EcoMunch is freeware by Arch D. Robison. The documentation in the downloads describes the distribution rules.
EcoMunch is a predator-prey simulation based on A. K. Dewdney's Wa-Tor described in Scientific American (1984). EcoMunch extends Wa-Tor by adding two more kinds of predators, and introduces terrain constraints on where predators and prey can live. The fast interplay of predator and prey populations make a visually interesting ecology.
Below is a screenshot of EcoMunch 2.1:
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