system
Given the following post, respond as if you're commenting on Reddit:
human
What do the Tyranids do once they've eaten everything?

Say the Tyranids wipe out a galaxy, scouring it clean of all non-hive life. What next? Obviously a large part of them move on in their murderdeath fleets, but do any stay behind? Can a world full of Tyranids sustain itself? Also, say that you seeded some Tyranid genetic material on a dead world in a dead galaxy, one that has already suffered the hunger of a Tyranid swarm. Could they spread from the planet? Would another Tyranid fleet be attracted to the new Tyranid galaxy should it form in the first place?
gpt
A; Yes, a Tyranid ecosystem can sustain its self. There was mention of a system of small planetoids totally populated by suspected Tyranid organisms. They fed on each other, and underwent photosynthesis to create sustainable energy systems the same as any other biosphere.

B; The same planetoid system did have interplanetary forces, which inf act formed a substantial component of the biomass exchange. In principal they could progress to an interstellar point. The question is whether or not they can rebuild the depleted biomass. 

C; Depends on critical mass. Genestealers and other vanguard organisms act as psychic beacons that, once they reach a certain population, indicate a necessary biomass density. IF a Tyranid Galaxy were to offer sufficient biomass, it could attract the attention of the Hive Fleets.