum, Myristica sp., Cullenia exarillata, Eleocarpus sp  in the study area. Initial survey of artificial analogies shows that plastic containers are colonized by Mosquitoes, Syraphidae, and Tipulidae and often visited by frogs.

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Dr. Hoo Zoo Lee (UNEP), one of the collaborators of the project during his visit to WRCT, Nilambur

This project is financially supported by:

For more details about the project contact Dr K.S. AnoopDas at:

dasksa@wrctindia.org, dasksa@gmail.com