Diversity of species assemblages of islands: predictions and their test using tree species composition of shola fragments

Abstract

Based on all possible combinations of species that can theoretically exist on islands of different sizes, we estimated similarities among islands of equal size in their species compositions. We found that similarity coefficients among islands of equal sizes increases and the coefficients of variation for similarities decreases with increase in the size of islands. Accordingly, it can be predicted that the species compositions of small fragments shall be more diverse compared to large fragments. We tested this prediction using the tree species composition of the shola fragments at BR Hills, Western Ghats. Similarities among sholas were measured as coefficients of correlation between the frequencies of species in them and also as proportions of species shared among sholas. Small sholas were less similar and shared fewer species among themselves, compared to medium and large sholas. Also, small sholas showed higher coefficient of variation for the correlation coefficients and for proportion of species shared than medium or large sholas. In other words, species assemblage diversity among sholas was found to decrease with their size and large islands appeared to converge towards a common assemblage of species. Based on our results, we argue that small fragments add structural and functional heterogeneity to the ecosystem and hence are as important as are the large fragments in conserving biodiversity.