The huge Reimu is the queen of the clan, and is responsible for reproducing babies. Individuals who has slower growth and/or visually unappealing are killed right before queen Reimu's eyes, and the dead bodies become the latter's food. This is called "culling".
And for some reason, Reimu is not catching anti-yukkuritis. Also, why not just use them as shit slaves? That can both solve the shit problem and the easiness problem, as looking at shit slave is one of the few options for wild to take it easy without breaking the balance. But ehh, maybe I expected too much from those paste-for-brain.