Changes between Version 1 and Version 2 of EwEugMixedTrophicImpact


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Timestamp:
2010-11-23 01:46:59 (9 years ago)
Author:
shermanl
Comment:

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  • EwEugMixedTrophicImpact

    v1 v2  
    66''MTI,,i,j ,,= DC,,i,j,, – FC,,j,i,,'' ''' Eq. 45''' 
    77 
    8 where ''DC,,i,j,,'' is the diet composition term expressing how much ''j'' contributes to the diet of ''i'', and ''FC,,j,i,,'' is a host composition term giving the proportion of the predation on'' j'' that is due to ''i'' as a predator. When calculating the host compositions the fishing fleets are included as ‘predators’. 
     8where ''DC,,i,j,,'' is the diet composition term expressing how much ''j'' contributes to the diet of ''i'', and ''FC,,j,i,,'' is a host composition term giving the proportion of the predation on'' j'' that is due to ''i'' as a predator. When calculating the host compositions the fishing fleets are included as 'predators'. 
    99 
    10 For detritus groups the ''DC'',,''i,j'',, terms are set to 0. For each fishing fleet a ‘diet compositions’ is calculated representing how much each group contributes to the catches, while the host composition term as mentioned above includes both predation and catches. 
     10For detritus groups the ''DC'',,''i,j'',, terms are set to 0. For each fishing fleet a 'diet compositions' is calculated representing how much each group contributes to the catches, while the host composition term as mentioned above includes both predation and catches. 
    1111 
    1212The diagonal elements of the ''MTI'' are further increased by 1, i.e.,'''''' 
     
    2020Figure 7.5 shows the direct and indirect impact that the very small increase of the biomass of groups mentioned to the left of the histograms (rows) have on the biomass of the other groups mentioned above the histograms (columns). The bars pointing upwards indicate positive impacts, while the bars pointing downwards show negative impacts. The bars should not be interpreted in an absolute sense: the impacts are relative, but comparable between groups. 
    2121 
    22 In this example, the apex predators will have a negative impact on their preferred prey, epipelagic nekton, and an indirect, if slight (0.03, not visible on Figure 7.5 but available on the mixed trophic impact table output by the program) positive impact on the prey of their prey, the larger zooplankton (this effect is known as a ‘cascade’). Further, the impact of the epipelagic nekton on the microzooplankton is slightly positive (0.01), even though the former feeds on microzooplankton directly. This is because the epipelagics also feed on the large zooplankton, and this overrules the direct impact. 
     22In this example, the apex predators will have a negative impact on their preferred prey, epipelagic nekton, and an indirect, if slight (0.03, not visible on Figure 7.5 but available on the mixed trophic impact table output by the program) positive impact on the prey of their prey, the larger zooplankton (this effect is known as a 'cascade'). Further, the impact of the epipelagic nekton on the microzooplankton is slightly positive (0.01), even though the former feeds on microzooplankton directly. This is because the epipelagics also feed on the large zooplankton, and this overrules the direct impact. 
    2323 
    2424Most groups have a negative impact on themselves, interpreted here as reflecting increased within-group competition for resources. Exceptions exist; thus, if a group cannibalizes itself (0-order cycle), the impact of a group on itself may be positive. 
    2525 
    26 The mixed trophic impact routine can also be regarded as a form of ‘ordinary’ sensitivity analysis (Majkowski, 1982). In this system, it can be concluded, e.g., that the impact of the bathypelagics on any other group is negligible: these fishes are too scarce to have any quantitative impacts. This can be seen to indicate that one need not allocate much effort in refining one’s parameter estimates for this group; it may be better to concentrate on other groups. 
     26The mixed trophic impact routine can also be regarded as a form of 'ordinary' sensitivity analysis (Majkowski, 1982). In this system, it can be concluded, e.g., that the impact of the bathypelagics on any other group is negligible: these fishes are too scarce to have any quantitative impacts. This can be seen to indicate that one need not allocate much effort in refining one's parameter estimates for this group; it may be better to concentrate on other groups. 
    2727 
    2828One should regard the mixed trophic impact routine as a tool for indicating the possible impact of direct and indirect interactions (including competition) in a steady-state system, not as an instrument for making predictions of what will happen in the future if certain interaction terms are changed. The major reason for this is that changes in abundance may lead to changes in diet compositions, and this cannot be accommodated with the mixed trophic impact analysis.