This is really cool! I made a fairly simple system with plants, herbivores, and carnivores... Plants can reproduce to any open square on the board. Herbivores and carnivores reproduce when they eat, and can die from overcrowding or randomly from disease/old age. Corpses provide fertilizer for nearby plants. Certainly not the most realistic model, but a lot of fun to mess around with!
http://ncase.me/emoji-prototype/?remote=-K4BOya31RZ3XEK5PTgy