Pollution also prompts them to attack.

Terrain can naturally absorb some of it and trees can absorb even more but will eventually die from it, so they're just a temporary stopgap. Eventually when it reaches the aliens, they too will absorb it until they reach their limit then they'll organize attacks.

That being said, I can't remember if it's specifically pollution they absorb that causes them to evolve into stronger forms or if it's any pollution you produce that does it.