Forcing functions represent physical or other environmental parameters that may influence trophic interactions. These forcing functions can be used to modify the Q/B ratio of the consumer groups in the model (see Linking mediation and time forcing functions to trophic interaction rates), or to directly influence primary production (see Primary production in Ecosim).
To apply a forcing function to trophic flows (i.e., predator-prey interactions), use the Apply FF (consumer) form (Time dynamic (Ecosim) > Input > Apply FF (consumer)). To apply a forcing function to primary productivity directly, use the Apply FF (primary producer) form. Before using either Apply FF form, you must first define at least one seasonal or long-term forcing function instructions for using the Forcing function form.
The Apply FF (consumer) form consists of a grid representing predator (j) / prey (i) interactions (Figure 8.18). Predators are represented in columns with prey in rows. Predator/prey interactions, as defined in the Diet composition are indicated by white cells.
To apply a long-term forcing or seasonal function, select the i,j cell representing the predator/prey interaction to which you wish to apply the function. This will open the Apply forcing and mediation functions dialogue box (Figure 8.19). The affected prey and predator group will be displayed at the top of the form.
Next, select the parameter to which the forcing multiplier (represented by the forcing function) is to be applied using the radio buttons at the bottom of the dialogue box. There are four possible options:
The selected modifier will be shown in the window Applied shape functions window (Figure 8.19).
Next, select the desired forcing or seasonal shape from the Available shape functions window at the left of the dialogue box by clicking the green arrow. This will move the shape function into the Applied shape functions window. You can remove a shape from this window by clicking the remove button immediately below the green arrow. Note that a common form is used to apply Mediation and Seasonal / Forcing functions. Do not select a mediation function.
Finally, click OK. You should see the number of the forcing or seasonal function you selected in the predator/prey cell (preceded by ?F?).
Note that you can apply up to five seasonal, long-term forcing and/or mediation functions to each predator-prey interaction.
You can clear all applied seasonal, forcing and/or mediation functions from the predator-prey grid by selecting Clear all from the top of the form.
To apply a forcing function to the interactions between a particular predator and all of its prey species, click on the number of the predator at the top of the form. When the Apply forcing and mediation functions dialogue box opens you will see a message at the top of the box reflecting that all the prey are affected.
Figure 8.18 The Apply FF (consumer) form'
Figure 8.19 The Apply forcing and mediation functions dialogue box. Select only Forcing shapes.'