Controls of spatial grain size and environmental variables on observed beta diversity of molluscan assemblage at a regional scale
Beta diversity, which quantifies the compositional variation among communities, is one of the fundamental partitions of biodiversity and is associated with abiotic and biotic drivers. Unveiling these drivers is essential for understanding various ecological processes in past and recent faunal communities. Although the quantification of measures of beta diversity has improved over the years, the potential dependence of beta diversity on methodological choices are relatively understudied. Here, we investigate the effect of the variable scale of sampling on different measures of beta diversity at a regional scale. The west coast of India bordering the eastern margin of the Arabian sea, presents a coastal stretch of approximately 6100km from 8-21{degrees}N. We used marine bivalve distribution data, consisting of live occurrence data from literature reports and abundance data from death assemblages collected from localities representing latitude bins. We tested if the observed variation in beta diversity is explained by variable sampling scales due to differences in bin sizes and unequal coastline length. We developed a null model to generate a beta diversity pattern with an increase in spatial scale of sampling by increasing the spatial grain size along the 14 latitude bins progressively. Our null model demonstrates that for the both live and dead dataset, the total beta diversity measured by Bray-Curtis, Whittaker and Sorenson indices decreases with increasing sampling scale. The species replacement (turnover) evaluated by Simpson index decreases and the species loss (nestedness) measured by Sorenson index increases with increasing sampling scale. A comparison between the simulated and observed beta diversity distribution using K-S test demonstrated that the observed pattern of beta diversity is significantly different from the pattern generated from the null model in both live and death assemblages. This implies that sampling alone does not generate the spatial variation in beta diversity in this region. The results show that environmental parameters such as salinity, productivity, and cyclones play a significant role in shaping the regional beta diversity along the west coast. Our study provides an approach for evaluating the effect of variable sampling scale on comparing regional beta diversity. It also highlights the importance of spatial standardization while inferring about processes driving spatial diversity changes.