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.

Deathworlds are fun, my last one I had frequently 100s of buildings on fire and increased production to such a high degree to replenish combat losses. It was only when I came up with an overkill wall design and started implementing that all around the base starting from high risk points that the losses came down. The power demand was insane, GW of power just for the tesla turrets.

Gleba first run as a deathworld was probably not the best idea, but it was fun.