When worldbuilding, I am trying to fill my world's nature with a lot of dangerous monsters. However, there was something I realized I forgot to do, and something that I notice many fantasy creators forget: Balancing fictional wildlife to match ecosystem.

In many fantasy worlds and urban fantasy, you will see many natural environments with normal animals, but also mythical beasts and monsters coexisting. Unfortunately, for consistency, they do not have their own prey to continously survive.

While this may be a really small detail, I am trying to make it so that monsters can exist in my world, BUT there is a consistent ecosystem for them to survive in. What my question is: How can I edit the environment, so that fictional creatures (deadly monsters in my case) can be like normal animals in it?

Note: For one solution, I am already doing it: Adding on more fictional creatures for the monsters to feed off of. While that is for a few environments, I am trying to make environments where they appear like regular forests and deserts, but can handle extra fauna, much like how those creatures exist in real-world mythologies.

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    $\begingroup$ what's a monster in this context? $\endgroup$
    – njzk2
    Commented Dec 16, 2022 at 18:41
  • $\begingroup$ @njzk2 By monster, I mean a giant, deadly beast that does not actually exist. In this context, and to make it simple, they are natural animals, nothing paranormal or magic-like $\endgroup$
    – Crafter
    Commented Dec 16, 2022 at 20:25
  • $\begingroup$ how big, and what do they eat and how much? $\endgroup$
    – njzk2
    Commented Dec 16, 2022 at 20:38
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    $\begingroup$ Voting to close. This question is answerable for a particular monster with describable habits but not for monsters generally. $\endgroup$
    – SRM
    Commented Dec 19, 2022 at 0:56
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    $\begingroup$ Voting to close, needs clarity. The real answer to this question would be a categorization of existing mythical creatures based on how easy it would be to justify its existence from an evolutionary perspective. If the platypus can exist, then an owlbear could exist. A pegasus blatantly violates the laws of physics, and genetics can't get you there. A siren or gorgon might be physically possible (plus or minus some creative story telling), but its defining characteristic is that it casts a spell. That would only evolve in a universe with spells, but would then be inevitable. $\endgroup$ Commented Dec 19, 2022 at 6:43

6 Answers 6


Remember the 10% Rule of Biology, which holds that each level of the food chain, 10% of the bio energy transfers to the next level of the food chain on consumption. The film Zootopia actually makes this a plot point, by explaining the population of the city is 90/10 herbivore to carnivore. In the real world, the largest predators tend to also be quite endangered owing to territorial coverage they need for prey and competition with other predators in the are (including humans). Some carnivorous species are smaller than most larger animals (Otters for example) while others are omnivores (Almost every species of Bear, with the Polar Bear being a notable exception. This was also one of the reasons humans became as successful as they did).

It should also be noted that lots of animals in the real world can be dangerous, but are entirely herbivores. The Hippopotamus is a very aggressive animal and is one of the most dangerous in it's environment, to such a degree that predatory Crocodile will avoid them at all costs, similarly Rhinoceroses are large grazers but are easily spooked and are prone to fighting off their would be predators. Elephants, especially males, are well aware that they have a size advantage.

And in many environments, there is more than enough prey to satisfy a predator species, but the predator species might not be able to handle the prey enough to keep the population in check. For example, in the East Coast of the U.S., the White Tail deer is becoming an environmental threat because it's natural predators, the Grey Wolf, are no longer extant in the environment. These Deer are still reproducing at a rate that would be viable with predation, which means the population has no natural check on it's growth save for environmental limiting factors. In this part of the world, it's not uncommon for cullings to occur to reign the population in.

This can actually be a reason for the monsters and the human main characters to meet, as the monsters would move into the area could be supported by the deer populaiton BUT humans are culling the sources because they don't know the monsters. As far as the monsters are concerned, the humans will be a good meal when the deer can't satiate them and hunters assisting with the cull would be in their territory. Also, audiences are less sympathetic to Deer Hunters because Deer are cute and "Man" is still one of the most evil Disney villains for shooting Bambi's mother.

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    $\begingroup$ That's a really good point and a natural way to keep the worst monsters from running amok. $\endgroup$
    – JBH
    Commented Dec 16, 2022 at 15:22

Two-Teir Ecology.

You want a mundane world with foxes and worms and wolves and hedgehogs, and a fantasy world with dragons and hydras and chimeras. The two worlds exist on top of each other. But they don't interact so the dragons burninate all the foxes and earthworms, or the wolves outcompete and extinctify all the dragons.

You need a mechanic to force non-interaction. I propose the following:

Mundane creatures survive by eating each other. Mundane foxes eat mundane mice and insects and eggs. Fantasy creatures on the other hand survive by consuming mana (or other buzzword). This is the same power-source that makes the creatures able to exist and fly and breath fire. Hence the monsters need a constant supply of mana to not starve.

The magic creatures are able to hunt the local mundane wildlife into extinction. They just don't because the mundane wildlife provides no mana. In fact it results in a net loss of mana, since the energy cost of chasing down a herd of deer is less than the mana gained by eating them.

So the best strategy is to ignore the local mundane wildlife, and instead focus on ambushing smaller fantasy creatures. Or set up shop at the nearest leyline and absorb the mana radiation. Or go after those pesky adventurers and their tasty magical items.

Mundane foxes and worms and wolves and villagers typically kill and eat each other, since the fantasy monsters are too strong. On the other hand, fantasy creatures and adventurers typically kill each other rather than the mundane wildlife, since the mundane wildlife does not provide them any energy.

Adapted from my earlier answer.

  • $\begingroup$ I assume you mean two-"tier" $\endgroup$ Commented Dec 16, 2022 at 20:40
  • $\begingroup$ @TheresaKay Yep it should be "tier". Thanks. $\endgroup$
    – Daron
    Commented Dec 16, 2022 at 21:39

You, my friend, need to play the game Ecologies

You're talking about creating biomes where your monsters play a role like animals do in real biomes. I believe this is very doable, but you're missing some information about how biomes work. However, a full and detailed explanation of how biomes work (which I'm not competent to provide) are not, IMO, what you need.

I hate to turn this into something of an advertisement, but I believe you would benefit from playing the game Ecologies from Montrose Biology. It's fun to play and does a great job of simplifying biomes in exactly the way you you need them to be.

The basic rules can be found here. For a half dozen biomes (there are expansion packs with more biomes, including fantasy biomes, if I recall) you need:

  • An identified biome
  • A "Producer," meaning a basic food source like lichen or mushrooms. This is the bottom of the food chain.
  • Four levels of "Consumers" (C1, C2, C3, C4). Each consumer feeds on the next level down. So C1 eats the producer. C2 eats C1, etc. C4 is the top of the food chain.
  • Finally, you need an SD creature (scavenger/decomposer/detritivore). Basically, the animal or organism that helps return the dead to Mother Earth.

A complete biome has at least one of each animal, plant, or organism type: producer, consumer, and scavenger (a total of 6).

I advocate this kind of simplification

I believe it would be straightforward for you to insert your monsters into various C1-4, SD and P positions depending on whether or not you want to "enhance" or modify an existing biome or build a fantasy biome. Remember that you can have multiple consumers at each level. There's nothing wrong with having a regular C4 bear and a C4 Bugbear that compete for territory but are of a type/size/capability that they can't simply be dialoged from the food chain.

I also advocate playing Ecologies

Because it's a pretty good way to get a basic handle on how biomes work.


Nothing but monsters.

muskie eats pike


You don't need to waste time with worthless unexciting animals of no interest to anyone. Everything is a monster! Monsters eat other monsters! Maybe there are some small monsters that eat grass and flowers. These little monsters will gladly lunge up and bite your junk.

  • $\begingroup$ Those small monsters already exist my friend. They are called dachshunds. $\endgroup$
    – Daron
    Commented Dec 17, 2022 at 11:33

Make a food web

enter image description here

For monsters to be in an ecosystem, they need to have enough food for them to survive. That means generally you need some plants to get energy from the sun to make biomass, animals above that to eat the biomass and make meat, and animals above that to eat those animals. The larger an animal is, the more food it needs.

This is good practice to show the impact of heroes. If the hero takes all the mushrooms because they want to make healing potions then the hawks will get more aggressive because their food supply is cut. The trees and the shrubs will get eaten more as the squirrel tries to substitute the food. Any change can have a whole chain of knock on effects.


Myths are actually ecologically sound

If monsters prey on humans, humans are their prey. Easy enough.

It's worth noting that oftentimes mythological monsters from all cultures are now identified as "personifications" of diseases and natural perils that humans had a hard time figuring, like storm winds for instance, and sometimes even bad human behaviour like hate and violence. At the time, human populations were a lot more prolific with children than today, and a lot of people would die by causes that are now much less dangerous or outright disappeared in the western world.

In your world, humans could have that kind of life expectation, being prey of a multitude of monsters, not unlike ancient people considered themselves, and that would be sustainable from an ecological point of view.


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