Our attention so far has been focused on modeling single populations. Although we have broken a single population into subgroups, such as by age or developmental stage, we have still treated it as if it is unaffected by the other species or populations with which it might share an environment. Although these models have provided valuable insights into how population sizes can change, we now move our attentions to interactions between species or populations.

Most living things interact with many coinhabitants of their environment. Preying on other species, whether plant or animal, is a common way of taking in energy; and most organisms are at risk of being preyed on themselves. But not all important interactions between species are so obvious. Species may find themselves in competition for limited resources, whether food or space, so that growth in one population is detrimental to another. Mutualism, where several species interact in a way that benefits both, also occurs in nature. A real ecosystem may have hundreds or thousands of interacting populations, with all sorts of direct and indirect interactions between them. How can we understand the effects of these interactions without being lost in the complexity of it all?

To begin to understand the dynamics of interacting populations in such systems, we start by focusing on only two populations and a single type of interaction. The questions that will guide us are modifications of those we have already asked when modeling a single population.