When Reasonably Stop Sampling? How to Estimate the Gain in Newly Recorded Species According to the Degree of Supplementary Sampling Effort

Aim: Register new species gradually becomes more difficult as sampling of a community progresses, addressing increasingly rarer species. Thus, although biodiversity assessments would ideally require complete samplings, only partial samplings are ordinarily achieved when species abundances distributions are highly heterogeneous within communities, which is often the case. Then, in the frequent context of partial samplings, answering knowingly whether to continue or stop an ongoing inventory require to tentatively assess the “profitability” of the extra sampling effort. That is, trying to estimate the number of species expected to be newly recorded thanks to a given further increase of the sampling size.

Methods: Such estimate may be conveniently derived on the basis of the recorded numbers f1, f2, f3, fx, of species already recorded once, twice, three, … x-times within the ongoing sampling. The derivation involves a Taylor expansion of the species accumulation curve, with the successive derivatives of the species accumulation curve being respectively expressed in terms of the successive recorded values of fx.

Results: A simple nonparametric estimator of the expected number of newly recorded species is derived as a function of the foreseen additive sampling effort. Depending only upon the directly recorded values of the fx within the ongoing sampling, this estimator is easy-to-implement and, in particular, does not require recording explicitly the species accumulation curve.

Conclusion: The practical interest of this estimator is to offer a convenient way to gauge the additional sampling effort required for a given increase in sample completeness, thus providing quantitative elements to determine whether further continuing an ongoing sampling looks appropriate or not, in the context of both limited available time expenditure and possible other competing priorities.