The earth is a non-isolated system. There is almost no exchange of matter with the outer space (the earth loses a little hydrogen and receives meteorites). To be able to utilize the matter many times during the evolution or from one year and decade to the next, cycling is necessary. Cycling implies that the ecosystem components are linked in an interacting network (Chapter 5).

Ecosystems must be, as the earth, non-isolated because otherwise they could not receive the energy needed to maintain the ecosystems far from thermodynamic equilibrium and even move further away from thermodynamic equilibrium. Ecosystems are actually open systems (Chapter 2) because they need to exchange at least water (precipitation and evaporation) with their environment. In addition, it is practical that suitable solutions (for instance species with new emergent properties that facilitate survival under a combination of new and emergent conditions) in one ecosystem can be exported to other ecosystems. Moreover, it is easy to observe that ecosystems are open systems.

The flow of energy from the sun to the ecosystems is also limited. It is important that an ecosystem captures as much sunlight as possible to cover its energy needs. Therefore, ecosystems, with increased biomass, can increase net primary productivity. But even the best photosystems can only capture a certain part of the solar radiation, which anyhow is limited to about 1017 W on average. Therefore moving further away from thermodynamic equilibrium requires that an ecosystem develop better utilization of the exergy that it is able to capture. Network development, where the components have been fitted together, provides improved exergy efficiencies (Chapters 4 and 5). Another possibility is to increase information in the form of better process efficiencies (Chapter 6). Increased sizes of the organisms imply also that the exergy lost for respiration decreases relatively to the biomass (Chapter 2).

While matter and energy flow limit evolution, the amount of information is far from its limit. It is, therefore, understandable that information embodied in genes and in ecological networks has increased throughout evolution (Chapter 6).

On the one side, we do know the characteristic ecosystem response to changed conditions and can make predictions. On the other side, ecosystems are so complex that very accurate detailed predictions always will be impossible and "surprises" should always be expected; see Chapter 3.

The development of the life forms that we know from the earth has been possible because the earth has the elements that are needed to build the biochemical compounds that explain the life processes. It includes water that is an ideal solvent for biochemical reactions. In addition, the earth has the right temperature range that means that the biochemical reactions proceed with a certain rate and that the decomposition of particularly proteins is moderate. The right balance between formation and destruction of high molecular proteins that are the enzymatic compounds controlling the life processes is thereby obtained.

The life processes take place in cells, because they have a sufficiently high specific surface to allow an exchange rate with the environment that is suitable. Cells are, therefore, the biological units that make up organs and organisms. Nature must, therefore, use a hierarchical construction (Chapters 2, 3, and 7): atoms, molecules, cells, organs, organisms, populations, communities, ecosystems, and the ecosphere. The addition of units in one hierarchical level to form the next level gives the next level new and emergent properties (Chapters 3 and 7).

The variability of the life conditions in time and space is very high (Chapter 6). When an ecosystem has adapted to certain conditions, it can still be disturbed by catastrophic events. Ecosystems have due to their properties (see all the chapters but particularly Chapter 7) the adaptability and flexibility to meet these changed conditions and still maintain the system far from thermodynamic equilibrium. The disturbances call for new and creative solutions for life to survive. Disturbances may therefore also be beneficial in the long term for ecosystems (Chapter 7).

We can explain the ecosystems, their processes and responses, and evolution by the properties presented in this book. The discussion throughout the book has clearly shown that the properties are sufficient and the discussion in this last section has demonstrated that the properties are also necessary. It may, however, not be the only possible explanation to life in general. We cannot exclude that we will find other life forms somewhere else in the universe, for instance based on silica or carbon but with another biochemistry, better suited maybe for a different situation. The properties presented above are, however, very consistent with both direct and indirect observations, which render them a good basis for an ecosystem theory applicable on Earth. Chapters 8 and 9 have, not surprisingly, shown that the ecosystem theory presented in this book can be used to explain other ecological rules and hypotheses and have potential for application in environmental management.

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