| 1 | == 7.11 Electivity == |
| 2 | The electivity (selection index) describe a predator’s preference for prey. It scales from -1 to 1; where -1 indicates total avoidance of a prey; 0 indicates that a prey is taken in proportion to its abundance in the ecosystem; and 1 indicates total preference for a prey. The electivity values are highlighted using a colour scale for the background, scaling from –1 (white) to 1 (red) using shades of red for intermediate values. The electivity index displayed is the standardized forage ration of Chesson (1983), see below. |
| 3 | |
| 4 | One of the most widely used indices for selection is the Ivlev electivity index, ''E,,i,,'' (Ivlev 1961) defined for a group (''i'') as: |
| 5 | |
| 6 | ''E,,i,, = (r,,i,, – P,,i,,) / (r,,i,, +P,,i,,)'' |
| 7 | |
| 8 | where ''r,,i,,'' is the relative abundance of a prey in a predator's diet and ''P,,i,,'' is the prey's relative abundance in the ecosystem. E''i'' is scaled so that ''E,,i,,'' = -1 corresponds to total avoidance of, ''E,,i,,'' = 0 represents non-selective feeding on, and ''E,,i,,'' = 1 shows exclusive feeding on a given prey'' i''. Note that within Ecopath, ''r,,i,,'' and ''P,,i,,'' refer to biomass, not numbers. |
| 9 | |
| 10 | The Ivlev electivity index was included in the DOS version of Ecopath because it often shows up in the literature. This index has, however, a major shortcoming, seriously limiting its usefulness as a selection index: as shown by several authors, e.g., Jacobs (1974): the Ivlev index is not independent of prey density. |
| 11 | |
| 12 | A better approach is to use the standardized forage ratio (''S,,i,,'') as suggested by Chesson (1983). This index is independent of prey availability, and is given by: |
| 13 | |
| 14 | [[Image(wiki:EwEugImages:08000077.png)]] |
| 15 | |
| 16 | where ''r,,i,,'' and ''P,,i,,'' are defined as above, and ''n'' is the number of groups in the system. The standardized forage ratio as originally presented takes values between 0 and 1, with ''S,,i,,'' = 0 representing avoidance and ''S,,i,,'' = 1 exclusive feeding. |
| 17 | |
| 18 | As implemented in Ecopath, the forage ratio has been transformed (linearly) such as to vary between -1 and 1, so that -1, 0 and 1 can be interpreted as for the Ivlev index. |