What is the theme in A Sand County Almanac by Aldo Leopold?

The theme revolves around the atom in nature. The atom in nature used to be free to roam about as it pleased. This is no longer the case. Now it is subjected to restraints and boundaries that are completely unnatural. Yet, are these boundaries enough to truly constrain the atom? An atom leaves a group of rocks where it has been laying for a couple thousands of years, then it goes into the soil and is absorbed by a plant. It moves to the leaf and is eaten by a deer who is then shot by a person and is eaten by them. Afterwards, the person dies, and the atom goes back into the soil.