LaXnyd said
So the Tyranids will consume each planet they go to, huh... looks like Gaia got her "threat to the survival of the planet"
A very big threat they consume planets till theres nothing left not even an atmosphere.
Edit:Day 01: Mycetic spores are dropped, mostly containing Lictors and Genestealers. As soon as they hit the ground, the reproduction of Tyranid creatures likely begins immediately.
Day 09: By now, the Tyranid infestation will have expanded to around 250 km around the drop point, and will likely present a significant threat to planetary defense force troopers and the resident Imperial Guard.
Day 13: Tyranids have expanded to 700 km from the drop point; some may begin infesting local water sources.
Day 37: Tyranids completely control the area within 2,000 km radius of the drop point and basolithic infestation has begun to reach a 5,000 km radius from the drop point of the first mycetic spores.
Day 48: Tyranid population growth becomes exponential, with population doubling approximately every 3 days.
Day 50: The main Hive Fleet arrives, with the number of Tyranid craft in the swarm generally numbering around 1.5 billion. All psychic contact with the planet through the Warp is cut off by the Warp shadow of the Hive Mind. Any attempts to escape are quickly stopped by the Hive Fleet. Any remaining surface life is eliminated by Gaunts.
Day 51: Primary consumption of planetary biomass begins (any sentient resistance has generally been eliminated by now). Brood ships land, releasing Ripper swarms, which consume all organic material (even the other Tyranids) and deposit it at the reclamation pools. Capillary towers (and the Brood ships) send the material into orbit for use by the Hive Fleet.
Day 80: Ripper swarms board the Brood ships and return to the Hive Fleet. The hive ships descend into the upper atmosphere and begin collecting it. Reduction in atmospheric pressure causes oceans to boil away, which are also collected so that their free water can be used by the fleet. The loss of ocean mass causes plate tectonic shifts, dramatically increasing volcanic activity across the victimized world. Upon completion, the Hive Fleet leaves the planet and reenters the Warp in search of fresh prey.
Day 100: The Imperial Navy arrives in response to the planet's original Astropathic distress call, only to find a destroyed, lifeless, uninhabitable planet.
This is pretty much what they do.
Edit2: But this cycle depends on many variables like strength and size, as well as how fast the genestealers act to get into power.