The system theory or system approach is a model to describe relations as a system with its interactions including feedback. It is used in different sciences including biology (ecology) and cultural anthropology (cultural ecology) where an important subject is the ecosystem. Cybernetics is the science that treats any system and mainly the feedback.
We already learn about systems when as children we play with boxes with wooden logs. Let's take the box as a system. However much we change the position of the logs within the box, their number does not change. The number of logs in the box changes when we take logs out or put logs in from outside. That way we transfer logs from one system to another. The processes within the system change the position and relation between the logs but its number stays the same. If we apply what we have learnt here to other contexts, we already understand quite a lot of systems.
How do rabbits change the balance of nutrients in a forest? Let's take the forest as the system. The rabbits in the forest can change the plant stock by selection pressure, they can accelerate the circulation of nutrients. Like we already know from the logs, the calcium potassium, phosphorus etc. atoms can change their positions, they can participate in chemical reactions, but their quantity does not change by processes within the system (forest). When a rabbit moves out of the forest (no matter whether a living rabbit goes out on its own or a hunter carries out a dead rabbit), with it some nutrients are moved out of the system. And when a hunter brings food for the rabbits from outside the system, he (or she) adds nutrients to the system.
The same holds true for energy. When we put energy in an electrolysis of water to obtain hydrogen and later burn the hydrogen to gain energy, we'll get the same amount of energy that we have put in. Exactly like the logs the energy cannot increase in quantity without anything put in from outside the system (principally logs can be created out of trees, outside of children's everyday life, but energy cannot be created). The idea that it's possible to take energy out of a system (for example a system of magnets) constantly without ever adding any, is as absurd as the idea of taking water out of a bottle constantly without ever adding any.
Many people believe that the use of fossil fuels can be compensated by planting trees. If we take the bisophere as a system, we see that the fossil fuels add carbon to the biosphere (apart from other chemical substances). The trees pull the carbon temporarily out of the atmosphere but it stays within the circulation of the biosphere. If we understand the carbon circulation (one of the basics of ecology), we cannot believe in such indulgence.
And when finally we take a society as a system, we'll see that the creation of employment does not create wealth within the system. What creates wealth, is productive work, that kind of work which creates something that is considered wealth. If someone does an improductive work that has only been created in order to keep people employed, that can change the distribution of the goods, but it does not contribute to the total wealth of the society. A machine that breaks easily, contributes to employment and increases the gross domestic product (which only refers to transfer and not to productivity), but a machine that works without problems for a long time, contributes more to the society's wealth. Many economists do not understand that because they have never learnt to think in systems, and so they keep thinking that a lack of employment is a problem which can be solved with machines that break easily. And as their diagnosis of the problem is wrong, they cannot solve it. If an economist learns to think in systems, first of all he will know that an unlimited growth in a limited system (like the Earth) is not possible.
French: analyse systémique