If you want to make a realistic sort of encounter table for a wilderness (or indeed a dungeon) there’s a neat little rule about biomass you can follow.
Basically, you need to create a pyramid, where each level is 10 times the amount of the level above it.
- Apex Predators
- Other Predators/Large Prey
- Smaller Prey
- Baseline Food
10 times is a basic rule of thumb, you can measure it by weight (mass) if you have those statistics, or by size.
Let’s abstract it and call it ‘size units’, with each one being about the size of a ‘Medium creature’ in D&D terms. Each scale up or down in D&D can be approximately four times the size (doubling each axis).
So, to survive with enough prey, a LARGE red Dragon would need 10 LARGE prey animals.
So for every LARGE red dragon we need 10 LARGE bison.
Those bison need 100 5×5 squares of grass to feed upon, save that foliage probably isn’t going to fill the whole square, so let’s multiply that by 5 to reckon on wild grasses on average 1ft high. So that’s 1 dragon and 10 bison in an area of grass approximately 2500ft on each side. That’s about a half mile.
‘Grass’ doesn’t really count as an encounter, so an encounter table might consist of a d12 with 1: Dragon, 2-12 Bison, but only a 1/6 chance of encountering anything interesting each half-mile covered.
But you’re not just going to have dragons and bison in a plains environment. You can do the same working with other predators and prey and have multiple tables of possible encounters, with a dice used to differentiate between them.
For a dungeon example:
A Mimic is a medium creature. It’s going to need the equivalent of 10 medium prey creatures inhabiting the dungeon, in order to survive.
Let’s put some goblins in the dungeon, and they’re what ends up – mostly – as the mimic’s prey. Goblins and albino lizards. Goblins are small (4 times as many needed), and lizards are tiny (16 times as many needed. That’s 80 lizards and 20 goblins to sustain the mimic.
The lizards and goblins are going to need a mix of vegetation and insects ten times as big as 100 medium creatures.
Let’s give the goblins a mushroom farm, plagued by pests, taking up 50 medium creatures (giant fungus) with a couple of shrieker fungus to act as an alarm. That’s a nice room for our dungeon too, The remaining 50 is taken up by 250 ‘spaces’ of slimy walls, moss, rotting material and squirming bugs – and otherwise empty.
Dungeons don’t have to be realistic of course, and many of these creatures aren’t going to wander (in many cases because they get eaten by mimics), but following realism can help the creative process.