one word... pathogens, some say the reason for the human population
explosion is due more to sanitation than anything else.. but that's
like saying what was the most important factor in winning a war.
keep googling, energy cycle, nitrogen cycle, food chain, food cycle..
when you get all done, dig a hole and bury it and run the kitchen water
out in the yard. There is a reason the grass is greener over the
lateral lines of a septic tank system.
a starter,...
http://www.fao.org/docrep/003/X9199E/X9199E04.htm