The p- and q-axes in the graph above run from 0 to 500 with grid line spacing 100. The p-axis is the horizontal axis along the bottom of the graph and the q-axis is the vertical axis along the left edge of the graph. The two blue lines indicate points at which q' is zero and the two red lines indicate points at which p' is zero. Notice there are two equilibrium points -- at (0, 0) and (200, 200).
Click anyplace within the green graph to set the initial conditions for the populations p and q. A black dot will appear marking the initial condition.
This applet has five buttons at the bottom. From left to right they do the following.