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Niche Partitioning and Population Structure of Sympatric Mud Snakes (homalopsidae) from Bangladesh

Shahriar Caesar Rahman
School of Environmental Science and Management, Independent University, Bangladesh. Bashundhara, Dhaka, 1229, Bangladesh

AHM Ali Reza
Division of Biological and Physical Sciences, Delta State University, Cleveland, MS 38733, USA

Rupa Datta
School of Environmental Science and Management, Independent University, Bangladesh. Bashundhara, Dhaka, 1229, Bangladesh

Chris L. Jenkins
The Orianne Society, 579 Highway 441 South Clayton, GA 30525, USA

Luca Luiselli
Centre of Environmental Studies Demetra, via Olona 7, 00198 Rome, Italy

Abstract/ Executive Summary:

Despite the greatest diversity of snakes being in the tropics, detailed ecological studies are rare, especially in tropical Asia. We studied the ecology of a coastal marine homalopsid (rear-fanged, aquatic snakes) assemblage in southeastern Bangladesh. Data were collected on community structure, resource partitioning (diet and habitat), body size and sexual size dimorphism. A total of 653 specimens belonging to three species were collected: Cerberus rynchops (81% of total capture), a mediumsized piscivorous snake, found to be the most abundant species in the study site followed by two crustacean eaters, Gerada prevostiana (13%) and Fordonia leucobalia (6%). The three species were relatively similar in terms of body size but were inconsistent with each other both in terms of morphological patterns and demography characteristics, with sex-ratio being equal in two species but female-biased in G. prevostiana. There was no apparent non-random resource partitioning along the microhabitat axis but a clear pattern of niche partitioning was observed along the food axis. Despite the very unusual evolutionary history of the Homalopsidae inside the group of the Colubroidea, our snake assemblage very closely resembled other communities of snakes worldwide.

Keywords: Bangladesh, Food niche, Habitat, Homalopsidae, Mud snakes, Population structure

Location: Sonadia Island, located in the far southeastern corner of Bangladesh

Start Date:

End Date:

Program Area: Animal Health and Management

Commodity: Snakes

Objectives:

In this paper we aim to (i) provide ecological data on the community structure, food and habitat resource partitioning, and body size and sexual size dimorphism in an assemblage of homalopsid snakes from Bangladesh, and (ii) discuss these data in the light of general findings of community ecology of snakes.

Methodology:

Study area Our study was conducted at Sonadia Island, located in the far southeastern corner of Bangladesh, approximately nine kilometres northwest of Cox’s Bazaar town. Sonadia is a ~4,900 ha barrier island separated from Moheshkhali Island by a tidal canal. The elevation of this island ranges from 0–4 metres and the depth of the mud varies from a few centimetres to few metres (CWBMP, 2006). The climate of the island is moist tropical maritime with high temperatures year round. The island receives high rainfall concentrated in the monsoon period (June–September) and a dry period from November–March (CWBMP, 2006; average recorded annual rainfall 2,867 –4,684 mm). The mean annual maximum and minimum temperatures recorded were recorded as 30.3–33°C and 19.3–22.4°C, respectively between 1987 and 1996 (CWBMP, 2006). Sonadia Island comprises of a wide variety of habitat types, including mudflats, mangroves, intertidal mixed grassy vegetation, salt marshes, sand dunes, lagoons and sandy beaches (Chowdhury et al., 2011), and supports some of the last remaining patches of natural mangrove forest found in southeastern Bangladesh. The mangrove area is dominated by Avicennia officinalis, A. marina, A. alba, Sonneratia apetala, Aegicerus corniculatum, Ceriops decandra and Aegialitis rotundifolia. The salt marsh mostly consists of Porteresia coarctata and Myristichia wighthenia (CWBMP, 2006). We categorised the whole island into five different habitat types: 1) Open mudflat: open mudflat with no vegetation adjacent to tidal rivers inundated during high tides twice daily; a mean mud depth of 30 cm; 2) Mangrove mudflat: adjacent to open mudflats with a mean mud depth of 30 cm with densely planted secondary mangrove trees; 3) Intertidal mixed grassy vegetation: early succession mangrove forest dominated by mangrove saplings of less than 30 cm and mangrove shrubs and bushes with an average mud depth of 5 cm, mean elevation of 4 m asl. This area is inundated by flood water only during the spring tides; 4) Creek: narrow tidal creeks that passes through different habitat types; and 5) Salt marsh: grassy marshes adjacent to open and mangrove mudflats. We conducted snake surveys at Sonadia Island on a total of 20 field days between 15 June and 17 August 2012. Snakes were surveyed during low tides during the day and at night. Snakes were collected using two different methods commonly used for surveying aquatic snakes (Mcdiarmid et al., 2012): 1) opportunistically collected (OC) from the fishermen’s stake net placed in the middle of a tidal canal during spring low tides, and 2) active searching in the intertidal zones using visual encounter survey techniques (VES, Doan, 2003), in two rectangular plots of dimension 1000x225 m and 550x700 m located near a fishing village. The dimension and location of the study plots were selected based on logistics and accessibility. The routes for VES inside those plots were selected using randomised walk designs (Mcdiarmid et al., 2012) and all efforts were made to cover different microhabitat types. In each plot, two surveyors walked the area at a standard pace visually examining the habitat for snakes. Snakes were captured by hand, placed in snake bags and taken back to the field station for detailed data collection. Snakes captured during the day were processed the same evening and those captured at night were processed the following day. Diet was examined by palpating the snakes’ abdomen until regurgitation of ingested food or defecation occurred. Food items were preserved in ethanol for later identification. Snakes were measured for snout-vent length (SVL) and tail length (TL) to the nearest 1 cm using a measuring tape, head length (HL) and head width (HW) to the nearest 0.5 cm using a slide calipers; and weighed to the nearest 1 g using a digital scale. Each snake was sexed using cloacal probing and individually marked by ventral scale-clipping following the protocol given by Dorcas & Wilson (2009). All snakes were released at the site of capture within 24 hours. All statistical tests were two-tailed, with alpha set at 0.05. Eventual departure of adult sex-ratio from equality was evaluated by an observed-versus-expected χ2 test; mean body size differences between sexes were assessed by a Student t-test (means are followed by ±1 standard deviation). A one-way ANCOVA was used in order to test whether, for a given body length, males differed from females in tail length; a few evident outliers were removed from analyses due to the risk of biasing the analyses.

Source of Information: Herpetological Journal Volume 24 (April 2014), 123–128

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Achievement/Findings:

We conclude that the apparent resource partitioning along the food axis was not caused by interspecific competition, but merely by the divergent evolutionary history of the three genera studied in this paper. The lack of structure along the microhabitat niche axis (consistent with other snake community studies; see Luiselli, 2006a) further suggests that interspecific competition should not be strong within mangrove snakes (Homalopsidae) assemblages.

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