I’m seeking some guidance on how to set parameters for the Farms shapefile input to the InVest Pollinator model.
First, I’m wondering if you have any recommendations for setting the half_sat parameter. Is there any literature available I can reference for a default value? My initial thought is that the half_sat value should be half of the pollinator dependence of a crop. For example, if soybean is modestly dependent on pollinators and has a p_dep value of 0.25, then potentially the half_sat should be 0.125?
My second question is how to interpret the y_tot output attribute from the model. I understand its a relative index, so it does provide an absolute value for yield. However, would a change in y_tot between two model runs indicate a percentage change in yield? Is the y_tot index linear? Based on the video explanation of this model, the indices are not necessarily linearly correlated with yield/abundance.
Thanks for your questions about InVEST’s Crop Pollination model.
As defined in the User Guide, a crop’s half saturation coefficient, “is the wild pollinator abundance needed to reach half of the total potential pollinator-dependent yield. This is a tunable parameter that may be most useful to adjust following an initial run of the model and an examination of the results.” This value is not necessarily half of a given crop’s pollinator dependence. In fact, a crop’s half_sat value is often greater than its p_dep value. “The actual contribution of wild pollinators to pollinator-dependent yield depends on the degree to which pollination needs are already being met by managed pollinators.” This paper may prove useful to you as a source. But, absent managed pollinators and more precise knowledge of the crop and location, I suggest starting with a value of 0.5 for half_sat and then adjusting it after examining the results of an initial model run.
For y_tot, yes, you can compare the difference between values to calculate % changes in crop yield. Just please do not interpret any absolute differences in results (don’t use volume or mass), as the index is only meant for comparing relative changes. My interpretation is that the relationship between a crop’s dependence on pollinators and its total yield is nonlinear.