The Hierarchical Structuring of Species Abundances within Communities: Disentangling the Intensity of the Underlying Structuring Process behind the Apparent Unevenness Pattern

Besides species richness, the hierarchical structuration of species abundances is the second major characteristic that numerically specifies a community of species. However, while the meaning of species richness is simple and straightforward, the hierarchical structuration of abundances is a less simple concept, where the pattern – i.e. the straightforwardly observed level of unevenness of species abundance distribution – does not reliably mirror the genuine intensity of the structuring process itself. This is because the level of unevenness is also mathematically dependent upon species richness. Accordingly, when specifying numerically a community of species, I advocate not to be satisfied, as usual, by considering only the total species richness and the degree of unevenness (whatever the expression chosen to quantify unevenness). A third parameter should be further considered: the genuine intensity of the structuring process itself, defined freed from the purely mathematical influence of species richness and, thereby, accurately reflecting the functional contribution to the hierarchical structuration of species abundances. The level of unevenness is thus only granted a simply descriptive goal, while the intensity of the structuring process relevantly speaks for the biological background behind the apparent hierarchical structuration of species abundances in communities. An additional requirement to warrant the reliable evaluations of these three parameters is, of course, to work with (sub-) exhaustive samplings of the studied communities or, when not possible in practice, to consider the least-biased numerical extrapolations of partial samplings (when only the latter are available). The benefits of this renewed methodological way to quantify the internal organization of species communities, as well as the potential pitfalls to which one may be exposed by considering only species richness and (apparent) abundance unevenness, are argued from a theoretical point of view and then highlighted concretely in a series of examples.