*basically less and less animals will be bred to be killed
THIS ^
We breed animals in huge numbers in order to consume them.
In a wild situation, animal numbers are kept in balance - huge herds of wildebeest support large numbers of predators which keep their numbers stable. If the wildebeests die back (eg disease) predators do to because they starve,
We are really the only animal which utbreeds its food source, because we have found a way of increasing it artificially. But it isn't sustainable.
there are far too many people on this planet.
Everyone feels they have the "right" to have children (we don't - children have the right to have loving parents who care enough to protect the world they bring them into so that they have a future) - even in the west today, where there is no problem with contraception, a lot of families have many children (I'm a firm believer in a two-child rule, personally -or at least a two-successful-pregnancy rule).
A falling population will cause problems for a couple of generations, but then will right self and if people have the sense to keep it stable, would be sustainable.