They have the simulation setup so if you run it, it eventually settles into a volatile but sustainable steady state between grass, wolves, and sheep.
The interesting part is changing some of the parameters to see what happens. When you do, you quickly learn that intuition takes a holiday in system dynamics. For instance, I changed how much energy the sheep gain from eating grass from 4 to 5 (for whatever reason, they improve their utilization). What do you think happens in the steady state values of grass, sheep, and wolves? I expect that the same amount of grass would sustain more sheep and wolves due to the sheep extracting more energy every tick.
Turns out, the wolf population goes down! Change the sheep gain to 6 and the wolves die off totally after a while. What happens is the additional sheep create increasingly large oscillations in the wolf population that eventually wipes them out. Certainly gave me something to think about.