Resource Use and Foraging Tactics in a South Indian Amphibian Community
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J. South Asian nat. Hist., ISSN 1022-0828. February, 1996. Vol.2, No. 1, pp. 1-30,12 figs., 13 tabs. © Wildlife Heritage Trust of Sri Lanka, 95 Cotta Road, Colombo 8, Sri Lanka. Resource use and foraging tactics in a south Indian amphibian community Indraneil Das* Abstract This study looks at resource (trophic, spatial and temporal) use and foraging tactics in a community of eight species of anuran amphibians at a seasonal locality in south India. Within the community, the species are differentiated into a sit-and-wait group, which are large, cryptic and sedentary foragers showing a relatively wide dietary spectrum; and a widely foraging group, whose members are aposematically coloured, and actively forage on a few prey types. However, there are indications that these modes represent ■ two ends of a continuum, with some species showing greater plasticity in prey use than others. Sympatric species, except dietary specialists, were found to generally overlap broadly in diet. Microhabitats are partitioned to a greater degree than food, the most closely related species, which tend to show similar diets, selecting different foraging areas. Seasonality affects the activity of two of the three non-sit-and-wait species, but none of the sit-and-wait ones, possibly because sedentary foraging is more energetically effi cient during the resource lean season. Net gains per unit energy spent are presumably lower than for active foraging. In general, both trophic and spatial niches increase in breadth with body size across species, with larger species taking more types of food and using more different microhabitat types than smaller ones. Smaller species take smaller prey, but the mean number of prey harvested is higher than in larger frogs. Differences in the use of envi ronmental resources are thought to be a factor in determining species composition in a community, within which larger species tend to be generalists, while their smaller sympatrics are more specialized in their use of environmental resources. Key words: resource use, community, niche, foraging, amphibians. * Animal Ecology Research Group, Department of Zoology, University of Oxford, South Parks Road, Oxford 0X1 3PS, United Kingdom. Present address: Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cam bridge, MA 02138, USA. DAS Introduction Ecological differences in the use of trophic (food), spatial (place) and temporal (time) resources have long been associated with the structure of biological coifi- munities because of their potential to reduce competition/ thereby apparently facilitating coexistence (Pianka, 1975). Amphibians and reptiles exploit adaptive zones based on low energy flow that are unavailable to endothermic tetrapods, because their modest energy requirements allow the exploitation of niches not used by birds and mammals (Pough, 1980). Niche relations among these groups of animals are thus wor thy of study, particularly because most of the current theories on vertebrate community-structure are derived from investigations on avian and terrestrial mammalian communities (but see the works on saurofaunas, reviewed by Pianka, 1986; and a review of resource partitioning patterns in herpetofaunal communities, by Toft, 1985). Among amphibians and reptiles, anurans are undoubtedly the most speciose group (approximately 4,000 described living species), and compose a major component of the vertebrate faunas in many parts of the world. Relatively few community-level studies have been directed at adult frog assemblages, most of these being conducted in the New World tropics (e.g., Duellman, 1978, 1989; Toft, 1980, 1980a, 1981). Studies on the Asian fauna have been rarer still, all conducted by Robert Inger and co-workers, in Thai land (Inger and Colwell, 1977), Sarawak, Malaysian Borneo (Inger, 1969) and the Western Ghats forests of south-western India (Inger et al., 1987). This paper is based on an eighteen-month field study on patterns of re source use and foraging tactics in a community of eight anurans at a seasonal locality in south India. Study area Field work was conducted in Chengai-MGR (formerly Chengleput and Chengai) District, Tamil Nadu State, south India, centred around the village of Vadanemmeli (12° 45'N; 80°12/E), approximately 42 km south of the city of Madras, on the south-east coast of India. The area is a stretch of coastal scrub land, interspersed with a large number of permanent or ephemeral water bod ies. Most of the waterbodies are along a 1.25 km stretch of low-lying land, in an almost continuous series. Three broad categories were identified, based on surface area and permanency: Large permanent (N=2), 1326-2728 (mean 2027) m2, small permanent (N=4), 1.0-27.5 (mean 8.5) m2, and ephemeral (N=4), 302.5- 1210.0 (mean 759) m2. The mean maximum depth of these bodies of water varies between 3.0 m for the large permanent ponds, through 1.5 m for ephem eral ponds, to < 1.0 m for the small permanent bodies of water that are exca vated by the human residents to entrap rainwater. All measurements of sur face area and greatest depth were made during the peak of the Southwest monsoons, and depth of the large permanent waterbodies was observed to vary greatly seasonally. A variety of aquatic macrophytes and riparian veg etation occur in association with the larger waterbodies, including submerged (Ceratophyllum demersum, Vallisneria spiralis, Hydrilla verticillata, Phyla nodiflora, Alysicaiyns manilifer, Ottelia alismoides, Oedogonium sp. and Zygnema sp.), float ing (Nymphaea stellata and Lemna perpusilla) and emergent (Aponogeton natans) 2 J. South Asian nat. Hist. R eso u r c e u s e a n d fo r a g in g ta ctics in s o u th In d ia n a m ph ibia n s types. The sedge Cypenis sp. frequently forms the major vegetation of the shores of the permanent waterbodies, while the ephemeral ones have barren shores, o Other major amphibian habitats include Casuarina forests, which are ex tensive on the seab^aches and typically lack undergrowth. Human habita tions also occur in the area and comprise thatched huts and a few concrete structures. An eight acre area of land belonging to the Madras Crocodile Bank Trust, which has been carefully planted with many local tree species, espe cially neem (.Azadimchta indica), is the most densely vegetated patch at the site. Mean daily maximum and minimum ambient temperatures for the hottest (May-June) months are 36°C and 28°C, respectively; corresponding figures for tih.e coldest months (December-January) being 29°C and 21 °C, respectively. The dry season is long, extending for about four months from February to mid-June. This is followed by the summer monsoon which extends until Sep tember, but brings comparatively less precipitation than the winter monsoons, which occur between the end of October and mid-December. The area received 1147 mm of rainfall in 1989, the figures for the following year being 47.7% greater (1694 mm), which can be attributed to a cyclone which hit the area in May. Study species Eight species of anuran amphibians occur at the study site— Indian green frog, Rana hexadactyla (Lesson, 1834). Unique among anuran am phibians in being primarily folivorous throughout adulthood, the frog feeds on a variety of aquatic macrophytes. This is the largest as well as the heaviest species in the study site, attaining 132.2 mm in snout-vent length and weigh ing up to 270 gm. Newly metamorphosed frogs, between about 15-30 mm snout- vent length feed on various kinds of insects. The species is restricted to the comparatively deeper (> 2 m depth) waterbodies. Jerdon's bullfrog, Rana crassa (Jerdon, 1853). This species attains 93.1 mm in snout-vent length in the study site and inhabits the edges of ephemeral waterbodies, where they sit in wait for invertebrate, and very occasionaly, small vertebrate prey. Skipping or skittering frogs, Rana cyanophlyctis (Schneider, 1799). Two distinct colour morphs occur in the site, a dark-blotched, large-sized (to snout-vent length 70.7 mm) form that inhabits permanent ponds, and an unpatterned, small (to 40 mm) form restricted to temporary ponds. Aquatic insects, espe cially beetles, constitute the mainstay of the diet of this species. South Indian burrowing frog, Tomopterna rolandae (Dubois, 1983) . Encountered in the Casuarina forests and agricultural fields especially during the rains, the species reaches a snout-vent length of 43.2 mm and forages on beetles and many other groups of invertebrates. Marbled balloon frog, Uperodon systoma (Schneider, 1799). A secretive and prob ably rare species that is encountered only during rainy nights, at the height o f the monsoons. The species reached 54.7 mm in snout-vent length and special izes on termites and ants. Vol. 2., No. 1. 3 DAS Red narrow-mouthed, frog, Microhyla rubra (Jerdon, 1854).. A relatively com mon, brightly coloured ant and beetle specialist, this species reaches a snout- vent length of 26.5 mm. Ornate narrrow-mouthed frog, Microhyla ornata (Dumeril & Bibron, 1841). A rather rare species in the study area and a small insect-specialist. This species attains only 17.4 mm, making it the smallest frog species in the study site. Common Indian tree frog, Polypedates maculatus (Gray, 1834). With microhabitat requirements quite different from those of the species already described, the common Indian tree frog rests on trees or inside human habitations during the day, emerging after dark to forage on the ground or on trees. In the dry season, activity is greatly reduced, when it hides under leaf sheaths of banana trees, and may also take shelter in cracks inwalls inside human dwellings. This spe cies attains a snout-vent length of 79.2 mm. Its broad diet includes both verte brate and invertebrate prey. Methods Data on which this paper is based were obtained between March-August and November-December, 1989, and January-December, 1990.