A Universal Key to Rationally Select Which, Among Nonparametric Species Richness Estimators, Performs Best According to Each Particular Incomplete Sampling

Since most samplings of local species communities are bound to remain substantially incomplete for practical reasons, a wide variety of nonparametric estimators of the number of unrecorded species have been proposed over the past fifty years. Unfortunately, the distinct formulations of each of these estimators naturally lead to substantially divergent estimates. The will to try to select, in each case, the estimator expected to be the more accurate has long been carried out only on a purely empirical, even arbitrary, basis (as is evident from the extensive consultation of much of the past literature on estimating species richness of incompletely sampled communities). So that extrapolating the true species richness of a community from its incomplete survey has long remained quite unsatisfactory. Indeed, the definition of a truly rational procedure for selecting the most accurate (least-biased) estimator actually requires a solidly established theoretical framework, involving to conform, as best as possible, to the general mathematical characteristics of the Species Accumulation Function. Accordingly, unveiling, first of all, these mathematical characteristics of the Species Accumulation Function was a decisive step forward in this perspective. Thereby making it now possible to propose an objective key to rationally select the one, within the series of various estimators, which, depending on each particular sampling, happens to be the least biased in this particular case, thus providing the most accurate estimate of the number of still unrecorded species. And, consequently, making it possible, now, to deliver the best estimate of the true species richness of a local community, despite its being incompletely surveyed.

Damping the Impact of Intraspecific Variability of Whorl Growth-rate on the Range of Intraspecific Variation of Adult Shell-size in Gastropods with Determinate Growth: Evidence of a Finely-tuned Regulating Process

Many species among shelled gastropods � either land snails or marine snails � exhibit determinate growth and, therefore, are expected to implement some regulating process aiming at limiting the impact on the final (adult) shell-size of the (possibly excessive) intraspecific variability in the rate of shell-development. Indeed, a usually more or less limited range of variation is allowed for adult shell-size, in those species having determinate growth. Mollusks are expected to be no exception in this respect and, up to now, the occurrence of such a regulating process has actually been reported systematically, despite a still too limited number of investigations. Yet, the question remained of the ability of this regulatory process to finely adjust the end of the final development of both body and shell (specifically in term of the final number of whorls), so as to limit the variations in adult shell-size, despite the amplitude of intraspecific variability in shell development rate. I provide here preliminary empirical evidence for such a 揻inely tuned� regulating process � the strength of which is mirrored by the degree of 揋ouldian� negative covariance between whorl growth-rate and the number of whorls at adulthood. As expected, the strength of the regulating process reveals systematically increasing (i) with the amplitude of the intraspecific variability in shell development-rate and (ii) with the narrowness of the range of variation allowed for the adult shell-size (narrowness which depends upon the identity of the particular species under consideration). In addition to the already reported usual occurrence of this regulating process among shelled gastropods, its 揻inely tuned� character provides here still further evidence as regards its major importance in controlling the species-specific adult body mass, and this even for typically soft-bodied animals.