Commenter Heteromeles suggested, with respect to the design of closed-circuit biospheres for long-duration starships:
[This] isn't a design for a livable formerly-known-as-starship. Rather, it's a test to see if the design will work.Okay, let's take this test seriously:
Here's the test: the closed ecosystem within the ship has to be able to produce enough beer to satisfy all the beer consumers within the system.
The reason? Beer's pretty central to culture (see How Beer saved the World for a humorous and fairly accurate take on the role of beer in history). If things are so tight that there's not enough acreage to produce alcohol (beer or equivalent), then there's probably not enough surplus capacity within the system to withstand the inevitable problems they'll face. Ditto if everyone has to be sober all the time just to keep the ship running. In the later case, people will distill alcohol anyway, and it will be a problem, rather than a central part of the ecosystem.
Something to think about. If they can't drink, it probably won't fly.
What is the minimum number of species necessary in order to produce beer aboard a generation ship?
Note 1: An absolute minimum is compliance with the Reinheitsgebot; however, I'd also like to see analyses for a British bitter, a Belgian Lambic, and (even though I wouldn't be seen dead drinking it) a mass-produced American rice-based 'lager' style beer.
Note 2: This is a trick question; it's not about the specific ingredients that go into the beer, but about the food webs that sustain those crops.
Suggested additional reading: The Makeshift Rocket by Poul Anderson. (Probably not a useful reference, but amusing.)