Inferring Total Species Richness and the Exhaustive Hierarchical Structuring of Species Abundances in Tropical Sea-stars Communities (Asteroidea), Using Numerical Extrapolation of Partial Inventories

Even when remaining substantially incomplete, the partial inventory of a species assemblage can provide much more information than could be expected first. Indeed, retrieving this information is made possible by applying a rigorous procedure of numerical extrapolation to the partial inventory. This numerical extrapolation will deliver reliable estimates of the number of the still unrecorded species and, furthermore, of the distribution of abundances of these unrecorded species. As a result, the full range of the Species Abundance Distribution is finally made available, despite dealing with data from partial sampling only. In turn, this allows to address a series of descriptive and functional aspects of the internal organization of species assemblages, which otherwise would have required disposing of exhaustive samplings. The latter being, however, often impossible to achieve in practice. Fortunately, mathematic and algorithmic basis for a reliable numerical extrapolation of incomplete samplings have been developed recently, so that partial inventories no longer remain an obstacle to gain access to the true (total) species richness and the full-range pattern of hierarchical structuring of species abundances.

This approach is applied here to the previously reported partial samplings of two communities of tropical sea-stars associated to coral-reefs in the Central South China Sea. Among the main new results derived from the numerical extrapolations of these partial samplings, the following are highlighted:

- the extrapolated true (total) species richness of each of the two studied communities largely exceed the recorded figures, thereby confirming the limited completeness of these partial samplings: 53% and 67% completeness only;

- once properly completed by numerical extrapolation, the Species Abundance Distributions of both communities prove best fitting the “log-normal” than the “log-series” model, thereby suggesting that many independent factors (rather than only one dominant factor) contribute together to the hierarchical structuring of species abundances within these sea-stars communities;

- the intensity of the process of hierarchical structuration of species abundances proves being close to what would be obtained for the corresponding “broken-stick” model, which means a rather moderate level of structuration intensity, as compared to the range of values typically obtained for other kinds of marine invertebrates.