What do all the monsters in a megadungon eat? There's an awful lot of mouths to feed in the typical megadungeon. Surely many of the monsters feed on each other, the whole place certainly gets noticed by adventurers and other surface dwellers when the megadungeon dwellers go foraging on the surface but all the humanoid hordes, fearsome grues and giant rats are goign to need a fair bit of food and they aren't likely to find all they need outside the megadungeon. This post shall explore where the food comes from.
Every food web starts with some form of energy input, most often light and this is what we are most familiar with. Most simply light to plants, plants eaten by animals, animals eaten by bigger animals. It’s often a lot more complicated then that with multiple types of life making use of the energy introduced to the system and being preyed upon; a host of bacterial life can grow prolifically and be fed upon as are plants by other life forms. The typical RPG world has many more energy inputs than the sun alone.
In the real world food-webs are supported by the decomposition of organic matter transported from elsewhere adding a step to the often assumed food web so we have a more involved one of light to plan to animal to waste (plant or animal) to bacteria to (plant or animal). This is really just a complication of the main food chain but the stage where waste is consumed by bacetria opens ripe territory for a host of MegaDungeon scavengers. All organic matter that finds it’s way into a MegaDungeon is eventually going to be eaten and this food web will be one step removed from the light of the sun. Waste will be devoured by bacteria, fungus, various small invertebrates, the odd vertebrate, slimes/jellies, and the occasional awful horror with each of these in turn beign preyed upon by each other or some other creature.
Chemistry and heat produce energy, this energy is often consumed by life that never see the light of day. The deep sea geothermal vents and the bizarre life found near them are a classic example of a food web away from the light of the sun. Active geothermal/hydrothermal environments are ripe for anchoring a food chain far from the influence of the light of the sun.
Another food web not chained to light found underground is that of sulfur-oxidizing bacteria in sulfidic streams growing in such abundance it is a principle foodstuff of cavefish. These sulfidic waters can exist wherever limestone cave formation is found as the entire process fo limestone cave formation is moved along by the presence of hydrogen sulfide gas that permeates and the water and covers surfaces all over a cave system in a fine film eating away at the surrounding rock, creating an environment where more sulfide gas is created eventually becoming so rich in the waters the chemical reaction encourages blooms of bacteria which accelerate the process even further so sulfide streams exist with populations of bacteria high enough to form mats on the surface of rocks that are sufficient to feed fish the majority of their nutrients with no input from the sun.
Radiation is found everywhere and may be found in relatively high levels in some subterranean environments in such a level it may provide life with enough energy to grow and thrive. Radiotrophic fungus on earth derive nourishment from radiation in essentially the same manner plants do with light. Whether that radiation is the result of natural phenomenon or ancient technological civilization it can be fueling a food chain in a megadungeon.
Magical energy is a source RPG worlds get to tap into to anchor a food web. Magical energy could fuel species or supplement their nourishment from other sources (ever wonder where dragons get enough energy to do dragonish things?). The magical energy need not be as elaborate and intentional as a spell or the will of a powerful being it can be the leakage of an elemental vortex or some other fanciful phenomenon. All those giant animals traditionally found in megadungeons might depend on a link to a magical source in the food chain.
The Megadungeon food web may include forage, reduction of waste, thermophiles, the sulfidic subterranean food-chain, radiation thriving organisms, and magical phenomenon of all sorts. There can be adequate energy down in the MegaDungeon to explain why it’s there and what everyone is eating.