As you can see, both the fish population and the plankton population increase and decrease over time. As the fish eat plankton, they reproduce and make new fish. Soon, they have eaten most of the plankton, and start dying off. When there are few enough fish, the plankton can reproduce fast enough to grow back. When there are enough plankton, the fish start reproducing again. And so on. The populations cycle, with each peak in fish population followed by a peak in plankton population.
Also, over time the number of different colors of fish decreases. Eventually there is only a single color of fish. This is not because there is any advantage to having a particular color. In fact, with each run of the model it's impossible to predict which color will predominate. The number of colors decreases because of a bottle-neck effect: each time the fish population drops very low, only a few fish are left, and all the fish born after that are their offspring. Fish inherit the color of their parents, so all the fish after a bottle-neck have the colors of the few fish that were alive during it.
This is a very simple model of an ecosystem, but its behavior is quite complex. Simple models like this are useful, because these same patterns of change over time can be seen in many other ecosystems. Many other interesting phenomena can be observed just be changing the parameters of this model or adding small pieces to it.
Click here to download StarLogo TNG, which comes bundled with the fish and plankton model.
