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RESEARCH PRODUCT

Extrapolation des inventaires de biodiversité incomplets : comment estimer au mieux le nombre d’espèces manquantes et prévoir l’effort additionnel d’échantillonnage requis pour réduire ce nombre.

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While approaching or even reaching total completeness remains the idealgoal of any species inventory, incomplete samplings are deemed to become more often the rule, that are significantly more difficult to cope with (such as (micro-) invertebrates, for example).However, once post-operated appropriately, incomplete samplings may provide farmore data than it would seem and it is indeed possible to obtain valuable extra information froma reliable mode of extrapolation of the so-called Species Accumulation Curve.In fact, numerous tools designed for the exploitation of incomplete samplings havealready been proposed in the literature but the concern, with these numerous different tools, isprecisely the disparity of the results they provide respectively. Fortunately, the recent derivationof a specific mathematical relationship strongly constrains and restricts the range of relevantformulations of the Species Accumulation Curve. This, in turn, allows for efficient selectionamong the various tools proposed, thus highlighting the more reliable type of extrapolation ofthe Species Accumulation Curve beyond the ongoing sampling.Accordingly, this makes it possible:(i) to estimate, with fairly good accuracy, the number of missing species that escapedsampling and hence the total species richness of the sampled assemblage;(ii) to extrapolate the (gradually slowing) pace at which new species are expectedto add progressively to the list of already recorded species, according to the level of additionalsampling effort and, thus, to objectively gauge the expected gain in terms of newly collectedspecies according to the degree of additional sampling effort required.All these are essential information to help making a rational decision about theopportunity either to continue or to stop sampling operation, given (i) the ratio between theexpected gain in new species and the required additional sampling effort, (ii) the degree ofcompleteness of the current sample (with respect to the estimated total species richness), (iii)the usual co-occurrence of several other competing priorities of investigations, involving eitherother sites and/or other taxonomic groups to be further surveyed.Hereafter, I illustrate the implementation of the suggested procedure, aiming atreliably extrapolating the Species Accumulation Curve so as to “extend” numerically alreadyachieved samplings. For this purpose, I consider a recent survey of the gall-forming fauna ofthe “Ecrins National Park”, located in south-west French Alps, including partial samplings ofeach of the four main orders of gall-forming arthropods: Acari-Trombidiformes, Hemiptera,Diptera and Hymenoptera.