Biodiversity is commonly defined as the variety of life
(Gaston and Spicer 1998) from the genomic to the ecosystem
scale. There are thus many ways to measure
biodiversity and there is no single scale in which it should
be measured (Levin 1992). As far as spatial scales and
species diversity are concerned, this led to the classic
distinction between a (i.e., the diversity of species occurring
at a single site) and c (i.e., the diversity of species
occurring at the regional scale) diversity (Whittaker 1960).
The a-diversity accounts both for species richness and
dominance (number of species per site). Many indices
accounting for different proportions of these two parameters
have been proposed to assess a-diversity (e.g.,
Shannon and Weaver 1949; Pielou 1966; Hurlbert 1971).
The main difficulty in assessing c-diversity is related toinsufficient sampling. Several estimators (e.g., Chao 1984;
Grassle and Maciolek 1992; Karakassis 1995; Gray et al.
1997) as well as several extrapolation techniques (e.g.,
Ugland et al. 2003, 2005) have been proposed to infer the
true number of species present in a given area. However,
this problem is far from being solved yet. At last b-diversity
corresponds to the turnover of species along a gradient
(or between communities). It is thus of different dimensional
character from a and c-diversity; b-diversity is most
often assessed through the ratio between c-and average adiversity
(Whittaker 1972). Measures of b-diversity are
clearly needed to enhance our understanding of the structure
of ecosystems, which could have important
implications in conservation planning