Edit page ← Previous ChangeWiki History

Changes between Initial Version and Version 1 of EwEugElectivity


Ignore:
Timestamp:
2010-02-07 15:12:27 (14 years ago)
Author:
varunr
Comment:

Added page and all images.

Legend:

Unmodified
Added
Removed
Modified
  • EwEugElectivity

    v1 v1  
     1== 7.11 Electivity == 
     2The 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 
     4One 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 
     8where ''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 
     10The 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 
     12A 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 
     16where ''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 
     18As 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.