Humanity Has Declined

Jinrui wa Suitaishimashita (人類は衰退しました, lit. "Humanity Has Declined"), or Jintai (人退) for short, is a Japanese light novel series written by Romeo Tanaka. The novels originally featured illustrations by Tōru Yamasaki for the first six volumes, but Yamasaki was replaced by Sunaho Tobe in 2011. The first volume was published in May 2007, and six volumes have been released by Shogakukan as of February 2011. Two manga adaptations began serialization in 2012. An anime adaptation produced by AIC A.S.T.A. and directed by Seiji Kishi began airing in July 2012.

Plot
Jinrui wa Suitaishimashita is set in a post-apocalyptic world where the human civilization has regressed and mankind keeps decreasing in numbers. The story follows an unnamed girl who acts as a mediator between humanity and the "fairies" who are small dwarf-like creatures attracted by sweets and happy things, but also have the habit to cause trouble to her with their powers in their endless search for amusement.