Understanding the Indian mainland–island biogeography through plant dispersal mechanism

The plant–disperser relationship and its key predictors in Indian tropical archipelagos are understudied. East of the Indian mainland (the Eastern coast, EC) are the Andaman and Nicobar Islands (AN, about 1221 km away at the shortest distance), which exhibit a great plant diversity, with 848 unique species. West of the Indian mainland (the Western Ghats, WG) are another group of islands, the Lakshadweep Islands (LD, about 320 km away at the shortest distance), which exhibit less diversity (102 unique species) than do the AN. We compared these two mainland–island pairs (EC–AN, WG–LD), which exhibit different insular isolation conditions, to understand the plant dispersal mechanisms and their determining predictors (plant richness, geographic area, perimeter, elevation, and shortest distance). We found epizoochory (adherence to an animal surface), ornithochory (birds), and hydrochory (water) to be the dominant dispersers of both the pairs. Additionally, the plant dispersal in the WG–LD pair is predominantly driven by anemochory (wind), possibly denoting the effects of the northeasterly trade winds. The major role of biotic dispersers in the EC–AN pair may be explained as resulting from plant dispersal to more remote islands through biotic dispersers being more rapid and efficient. The plant commonality was found to be positively correlated with the plant richness, geographic area, and perimeter; whereas it was negatively correlated with the shortest distance and elevation of the selected mainland–island pairs. Both linear (R2 = 0.85) and non-linear (R2 = 0.95) regression with maximum accuracy found island plant richness, geographic area, and shortest distance to the nearest mainland pool to be the determinants in explaining plant-dispersal mechanisms. These predictors are important to maintain the habitat suitability and connectivity and thereby support more newly dispersed plants. The plant–disperser relationship (epizoochory, ornithochory, anemochory, and hydrochory) and the key predictors (plant richness, geographic area, and shortest distance) found in this study could be useful for conservation, planning, and monitoring of biodiversity.