Why is our eco-column self sustaining?

The eco-column is self sustaining because there is a terra, decomposition and aqua layer. In the terra layer, there are small weeds which, using sunlight, will photosynthesise and produce more oxygen and water. The small centipede living in the terra layer can live off the many woodlouse in the layer which will hopefully reproduce to create more food. Therefore the cycle repeats and is self sustaining.