Jacob’s selectivity index

Ivlev’s selectivity or Jacob’s index (E_i) was used to calculate the selection of each woody species using the formula: E_i = (U_i—A_i) / (Ui—Ai), where U_i represents the proportion of use of woody species i and A_i its proportional availability in the rangeland [34]. The Jacob’s index was selected because it is simple and has been reported to be comparable with more complex methods [6]. Jacob’s selectivity index factors the differences in the relative abundance of feed resources when evaluating diet selection [35]. The selectivity index (E_i) varies from -1 (indicating lower use relative to availability of woody species) to +1 (indicating higher use relative to availability of woody species) and a zero value indicating proportional use of woody species in relation to availability [36].