All right, I admit that I'm postulating a pretty simplistic planetary ecology here. There's no question that a more complex one is likely. But I'm constrained by the exigencies of fiction; To be more realistic about ecological diversity would require much more literary space, and would present me with a conundrum-how to describe it adequately while still leaving room for the story I'm trying to tell. I've settled for what I considered a necessary compromise. I hope those readers with greater biological knowledge will accept my "happy medium" without too many quibbles.