Variables and Parameters

What is the difference between a variable and a parameter?
In the context of these population-modeling equations, a variable represents the level of a particular population. In the mutualism section, there are two variables, x and y, which represent the two species that are helping each ot her. When we write x' or dx/dt we mean the rate of change of population x.

A parameter, on the other hand, determines how much of an effect a particular phenomenon has on the populations.