Evenness is complementary to the species number component in that it considers how evenly individuals are distributed among the species, and can provide complementary assessment in both monitoring species diversity and ecosystem functioning studies (Magurran, 2004; Hillebrand et al., 2008; Chiarucci et al., 2011). Evenness is itself considered as a multi-component concept which led us to use complementary indices to better capture this diversity component (Smith and Wilson, 1996; Beisel et al., 2003). The statistical properties of evenness indices have already been extensively studied for most of the indices available, notably regarding their sensitivity to variation of rare and dominant species, and to species richness (see for instance Smith and Wilson, 1996; Ricotta et al., 2001; Beisel et al., 2003).
In this context, we used Heip’s evenness index EHeip (Heip, 1974) which is mainly sensitive to variation in rare species (Beisel et al., 2003). In contrast to the most widely used Pielou J’ index, (Pielou, 1966), also based on the Shannon-Wiener index H’ (Shannon and Weaver, 1949), EHeip is less sensitive to variation in the number of species (Smith and Wilson, 1996).. Otherwise, in a complementary way to EHeip, we used the d Berger Parker index (Berger and Parker, 1970) which is only sensitive to variations in the most dominant species (Magurran, 2004, Chiarucci et al., 2011). We computed 1/d which increases when abundances are evenly distributed (maximum diversity) among the species and decreases with dominance.