Extrapolation of Total Species Richness from Incomplete Inventories: Application to the Gastropod Fauna Associated to Coral Reefs in ‘Mannar Gulf Biosphere Reserve’, India

Tropical coral reefs are known to harbor considerable biodiversity, especially among invertebrates. Gastropod fauna, as an important component of this biodiversity, yet remains poorly surveyed across most tropical reefs. Moreover, the few published inventories are generally far from being exhaustive, as is almost inevitable in practice with species-rich faunas. Hence the necessity of implementing a numerical extrapolation of species accumulation, providing both (i) estimates of the total species richness of the partially sampled sites and (ii) a way to predict the additional sampling effort needed to achieve a given additional gain in sampling completeness. Such numerical extrapolations were applied here to three partial inventories of Gastropod fauna associated to coral reefs in ‘Mannar Gulf Biosphere Reserve’. To ensure the maximal possible accuracy for extrapolations, a newly derived procedure was implemented, designed to select the least-biased among nonparametric estimators of the number of still unrecorded species. The total species richness of Gastropods at each site was estimated between 49 and 53 species according to sites. Accordingly, the completeness of the reported inventories was estimated between 71% and 78%. This, in turn, calls for further effective sampling but, also, immediately raises the question of how far to extend the extra effort with, in return, a reasonable expected benefit, in terms of the ratio between the expected number of newly recorded species and the corresponding additional sampling effort required. The least-biased extrapolation of species accumulation curves proves a convenient tool for rationally addressing this important question.