A circle would imply a gradual change, which doesn’t exactly happen. You start out with Life as grass, then it gets eaten and becomes Life as an antelope, then it gets eaten again and becomes Life as a lion, then it dies, decomposes, and returns to being Life as grass.
I grant you, “The Triangle of Life” may not pack the theaters for Lion King 3, but that would be a more appropriate description, don’t you think?
And that’s a pretty simplistic scenario. You could have shapes without right angles (The Rhombus of Life), or with as many steps as you can imagine (The Dodecahedron of Life).
And why limit ourselves to two dimensions? That antelope could be split amongst multiple lions. Why even limit ourselves to three? I heard a recent theory that the universe is a 12-dimensional super-torus, or something like that. That moves well beyond my ken for naming, but that’s Billy Joel’s problem.