It’s a strange relationship, not being circular, as one would expect. "Without oaks there could be no mice, without mice no cats, and without cats no oaks." Oaks feed mice, mice feed cats, but cats don’t feed oaks. So without cats and mice, there still could be oaks. But if you had just oaks and cats, that would be fine (for the oaks) although the cats could not exist because they would have nothing to eat. If you had oaks and mice, the oaks would be okay until they were gnawed to death by the overabundant mice, which (having taken over the world) would subsequently die, so that scenario wouldn’t happen. There would be a predator-prey relationship between the mice and the oaks, like there is between sheep and wolves (the classic example). Both would coexist fine, with a balance in both populations being struck. Now, if you had just cats and mice, the mice would die out first (of starvation), followed by the cats. With all three entities (oaks, cats, and mice) in existence, a reduction in any one of them would hurt one of the other two, but the only combinations under which the entities could not coexist is oaks and cats, and cats and mice. Here are all the combinations: Oaks alone: Would thrive. Cats alone: Wouldn’t have anything to eat; would die out. Mice alone: Wouldn’t have anything to eat; would die out. Oaks and cats: Cats would die; oaks would thrive. Cats and mice: All would die (mice would starve to death; followed by cats). Mice and oaks: Predator-prey; both would coexist. Oaks, cats and mice: It’s an ecosystem! Now I’m tempted to make a diagram. Of all the useless things to do… Well, I think I’ll go play Minesweeper instead. No, I hate that game (because you often get into situations where logic alone won’t help you, so you just have to guess).