Basically title. If a swarm of spiders overlapped with a swarm of rats and they were enemies (as far as swarms can go) would they be able to deal damage to each other?
I tried this; the GM houseruled it to be ok, but it ain't RAW. (but it IS awesome)
It seems to me that swarms can hurt other swarms.
Sure, swarms either take half damage or are immune to "weapon" damage, but swarms also don't deal weapon damage - a swarm's damage is automatic damage that has no type and no attack roll. That doesn't sound like "weapon damage" to me.
Personally, I think if you put a swarm of flies in a room with a swarm of spiders and give them a little time, you will only have spiders left. Sure, IRL that would take hours, or at least lots of minutes, while in Pathfinder that would only take a couple rounds up to maybe just about a minute at worst case, but it should definitely not be impossible.
The trick is to make them enemies. Unless you can control them somehow, my guess is swarms would avoid each other in most cases.
To GM Lamplighter:
I ask this because one of my players used a summon swarm spell to fight a swarm of rats. Since the summoned swarm attacks all living creatures and he summoned it in the same area as the rat swarm I ruled that they fought each other. My issue was whether they damaged each other.
In the end I like the creativity of his idea and let it work, but for some of my more RAW players I'd like to know for future situations.
"A swarm takes half again as much damage (+50%) from spells or effects that affect an area, such as splash weapons and many evocation spells."
Swarm damage effects an area - anything inside the swarm - so there's a case for giving it increased damage.
True that