People say 'It's as plain as the noise on your face.' But how much of the nose on your face can you see, unless someone hold a mirror up to you? - Isaac Asimov

Human beings, in terraforming a world, leave out the thorns and thistles, whatever they may be, and human beings then have to labor to keep the world going. It isn't a self-supporting organism as Gaia is. It is rather a miscellaneous collection of Isolates and the collection isn't miscellaneous enough to allow the ecological balance to persist indefinitely. If humanity disappears, and if its guiding hands are removed, the world's pattern of life inevitably begins to fall apart. The planet unterraforms itself.

An Earthman will give you anything as long as it costs nothing and is worth less

And one day he said, "I'm going to see Hallam again."
Bronowski's eyebrows lifted. "What for?"
"To have him turn me down."
"Yes, that's about your speed, Pete. You're unhappy if your troubles die down a bit."