[KannO]

Humans are more complex than flies and social atmosphere would need to be healthy relative to dramatic population sizes.

John B. Calhorn's mouse utopia experiments give some indication of complexity in large communities beyond food resources:

https://en.wikipedia.org/wiki/John_B._Calhoun#Mouse_experime...

https://www.youtube.com/watch?v=0Z760XNy4VM

  There was no shortage of food or water or nesting 
  material. There were no predators. The only adversity was 
  the limit on space.


  Initially the population grew rapidly, doubling every 55 
  days. The population reached 620 by day 315, after which 
  the population growth dropped markedly. The last 
  surviving birth was on day 600. This period between day 
  315 and day 600 saw a breakdown in social structure and 
  in normal social behavior. Among the aberrations in 
  behavior were the following: expulsion of young before 
  weaning was complete, wounding of young, increase in  
  homosexual behavior, inability of dominant males to 
  maintain the defense of their territory and females, 
  aggressive behavior of females, passivity of non-dominant 
  males with increased attacks on each other which were 
  not  defended against.[2] After day 600, the social 
  breakdown continued and the population declined toward 
  extinction. During this period females ceased to 
  reproduce. Their male counterparts withdrew completely, 
  never engaging in courtship or fighting. They ate, drank, 
  slept, and groomed themselves – all solitary pursuits. 
  Sleek, healthy coats and an absence of scars 
  characterized these males. They were dubbed “the 
  beautiful ones.” Breeding never resumed and behavior 
  patterns were permanently changed.