Mutualism between vertebrates and Cullenia exarillata in Silent valley forest ecosystem of Kerala 

Candidate: S. Anil Kumar
Title: Mutualism between vertebrates and Cullenia exarillata in Silent valley forest ecosystem of Kerala
University: University of Kerala
Date of Award: 13 Sept 2010
Guide: Dr M Balakrishnan


Mutualism is a positive, reciprocal relationship at the individual or population level between two species. Mutualism has been a case of specialization versus generalization, especially in the tropical forests, where specialized pollinators and their respective seed dispersers have evolved simultaneously. Plants harbour pollination mutualism when animal mediated cross pollination takes place in habitats such as closed canopy rain forests, where animals seek and get food rewards, most commonly nectar or pollen from the host plants. The relationship between animal dispersed plants and their dispersers is also mutualistic in nature as both the partners derive benefits from their participation. Mutualistic relationships are vital and important as they affect community composition in forests. Mutualistic species reach higher densities when they co-exist than when only one of the species exists in the community. Local elimination of one among the mutualistic interacting species may lead to local extinction of the other.  Mutualistic interaction between vertebrates and the interacting tree species is one among the least studied in the tropics.

The nature of mutualism that exists between the vertebrate communities and Cullenia exarillata (family Bombacaceae) in the tropical evergreen forest ecosystem of Silent Valley (between11o 00’ – 11o 15’ N latitude and 76o 15’ – 76o 35’E longitude) was evaluated during the present investigation. The dominant vegetation in the forest area is Palaquim-Cullenia-Myristicaassociation. C. exarillata is a tall evergreen tree reaching up to 40m in height with a conical crown extending as emergent canopy. Endemic to the southern Western Ghats, it is intermittently distributed from Karnataka to Tamil Nadu. Nectarivorous and granivorous vertebrates depend on C. exarillata for food.

Observations were made by instantaneous scan method on C. exarillata and other associated species present in three transects of 2000x25m2 area each, besides on plant species in a transect of the same area devoid of C. exarillata to know the phenological details of individual trees and the vertebrates interacted, parts eaten, rates of feeding, total time and mode of feeding. Flowers, nectar, fruits and seeds were quantified and correlated with the consumption rates of the interacting vertebrates. Floral traits, attractants and rewards were analysed to understand pollination mutualism. Pollination and exclusion experiments were conducted to identify the pollinators and to determine the nature of pollination. Fruit traits, primary and secondary dispersal of seeds, germination and seedling demography were studied to understand the effective dispersers of seeds of C. exarillata.  Data were analysed to estimate preferences and degrees of mutual dependence of C. exarillata and vertebrates.

Analyses of the data have revealed that animal mediated cross pollination was basal in C. exarillata. C. exarillata harboured pollination mutualism with the Malabar giant squirrel (Ratufa indica), lion-tailed macaque (Macaca silenus) and Niligiri langur (Trachypithecus johnii), bonnet macaque (Macaca radiata), bats and birds. Besides the above vertebrates, 15 species of birds and three mammals such as sloth bear (Melursus ursinus), Viverrids and flying squirrel also used nectar of C. exarillata as food. However, pollination mutualism observed between the vertebrates and C. exarillata is facultative. There was no substantial evidence to prove the presence of mutualistic interactions between C. exarillata and its seed dispersers. Diurnal vertebrates involved in pollination mutualism behaved more as predispersal predators than as true seed dispersers during the periods of fruit maturity.

Based on the data gathered on pollination mutualists of C. exarillata with reference to their specificity of food preferences and degree of dependence on alternate food resources, the present study has revealed the hypothetical situation wherein the poor seed dispersal strategy of C. exarillata might results in local extinction of the species in the floristic composition of the ecosystem in Silent Valley. The absence of efficient seed dispersers might have been responsible for the endemic nature of this species in the southern Western Ghats of India, one of the hottest global biodiversity hotspots.

Key words: Animal – plant interactions, Cullenia exarillata, flower predation, mutualism, pollination, seed dispersal, seed predation, tropical rain forest, vertebrates.


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