As a biologist, in year 1 you learn why the world is not overgrown with bacteria that double every 30 mins. Restrictions. From there follow niches, evolution through natural selection on population levels working on existing variation and more slowly on new variation. Evolutionary arms races, etc.

Anyway, the question: "Why is there no Darwinian devil?" Is a nice starter for starting to understand the enormous variation found in our biosphere. Of course, we are as close to a Darwinian devil as any species ever came because we make our own niche by tearing other's down. Resistance is futile. Of course, tearing other's down will also tear our own niche down because they are all connected, at least, our old niche is. Can we build a new one fast enough? Or should we stop tearing our niche down?