Watch the movie, the intro speaks about the wave motion energy that flows through out the universe, and is created by all living things. The Protodevelin go as far as to detail that different types of people/living things generate different types of spirtia. The whole existence of the Protodevelin in this universe was due to the Protoculture running experiments on tapping energy sources from different dimensions.
Assumedly, the Protodevelin exist in a dimension comprised entirely of energy, though the details of whether it's all sentient energy, or whether the Protodevelin exist as seperate entities because that energy was tapped & placed into seperate entities is unclear. What is clear, is that like all living things, life has to feed on other life to sustain itself, and being pure energy beings, the Protodevelin feed off of similar energy sources. i.e. living things = batteries.
Anima Spiritia appears to be too strong a wattage for them to stomach, though as shown by Sivil, with enough effort they can adapt to it. Being living things, the Protodevelin give off their own spiritia as well, the solution that Basara found being to show them the ability to generate their own renewable energ source, so they wouldn't have to feed off of others.