You need some energy to come into the system from outside.

The bulbs get their energy by eating insects. The insects get their energy from eating plants, animals or dung. The energy in the animals and dung all came from plants or from things that get energy by eating plants. The plants get their energy from the bulbs, so it's all a big closed loop.

A bit of energy is lost at every point in the loop. The ecosystem will quickly die out unless there is some kind of energy coming in from outside the cave. The more energy comes in, the richer the ecosystem can be.

It could be chemicals seeping up from deep underground, or an underground river that brings food from the surface.

If stuff is going in to the cave then you will also need a way for stuff to leave the cave, so it doesn't fill up.

You need some energy to come into the system from outside.

The bulbs get their energy by eating insects. The insects get their energy from eating plants, animals or dung. The energy in the animals and dung all came from plants or from things that get energy by eating plants. The plants get their energy from the bulbs, so it's all a big closed loop.

A bit of energy is lost at every point in the loop. More is lost every time a life form walks, flies, grows tall, or in any other way expends energy on something other than being delicious. The ecosystem will quickly die out unless there is some kind of energy coming in from outside the cave. The more energy comes in, the richer the ecosystem can be.

It could be chemicals seeping up from deep underground, or an underground river that brings food from the surface.

If stuff is going in to the cave then you will also need a way for stuff to leave the cave, so it doesn't fill up.