Slide 19: Keeping it together!
When you build even a moderately complex food web model, you soon find out that it is very hard to keep all the pieces coexisting. If you have two types of herbivore eating one grass resource, one will soon outcompete the other. So there must be some resource partitioning, although it is possible for each herbivore to share some of its optimal forage with other types.
You can balance this by making one more preferred by a predator. Predators are likely to prefer the faster-growing prey, for the simple reason that it is likely to be more available for consumption. This is included in the model by a preference term for predators, such that its preference for a species is a function of its representivity in the total herbivore population.
Finally, predators in general are much longer-lived than herbivores, and are also proportionally slower growing - if the predators have a fast growth rate, they will go into a 'boom-and-bust' cycle with the prey.