How do you get half a chicken? Or half a sheep?
Half a tree is easy - many food tries come in dwarf varieties, although these aren't necessarily the best way to have a lot of whatever you are growing. It may sound like a good idea, but if you are planning for the long haul, might as well get full size trees and be done with it. Dwarf trees are good for suburbia, or the inner city - but would you want to get stuck in either before, during, or after a cataclysm?
The easiest way is getting someone else to look after the animals, and set up a local trading scheme. Anyone experienced in breeding animals knows that there are certain population requirements - it is not enough to get a rooster and a couple of hens, and expect your grandchildren to be using the descendents of those few chickens for meat and eggs. Not just that you need to balance eating and breeding, but because genetics becomes a severe limitation. So, it is better to have a reasonably large collection of animals, or a few collections of animals, and have some monitoring of breeding. You get one person (or a couple of people) to look after the animals, compensate them for the time and care that is needed.
At least, you do if your neighbours are at all interested. Or you gather a few people, and assign tasks; again, finding people who will take a suggestion to join you early enough is difficult. You can always make sure that there is room for people to join you, but you're stuck with being the one to spend the time, money, and energy to get everything prepared.
But... it is either prepare or find yourself on the back foot.