Two Electivity Indices for Feeding with Special Reference to Zooplankton Grazing

Abstract

The electivity indices E_i and E_i′ of predator–prey interaction are currently used to quantify particle-size selection by grazers. Under conditions of passive, mechanical particle-size selection predicted by the leaky-sieve model, these indices yield electivity vs. particle-size curves that vary with the shape of the particle-size spectrum of food offered to the zooplankton. In addition to this bias, poor estimates of electivity will be obtained unless only a small fraction of the food is eaten in such experiments. The selectivity coefficient (W_i) used by modelers in feeding constructs and the electivity index E_i*, derived here, are recommended instead because they do not suffer from the shortcomings described for E_i and E_i′. Moreover, use of W_i′s and E_i*'s is recommended for quantifying selection for many other cases of predator–prey interaction. Key words: electivity indices, selectivity, selective grazing, predator–prey intraction