I've been playing Factorio for a short time and have just developed a tank. I figured it would be worthwhile to explore the zones around my base and scout out potential mineral sites. I travelled beyond the range of my base's pollution and was surprised to see that biter nests were becoming more massive than those closest to my base.
I had assumed the biters nearest to my base would be strongest, as they consumed pollution, and that biters far away would be weaker – but doesn't seem to be the case.
Is this unexpected nest population normal? If I travelled further would biter spawns thin out?
Best Answer
The other answer is technically correct, but in my opinion answers only the title of the question, not the actual question.
Over the course of the game, the enemies indeed get stronger - i.e., the aliens appear in bigger, stronger variants. This is indeed determined by the evolution factor, which is, among other things, indeed driven by the global pollution production. This applies to biters and spitters in all cases and to worms with some caveats. However, the area of the nests isn't affected by the pollution, nor do the nests grow when consuming pollution.
TL;DR the strength of biters and spitters (their variant) depends solely on the evolution factor, their numbers depend solely on the size of the nest. The size of a nest depends on the distance from the starting point, the only exception being nests founded by alien expansion.
Nests spawn all over the map when it is generated. The size of these nests depends only on the distance from the starting point, not on the evolution factor (which is 0 at the start of the game anyway). As you travel farther from the center of the map, the nests will increase in area, will appear more frequently (and thus tightly packed to each other). Furthermore, farther nests will spawn with stronger variants of worms. The number of enemies passively populating a nest is always directly proportional to the number of spawners.
Throughout the game, new nests can be created by alien expansion. Every 4-60 minutes a group of 5-20 aliens will set off from a nest to create a new nest nearby. These values increase with the evolution factor - i.e., the expansion will happen more often, and the groups will be bigger - meaning they will create slightly larger nests. Worms can spawn in nests during expansion and in this case their tier is determined only by the evolution factor, not the distance from the starting point.
Consuming pollution is a mechanic separate from the evolution and expansion and is how enemies launch attacks. When pollution reaches a spawner, it will start consuming the pollution to spawn new enemies. Every 1-10 minutes, the enemies spawned so far will launch an attack and will attack the source of the biggest pollution by ascending the pollution gradient (always choosing the direction where the pollution is the strongest). This ensures only the nests the pollution reaches will attack your base. Consuming pollution doesn't cause more nests to spawn, nor it itself increases the evolution factor.
I highly recommend going through the excellent official wiki if you want to learn about each of these mechanics in-depth.