The Nazgul were more dead than alive and didn't need to rely on biological processes to stay alive, and I don't doubt the same is true for Balrogs and other spirits (which may or may not include Dragons). Smaug certainly ate the people of Dale until it was thoroughly ruined and knew the taste of dwarf well enough to smell it their ponies. I'm sure he did some hunting in his time and his prey probably included larger mammals like wild horses, possibly deer, even some bears, or maybe wanderers from Lake Town who had lost their way. Of course, he also seemed to hibernate a good deal on his bed of gold and such a sedentary lifestyle does not require much energy.
As for the Ents...photosynthesis.
