Whacked out dungeons are fun, but sometimes you might want something a little more realistic to help with suspension of disbelief. The big problem with dungeons is coming up with ways to justify the kind of big-ass apex predators that actually present a threat to adventurers when they go delving.
If you stick to undead, golems, traps and intelligent monsters that can range beyond the dungeon to hunt, farm and raid then you have less problem, but when it comes to the lair of a bigger beast you’re going to have issues.
Constructing a dungeon ecology can be done by working according to the trophic pyramid.
Very, very, very roughly you can estimate that for each level of the trophic pyramid you travel down, you need to multiply the total biomass involved by ten – or so.
The base of the pyramid are typically the primary source of sustenance. That might be grass, fungus, scavenging creatures or anything that can turn background material or energy into biomass. In deep sea vents that might be chemosynthetic bacteria – which is a great way of creating biomass in more exotic fantasy locations.
As an example, let’s take a basic giant spider.
The giant spider is of medium size, which is 60-500 lbs and is described as man-sized. Let’s call it 100lbs for sake of ease.
The giant spider sits at the apex of our pyramid of dungeon ecology.
The next step below, the spider’s prey.
Of course, the centipedes and snakes need something to eat for themselves. Rats make good prey and we can combine them into swarms for more interesting encounters. We need 10,000lbs of rats to sustain our centipedes and snakes (and to, maybe, feed some baby spiders while they’re growing up) and let’s make them big, nasty, dungeon rats that weigh 2lbs each. That means this dungeon complex or cave system is going to be crawling with 5,000 rats.
What do the rats feed on? 100,000lbs of slime, fungus, moss, worms, beetles and carrion scattered through the dungeon complex. For comparison a cubic foot of hay weighs around 40lbs. To put that in perspective that’s 2500 cubic feet or, if you wanted to model a dungeon in minecraft, about 278 cubes that has to be producing food for your critters.
If you made that in minecraft as a tunnel with a 2×2 aperture and assumed every surrounding block produced food for your critters, it would be a tunnel 23 yards long. If it was just the equivalent of the floor – which is perhaps more realistic, it would be 138 yards long. So break out your grid paper and work out just how big that might be!
And all of that is just for one, solitary, CR1 giant spider…