Holy crap I played it 10 hours today... So addicted. The free play mode is way more fun. I built a huge base then got attacked because of pollution and then I noticed there were hundreds of enemy camps all over the map and I was basically surrounded. So I spent a few hours clearing out all the camps. Do the enemies keep attacking or do they mostly just stay put? I have only been attacked a couple of times and I think it was only because of some kind of pollution landmarks. I can kill a whole enemy base camp but there are so many of them, I don't want to go killing them all, and I don't want to just ignore them if they are waiting to do a huge attack sometime?
Yeah, the "campaign" are just some very minor extras. The sandbox is the actual game. I considered explaining this to you, but why bother when you were going to find out anyway?
Biters (or shitters, as I like to call them) attack your installations based on the reddish-purple pollution clouds your machinery produces. Shitter nests absorb pollution in much the same way beehives absorb pesticides, and each hive has an "irate-o-meter" that fills up slowly when exposed to light pollution, but more quickly when exposed to heavier pollution. Also, when you're producing a lot of pollution, not only is it heavier but it also tends to drift further afield and reach more distant shitter nests, although it dissipates with distance regardless.
When a nest's irate-o-meter fills up due to your environmental pollution, the shitters will organize a raiding party to pay a visit to the offending pollution source. I call these raiding parties "shitter litters." When they reach the approximate source of the pollution, they will attempt to destroy everything between you and them, and then will try their best to eat you. If you aren't around, they'll just try to trash your stuff until they either destroy it all or are killed.
Multiple raiding parties attacking the same/and or multiple locations simultaneously or concurrently are very possible. Each nest has its own irate-o-meter.
As for clearing out shitter nests: When you kill shitters and especially when you destroy shitter nests, their evolution-o-meter begins to climb very slowly. As they evolve, bigger, tougher, heavier-hitting shitters begin to appear around nests. When the evolution-o-meter hits 1.0, you'd better hope you're ready.
It's good to clear out some quantity of shitter nests around your pollution-producing installations, because they can't organize raids if they don't exist. You've got to balance this, however. Especially now that "Victory Poles" no longer work (don't worry about it; they don't work anymore, anyway), it's probably best to keep the immediate vicinity of your pollution sources clear, and deal with occasional raids from more outlying shitter nests.
Final note: As I recall, shitters will never, ever stop chasing you if provoked. If you drive by a bunch of shitter nests five miles away from your base in a vehicle (provoking some quantity of shitters in so doing), reach your base, park, have a coffee, and go about your merry business, eventually they will all show up to ask why you didn't stop to give them a ride.