Then all the Tyranids are placed in an inescapable area. This is sometimes on a hive ship or more often an asteroid in deep space, or sometimes an uninhabited planet without the level of nutrients for it to be worth digesting. All the Tyranids will then try to destroy each other until only one Tyranid remains.
If any animal of non-Tyranid origin is located, then the Tyranids will temporarily stop fighting each other to eliminate the threat. Once there is only one Tyranid left then it will give off a psychic signal and the hive ship will pick it up and reuse its DNA as a new base that has shown its effectiveness in battle.