Plant frugivore interaction and seed dispersal syndromes in shola forests of the western ghats India

Abstract

The Shola forests of the Western Ghats are ecologically rich, harboring a range of plant-animal interactions that are essential for maintaining the unique biodiversity in this ecosystem. Seed dispersal by frugivores is an essential process that influences forest regeneration, affects species diversity, and community composition. This thesis explores seed dispersal dynamics in Eravikulam National Park, examining how plant diaspore traits, frugivore feeding patterns, fruit production, and abiotic factors such as altitude and climate collectively function to shape dispersal processes in this tropical montane ecosystem. Diaspore morphology, including fruit pulp type, color, size, and seed size, is a key factor in determining the dispersal mechanism, with findings revealing that brightly colored, fleshy fruits are primarily dispersed by birds, whereas lighter seeds are more effectively dispersed by wind. By studying 105 tree species across 25 Shola forest patches, this study highlights how specific traits promote or limit seed dispersal which in turn contribute to forest structure and resilience. Detailed observations of frugivore interactions with fleshy fruits illustrated a substantial dietary overlap between species, highlighting a complex web of interactions between species. Frugivores such as birds in the Pycnonotidae and Sturnidae families and mammals such as langurs, civets, and squirrels were found to be particularly influential, with larger frugivores predominantly consuming larger fruits and playing a key role in dispersing seeds of greater size, which often support seedlings with higher survival rates Transect walks, camera traps, and fecal analysis were used to gather data for the construction of quantitative seed dispersal networks, which highlighted the ecological importance of frugivores with high species strength and interaction asymmetry, which frequently render them irreplaceable dispersers within the community. Rainfall and minimum temperature were strongly associated with seasonal fluctuations in fruit biomass production, with fruit production reaching its maximum during the wettest months. This seasonal fruiting pattern aligns with the "germination hypothesis," which posits that fruiting during the rainy season enhances seed germination success, and provides crucial food resources for a diverse community of frugivores. Across three years, fruit biomass averaged 384.69 kg/ha/year, comparable to other tropical montane ecosystems. A considerable amount of this biomass was made up of larger fruits that were dispersed by both mammalian frugivores and birds like Nilgiri wood pigeons. The final analysis explored how environmental gradients and abiotic factors drive frugivory, seed dispersal, and co-evolutionary dynamics within the Shola ecosystem. A study of 44 forest patches, each varying in altitude (1690–2024 m) and size, revealed that frugivore density peaked at lower elevations, where larger fruiting trees were more prevalent, while smaller frugivores predominated at higher altitudes. Correlations between body size, gape width, and fruit traits showed that large frugivores consume larger fruits, aiding the dispersal of seeds that benefit from more extensive dispersal distances at lower elevations. The findings suggested that as altitude rises, environmental constraints cause smaller, lighter fruits to be adapted to dispersal by smaller-bodied frugivores and by abiotic factors like wind. Abiotic factors like temperature and rainfall were found to have a significant impact on both frugivore abundance and fruiting phenology. In conclusion, this thesis offers a comprehensive understanding of how fruit characteristics and frugivores are highly adapted to the conditions found in Shola forests. These results highlight how diaspore characteristics have evolved to optimize seed dispersal, often in response to abiotic gradients and through specialized interactions with specific dispersers. This study provides important insights into conservation biology by highlighting the importance of preserving habitat connectivity and protecting diverse frugivore assemblages to promote effective seed dispersal and forest regeneration, particularly in tropical montane forests where environmental change and habitat fragmentation are becoming a growing problem.