Changes between Version 1 and Version 2 of EwEugMediation


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Timestamp:
2010-11-23 02:12:22 (9 years ago)
Author:
shermanl
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  • EwEugMediation

    v1 v2  
    11== 8.7 Mediation == 
    2 You can define a series of ‘trophic mediation functions’ that are used to modify vulnerabilities of selected prey types i to their predators j using Ecosim’s Mediation form (see Using the trophic mediation form below for instructions, although you should read this brief introduction first). 
     2You can define a series of 'trophic mediation functions' that are used to modify vulnerabilities of selected prey types i to their predators j using Ecosim's Mediation form (see Using the trophic mediation form below for instructions, although you should read this brief introduction first). 
    33 
    44It is not uncommon for some third type of organism (i.e., mediating organism) to affect the feeding rate of one type of organism j on another i.  At least two types of effects are possible: 
     
    66__Facilitation__: the third organism type behaves in some way that makes type i prey more available to predator j when the third organism is more abundant.  For example, pelagic piscivores like tuna may drive smaller fishes to the surface, where these fishes are then more available to birds. This is a concern for marine mammals and birds in the Eastern Pacific Ocean, at least in areas where heavy fishing has reduced abundances of tunas and billfishes. 
    77 
    8 __Protection__: the third organism provides protection for prey type i when the third organism is more abundant.  For example, juvenile fishes (as type i prey) may use corals, macrophytes, and/or sponges for protection from predators, and fishing may directly impact these ‘cover’ types. K. Sainsbury (pers. comm.) has emphasized the possible importance of this effect for evaluating impact of trawling on the Northwest Shelf of Australia.  For another example, increases in phytoplankton may reduce water clarity and hence search efficiency of visual predators on small fishes, which would tend to reinforce the ‘cascade effect’ of increasing abundance of small fishes causing reduced zooplankton abundance and hence increased phytoplankton abundance. 
     8__Protection__: the third organism provides protection for prey type i when the third organism is more abundant.  For example, juvenile fishes (as type i prey) may use corals, macrophytes, and/or sponges for protection from predators, and fishing may directly impact these 'cover' types. K. Sainsbury (pers. comm.) has emphasized the possible importance of this effect for evaluating impact of trawling on the Northwest Shelf of Australia.  For another example, increases in phytoplankton may reduce water clarity and hence search efficiency of visual predators on small fishes, which would tend to reinforce the 'cascade effect' of increasing abundance of small fishes causing reduced zooplankton abundance and hence increased phytoplankton abundance. 
    99 
    1010Each trophic mediation function is defined in the following steps (described in more detail in sections below): 
     
    1212 1. Sketch a functional form for the mediation effect (i.e. the mediation affect as a function of the biomass of the mediating organism); 
    1313 1. Define the ''x'' variable (i.e. independent variable) for each function by specifying relative weightings for the effects of different mediating organisms. Weightings can be specified for one or more Ecopath groups, so that __''x'' becomes a weighted sum of mediating organism biomasses__ (i.e., more than one group can have the same trophic mediation effect but to different degrees); and 
    14  1. Applying the ‘trophic mediation function’ to specific predator-prey interactions (using the [[Apply mediation.htm|Apply mediation]] form). 
     14 1. Applying the 'trophic mediation function' to specific predator-prey interactions (using the [EwEugApplyMediation Apply mediation] form). 
    1515 
    1616Sketching a function that increases with increasing x represents facilitation effects (prey more vulnerable when ''x'' is large), while sketching a function that decreases with ''x'' represents protection effects (prey less vulnerable when ''x'' is large).  The idea behind making ''x'' a weighted sum of third organism biomasses is to: 
    1717 
    18  1. Allow differential protective ‘quality’ for alternative organisms that provide cover for prey; and 
     18 1. Allow differential protective 'quality' for alternative organisms that provide cover for prey; and 
    1919 1. Recognize that several different groups may have different tendencies to generate facilitation effects, e.g., some tunas may commonly chase bait fishes to the surface while others may not. 
    2020 
     
    2727'''Using the trophic mediation form''' 
    2828 
    29 To implement trophic mediation in Ecosim, open the ''Mediation'' form (''Time dynamic (Ecosim) > Input > Mediation''; see Figure 8.9). Note you must have an [[Ecosim menu.htm|Ecosim scenario loaded ]]before you can use the ''Mediation'' form. There are three steps to the process: 
     29To implement trophic mediation in Ecosim, open the ''Mediation'' form (''Time dynamic (Ecosim) > Input > Mediation''; see Figure 8.9). Note you must have an [EwEugGettingStartedEcosim Ecosim scenario loaded]before you can use the ''Mediation'' form. There are three steps to the process: 
    3030 
    3131Step 1. Define trophic mediation shapes; 
     
    3737'''Step 1. Define trophic mediation shapes''' 
    3838 
    39 When you open the Mediation form for the first time, you will see several ‘blank’ mediation shapes (Figure 8.9). To define a mediation shape, first select one of these blank shapes. You can then define trophic mediation shapes in one of two ways: 
     39When you open the Mediation form for the first time, you will see several 'blank' mediation shapes (Figure 8.9). To define a mediation shape, first select one of these blank shapes. You can then define trophic mediation shapes in one of two ways: 
    4040 
    4141 1. Shapes can be drawn freehand using the blue sketch pad. You will see your sketched shape transferred to the thumbnail in the bottom pane;2. Alternatively, you can use standard mathematical shapes. Click ''Change shape …'' at the top of the form to open the ''Change shape'' dialogue box (Figure 8.10). Select a shape type (e.g., linear, sigmoid, exponential) from the menu in the window. You can tailor the shape to your specific requirements by setting the ''Y Zero'', ''Y End, Y Base'' and ''Steepness'' parameters (where appropriate) in the cells provided.