Salim Ali School of Ecology and Environmental Sciences, Pondicherry University, Pondicherry 605 014, INDIA.
Tropical forest harbour high diversity of frugivores and fruiting patterns of trees to a large extent determines this studies in fruit characteristics, fruit production and seed predation have received attention only recently. Though many studies have come forth from Neotropical forests, little was known, before this study, from the wet forests of the Western Ghats and much less from the mid elevation forests in the Southern Western Ghats.
Fruits of 53 species were sampled during the 3-year period. Forty-eight percent of the fruits were fleshy and the rest were hard, mostly comprising of dehiscent fruits which were more common than in many other tropical forests. Most of the fruits were green in colour. Fruit weights varied from 0.14 to over 2000 gms and seed weights from 0.04 to over 16 gms. Bird dispersed fruits were more common while mammal dispersed and dehiscent fruits were in equal proportions. Bird dispersed fruits had greater seed protection while most of the mammal and dehiscent fruits had a hard or spiny exocarp. Most of the large mammal or dehiscent fruits were multiseeded.
Fruit Biomass and Abundance
On an average the forest produced 645 kg/ha of fruits from nearly 53 species of canopy trees. Fruit biomass in the wet forest is influenced by the differential contribution of few fruiting species. Fruit biomass is largely influenced by the availability of mammal and dehiscent fruits while abundance of bird fruits are more. Fruit biomass in Kakachi is lower than in many wet forest sites in lowland neotropical forest. Some of these variations are probably due to plant species diversity and differences in fruit crop sizes of trees and irregular fruiting patterns.
Spatial variation of fruit production were largely due to few species which are either rare or few individuals which produced large numbers of fruits. Spatio-temporal changes in fruiting were not always significant.
Seeds comprised 28% of the fruit biomass estimate and over half the pulp was inedible. Seed predation varied from 1% to over 80% in some species. Most of the Meliaceae species suffered high predation compared to many Elaeocarpus spp. Major seed predators were the Giant Squirrels (Ratufa indica) and Nilgiri langurs (Semnopithecus johnii), followed to a lesser extent by lion tailed macaques (Macaca silenus). None of the avian frugivores were seed predators. Of the 49 species for which disperser/predator was known, 84% suffered seed predation by mammals, and even some bird dispersed species suffered high levels of seed predation by mammals.
High levels of seed predation are probably due to lack of fleshy fruits and predictable nature of seed availability. Seed protection help in lowering seed predation. Apart from this, community level strategies like mass fruiting help in seed escape.
Frugivore abundance and seasonality
Among mammal frugivores/seed predators Ratufa indica and Semnopithecus johnii were most abundant, while Macaca silenus was seasonal, commonly seed during the long dry season. There were no significant differences between years in the changes in abundances of these mammals. Among avian frugivores Hypsipetes madagascarensis was the most abundant while Columba elphinstonii was rare. All avian frugivores were seasonal occurring during the dry seasons and their abundance correlated with bird fruit biomass. Though many species are bird dispersed, avian frugivore diversity is relatively low than other sites in the Western Ghats and elsewhere. This is explained by the low density of bird dispersed fruits, irregular fruiting phenology of the species and lack of any dominant Ficus sp.
Fruiting was seasonal and peak fruit fall occurred in the wet season. Period of low fruit fall occurred between November and March in all the years. Zoochorous species produced fruits in the wet season while most of the dehiscent fruits released their seeds in the late dry season. Seasonality in seed predation was not related to fruit biomass, seed biomass or the number of species fruiting. Mass fruiting decreased seed predation between years. Rare trees and trees fruiting asynchronously with the rest of the population suffered significantly higher seed predation. Inter annual variation in seed predation was significantly different for some species but not for others.
General fruiting patterns in the two wet two dry forest at Kakachi are influenced by abiotic and biotic factors. Abiotic factors like minimum and maximum temperatures are important cues for the fruiting of few selected species or syndromes but do not have a general influence on fruiting patterns at the community level. Rainfall is not related to fruiting at all levels. Among biotic factors mass fruiting helps to dampen the effects of seed predation leading to aggregate fruiting in few species. For some species a combination of masting and climatic variables like maximum temperature are responsible for seed release. The possible influence of frugivore availability on selection of fruiting times of few bird dispersed species cannot be ruled out.
Fruit characters and fruiting patterns of plants in the wet evergreen forests of Kakachi, Southern Western Ghats appear to have been influenced strongly by vertebrate seed predation at the pre-dispersal stage. Some species are dispersed by seed predators, eg Cullenia exarillata while some are adapted for consumption by mammals. Other species which are bird dispersed avoid predation by arboreal mammals either by mast fruiting or by investing in seed protection. Therefore, seed predation must be considered when looking at plant/frugivore interactions as its effects on plant traits might be stronger than that of frugivores.