Jinrui wa Suitai Shimashita, translated as "Humanity has Declined", is a light novel written by Romeo Tanaka and an Anime animated by studio AIC A.S.T.A.For years, declining birth rates have forced what’s left of the human race to cede more and more territory to other beings who have appeared to take advantage of the emptying ecological niche. Now, only a handful of humans remain among the remnants of civilization and Earth is dominated by fairies – tiny, four-inch tall creatures of surprising intelligence. But humanity’s importance isn’t over quite yet, as the young heroine learns as she makes the decision to return to her hometown and assume her grandfather’s position as an arbitrator between the races. Unfortunately, the job isn’t going to be anywhere near as simple as she expected, and it’s going to take a wisdom far beyond her years to achieve her most important mission.Originally a light novel, an anime adaptation premiered as a part of the Summer 2012 Anime lineup and is currently being simulcast on Crunchyroll. It has been licensed by Sentai Filmworks with a home video release planned for 2013.
After the End: Humanity seems to be living in the aftermath of some previous catastrophe, with the technological level being reverted, knowledge having been lost, ruined and abandoned structures scattered all over the countryside, and a very small human population that is on the brink of extinction. While the catastrophe that caused this has been mentioned several times, it's never explicitly stated what it was.
The Heroine once alludes to the fairy about how the Old Humanity (homo sapiens) were destroying themselves through war and overexploitation of resources.
Aloof Dark-Haired Girl: Deconstructed. The reason the protagonist was this in school is because it's a psychological defense against a toxic environment. She's deliberately being nasty as a social firewall, which others somehow interpret as charming.
Y was also like this for somewhat different reasons. She genuinely did want friends who weren't as messed up as the Wild Rose Society and didn't want to reveal her hobby to others.
Anachronic Order: The arc order is Secret Tea Party (flashback) => Earth => Time Management => Homecoming => Secret Factory => Survival Skills => Secret Tea Party (frame) => Subculture.
Be Yourself: Inverted! Due to Assistant-kun's upbringing, he's a total Blank Slate, and thus seeks others to define him. With some help from the fairies, he manage to come up with a functional enough identity.
Beneath the Mask: The Wild Rose Society serves as the mask for its members, each of whom, barring Y and the heroine, are varying degrees of bat-shit crazy.
Brick Joke: The green-clad fairy from the Secret Tea Party arc. It's great that they essentially have infinite lifespan to pull this kind of joke.
The Chris Carter Effect: invoked Parodied heavily in Episode 4, where Y explains that the way to make a bestselling manga is not to craft a consistent plot, but to keep stringing viewers along with constant cliffhangers, since they won't realize the plot holes until the end. However, once the audience catches on, the popularity of their manga drops like a stone.
The greatest entertainer is the greatest swindler!
Hidden Purpose Test: Orchestrated by Y early on in Secret Tea Party arc. Having solved one riddle, the heroine sees the second. The only incentive being a key to an extra lock on her dorm room door, the heroine opts to break inside. Years later, it is discussed between the two: Y decided that the heroine didn't have the smarts to pass the test, the heroine claims she knew the answer, but wouldn't bother acting on it, seeing the risk of there being even more of these boring riddles.
Hikikomori: The city that's the main setting of the Homecoming arc was apparently a cultural version of this. They locked themselves in out of fear of EM waves.
No Name Given: All of the major characters, and a number of minor characters, almost to the point of being a Nameless Narrative. "Watashi" and "Assistant" aren't their actual names, obviously. The English-speaking fandom hasn't really settled on a nickname for the main character, using "Watashi" (which she's credited as), "Mediator", "the protagonist", "the heroine", and similar variably.
Our Fairies Are Different: Tiny, addicted to sweets, evolved from humans, don't act at all bright despite apparently having super-advanced technology, and have permanent vacant grins. No wings, either.
Personal Raincloud: The fairies get these. They really exist and can actually become the weather if you get enough of them in one place.
In Ep. 6, Oyage is briefly dressed as a Big Daddy.
Another pone at Ep. 10, when Watashi goes through the notes of her Grandfather's predecessor, they show in a display similar to the Resident Evil Files
Shown Their Work: Episode 6 references the Pioneer Anomaly, though the show explains that the effect was caused by the fact that Pioneer was self-aware, and hesitated because it didn't want to go out in the cold, empty void of space and leave Earth behind forever.
The Silent Bob: The heroine seems to be able to understand Assistant even when he isn't using signs.