I’ve mentioned that I don’t really like the idea behind “dungeons” because they don’t make a lot of sense to me. Tons of villainous creatures all living together in a confined space with traps preventing them from escaping - Why don’t they just eat each other? Sure - dungeons can be laid out in an intelligent fashion, but they rarely are. It’s like watching a bad movie filled with plot holes - Why doesn’t the dragon eat the orcs? Why don’t the stupid trolls wander into the trap area and die? Who’s feeding the wolves or the giant spiders or whatever?
But I do have underground races, and I worry about feeding them all the time. After all, there are only so many mushrooms that can grow in complete darkness. So when I was working on one odd race of humanoid turtles, I started thinking about how they really survived. When I first created them, I gave them mushroom fields, which still makes sense. There was also an area of their “kingdom” where the “roof” of their underground city was turf barely held together by roots and supported by a sunken city (buildings that had collapsed into an old mine). So here there was a small amount of sunlight getting through and thus they were able to grow a sort of cabbage/rhubarb hybrid - the huge leaves collected a large percentage of the minimal sunlight and thus were able to grow.
Like real turtles, I wanted them to be omnivorous. At first I thought maybe I’d give them a colony of giant ants nearby - something they could hunt and would probably never run out. I still think that’s a good idea, but I decided that they might not be mighty enough warriors to be constantly “at war” with a colony of giants ants, even if they were a food source. So I went with three sources of protein that I think anyone could use in any dungeon setting.
The first is a cop-out and used by nearly every GM and author: blind fish in stagnant pools.
The second I think is the easiest “hidden” food source. I had already given them mushroom fields and the rhubarb stuff, so why not have huge earthworms. Not only are these long (some nearly a foot), but they are fat! The largest of them is nearly an ounce. With these things both helping fertilize the gardens and serving as a food source, it seemed a very easy win, and the adventurers wandering through may never realize they are there. Though having a foot-long, hot dog sized earthworm in your sleeping bag with you when you wake up could be fun! (for the others)
Lastly I added some crickets. If my home is any indication, crickets can get into anywhere! I have only done a little cave exploration in my life, but I do recall finding crickets in caves at least twice. Now of course, this is high fantasy, so normal crickets won’t do. These need to be 5” long crickets. I tried to work it out mathematically and I believe that 18-20 of them would equal a pound. The turtle people catch them with butterfly nets, so the “hunting gear” and baskets filled with live crickets will be more obvious to the invading adventurers, but this really seemed like a decent idea. Being nocturnal, they would seem to appreciate the darkness. They likely need to have some means of reaching the surface world, but they could be flying in and out and still offer a decent amount of food to the underground dwellers.
I offer these examples, because I think they are the type of thing you can “include” in your games. You don’t need to make a big deal out of it, but if some players ask you how the turtle people could survive this far underground without any food sources, you can pretty easily say - “Didn’t you see the nets? The mushrooms growing along that one corridor? I told you about hearing crickets in the tunnels, right? I didn’t mention it, but there were worms slithering through the moss pit the mushrooms were growing in. Now, do you want to play the mission or do you want to discuss the ecology of the dungeon?” Maybe it’s silly, or maybe it’s just enough to support the willful suspension of disbelief.