Why would AI care about a balanced ecosystem? It wouldn't need one to survive and thrive.

You are assuming that the AI would suffer from the level of selfishness that mark the worst in humans. Higher levels of consciousness see the value in all life, whether or not it "gains" from that life. You're also assuming that the AI wouldn't be effected by the physical world, or have needs of it. Humidity, temperature, air content, solar radiation, and so on all effect electronics as well as living beings.

I don't think it would care much about the welfare of a lesser race like humans

You're once again applying the worst possible manifestations of human consciousness to something that would be immeasurably more complex, well-informed, and logical. Just because some humans wrongly believe the universe is anthropocentric & don't care about other species, doesn't mean that AI would suffer from that same mental disorder.