So as I thought one day I thought what do some monsters eat when they don't eat npcs cows or unlucky towns folk and adventurers. So I came up with some ideas what would an aankek eat why not a giant pill bug they are like crab and harmless so why wouldn't they be a easy meal that could be fun.
Here's a principle for designing an ecology. Useful for DIY.
Do your food web. Each population that's a step up is only 10% of the size of the step below. Make some bottom feeders of the magical world (dragons survive on magic right, but what's the krill or plant equivalent?). Think about the hunting ranges of beasts and how sparse populations are (see tigers). Are these assumptions that you can deal with and still have fun?
Keep fewer kinds of monster but go into more detail. After all, it's amazing what we get up to with only humans in our society.
And do allow for the occasional one off monster that a wizard did. They're rare, can't really breed or are just ecologically uncompetitive, but they can still exist. Like Frankenstein's monster.
You really can't do better than the collection of writing that has been officially published:
Monster Ecology is great it adds to the game and in the past my players have used it to notice things.
Recently I have my Kids running into a Dire Spider lair with baby spiders in an ruined Temple of Pelor. As they got closer I kept dropping hints, First the Druid noticed Nature Roll DC 10 that there were no squirrels in the trees and the sounds of the forest were missing, no birds chirping. As they got closer they camped out for the night, I let everyone know that the night seemed off, and after some rolls all DC 10 there were no bugs in the air and crickets we not chirping. My Daughter thought this was odd (She is an animal lover and plays the Druid) and asked me if she could look for animals in the morning. I had her and the Fighter looking around and they started noticing small spiders and webs in the trees. These were some of the baby spiders that had left the lair. Now everyone was on guard and thought spiders were the problem. Soon they found husks of animals and insects everywhere as they came upon the ruins, and it was confirmed, spiders were everywhere.
I use this all the time with Carrion Crawlers hiding and waiting for a battle to end, or giant rats being the food for other animals or goblins in a cave. Always find your big badly and think that it is the top of the food chain, then work down. Make sure that the critters get eaten by something smaller or weaker and remember that predators will eat prey. End your food chains with herbivores eating moss, mushrooms or plants in any ecosystem and make sure that they are more plentiful than predators. If the predators eat all the prey they will need to move to better hunting grounds, which is what the Dire Spiders were doing at night and eating cows from a halfling village.