Ventana software support forum

In the attached file I want the pest population and parasitoid population are positive always.
How can I do it?

If you don't want a stock to go negative, a good method is to use first-order negative feedback control. However, it is sometime difficult to formulate a correct equation for this.

You can also use the MIN function. For instance, "deaths" for pest population could be formulated as

MIN(parasitoid population*KILLS PER BEE PER DAY, pest population/TIME STEP)

Note that "KILLS PER BEE PER DAY" helps your model in correcting your units and making it more obvious the relation between parasite deaths and prey population.

Also, ensure that your TIME STEP is small enough to avoid integration error. A good rule of thumb is to choose a value at least half of your smallest time constant.

I hope this helps.

BP