As your factory grows, it produces pollution, which makes the aliens stronger. They become pretty dangerous, while you develop better ways to fight them, like the buggy, artillery, or spider mech.
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.
As your factory grows, it produces pollution, which makes the aliens stronger. They become pretty dangerous, while you develop better ways to fight them, like the buggy, artillery, or spider mech.
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.
Seems to be the latter:
https://wiki.factorio.com/Enemies#Evolution