DRIVERS OF BETA DIVERSITY IN TROPICAL MARINE ECOSYSTEM: INSIGHTS FROM THE BIVALVE ASSEMBLAGES OF THE WEST COAST OF INDIA

The relative importance of physical and biological drivers that shape biodiversity in diverse marine ecosystems through time is an area of intense academic debate. Beta diversity, the compositional variation between communities, is crucial for understanding the processes operating at a local scale (and affecting alpha diversity) and processes influencing regional or gamma diversity changes. Apart from environmental factors, historical processes operating in a region, biotic interactions, and traits specific to the studied organism may also shape the regional nature of beta diversity. Therefore, changes in beta diversity may provide important insight into the community response to physical and biological drivers in the past and present ecosystem. The nature of beta diversity in the tropics is of particular interest because of the tropical marine ecosystem's high species richness and intense biotic interaction. The tropical coastline of India has a wide latitudinal variation (8–22° N). The western coast, bordering the Arabian Sea, demonstrates a significant variation in the oceanographic variables, such as oxygen concentration, salinity, shelf area, productivity. We attempted to evaluate the variation in community composition with latitude using time-averaged bivalve specimens collected from 28 locations along the western coast of India. We used the species richness and evenness as indicators of alpha diversity. We evaluate the change of beta diversity with the oceanographic variables. We compared the results with a null model to assess the stochasticity of these differences.

Our results reveal that evenness increases from the south-western (SW) to the north-western (NW) sites. The beta diversity, measured by the pairwise proportional dissimilarity (PPD), did not change monotonically with latitude. The mean PPD of the NW region is slightly higher in comparison to the SW. There is no distinct clustering of beta diversity in the NMDS between the fauna of SW and NW regions. A significant positive correlation between community dissimilarity and spatial distance from south to north indicates distance decay. Therefore, between-habitat diversity decreases towards the north, whereas evenness or within habitat diversity increases. Null models show that stochastic processes only play a minor role in producing such patterns. The present study highlights the importance of beta diversity in understanding the influence of the regional environment in shaping the tropical marine community through time.