A more detailed introduction on screen saver programming and issues related to this article can be found at the following places:
Writing an OpenGL screensaver for Windows by Rachel Grey.
Probably the simplest possible OpenGL screensaver. All you need to know on OpenGL initialization and basic Windows screensaver design.
Wikipedia article on Lottka Volterra equations.
Find out more about the theoretical background of predator prey simulations at this Wikipedia article.
OpenGL tutorials by NeHe Productions.
Excellent page with a lot of OpenGL tutorials and their source code for a variety of platforms and languages.