AKA why vampire and were-creature “societies” make me cranky. The available energy just isn’t there.
Some background. Biology, so far as we know, works within the physical laws of reality. If you want your monsters to feel real, they have to at least give a nod to how real creatures work. Including how much energy they need to survive and what the likely prey/predator ratio is.
How much energy is available depends on your creature’s trophic level.
Long story very short, the realities of how many genes you can stuff into your DNA and how many different enzymes you can afford to build in your cells means that living organisms simply cannot extract every last drop of energy from an available food source. It’s more effective for survival to strip out the quickest and easiest nutrients and energy you’re adapted to get out of your food, and dump the rest. Other organisms, often microbial, take over breaking it down further and extracting what they can strip out. And so on.
So if you’re a multicellular critter, whatever you’re eating has to be several times more plentiful than you are. A useful biological rule of thumb is, you lose about 90% of available energy with each higher trophic level. So say you have 100 grass clumps, you could then have 10 zebras and one lion. This is why large predators are rare.
You’d think humans were the exception to this rule given how many of us there are, but we cheat. Like bears, we’re omnivores, only we can eat our way even farther up and down the trophic levels than any bear, keeping our population much higher than any available prey base would indicate. And this is why tigers are in serious trouble population-wise, while bears are usually not. Tigers are obligate carnivores. Great if there’s plenty of water buffalo and young elephants walking around, not so great if there’s just fields of grain and insects.
If there’s not enough prey in an area to keep tigers alive, how can your world justify a dragon?
There could be ways. Sleeping on their hoards for decades at a time, magical creations trapped by a spell broken by some poor fool… there are ways. But there’s one I’ve wondered about, given dragons often are magic.
If one world doesn’t have enough available prey for a population of dragons… what about ten worlds?
It’s classic predator behavior in biology. If there’s plenty of prey, you can afford to have more neighbors and guard a smaller territory. If there’s not – you roam over a larger one.
If a dragon eats ten cows in a sitting, and needs to eat even once a month, it’ll cut a bloody swath through even a Western range’s herd. But if it can visit a different world a month for those ten cows – that drops it from “utter disaster” to “dangerous and expensive predator”. Ecologically speaking, this is a more sustainable species.
It probably wouldn’t fit the tone of most fantasy stories, where the characters are mostly in one world, but it’s something to ponder. Especially if dungeons are implied to exist outside the usual space-time continuum.
BTW – while mammalian carnivores can’t subsist on just plants, mammalian herbivores can and will eat meat. Ruminants, in particular, eat the microbes they culture in their guts along with the plant matter, so they get a fair amount of animal protein. The animals just happen to be microscopic.
Take that, Thoreau, and all your grumping about people should be vegetarians because obviously oxen get by with vegetable-made bones….