Science does not make sense without a theory. Without a theory our observations become only a beautiful pattern of impressions. All our knowledge in a scientific discipline has to be coherent to be able to apply the underlying theory to explain our observations. Ecology has for a long time only partially been able to condense the systematic collection of observations and knowledge about ecosystems into testable laws and principles. The authors of this volume are convinced that an ecological theory is now available as a tool in ecology due to the contributions of many system ecologists during the last decades, mainly through the application of thermodynamics to explain the reactions of ecosystems. It has been difficult and has taken a long time to construct the theoretical building of system ecology, but nature has not been created to be easily understood by human beings. It has been necessary to break with the long reductionistic tradition in science and use thermodynamics in a new holistic approach to understand ecosystems. Reductionistic science has had a continuous chain of successes since Descartes and Newton.
Lately, there has, however, been an increasing understanding for the need of syntheses of knowledge into a holistic image to be able to grasp the sense of complex systems such as ecosystems and social systems. It is today considered by many scientists the greatest challenge of science in the XXIst century to put together our many observations of complex systems into a completely understandable holistic picture.
A number of ecosystem theories have been published during the last three decades. They are all attempts to capture the features and characteristics of ecosystems, their processes and their reactions to changed conditions, i.e. changed forcing functions. The different theories look at first glance not to be consistent, but when we examine the different theories more carefully, it becomes clear that they represent different angles and view points. It was asserted in the first edition of S.E. J0rgensen's book "Integration of Ecosystem Theories: A Pattern" (1992) that the various theories actually form a pattern, and the later editions (second edition 1997 and third edition 2002) have only enhanced the perception that the theories form a pattern and that to a large extent they are consistent. During 2000, there have been several meetings where the fathers of the theories met and discussed the pattern. It is clear from these discussions that we today have an ecosystem theory which is rooted in a consensus of the pattern of ecosystem theories. It is the intention of this volume to present this ecosystem theory as it has taken form in the beginning of the XXIst century, but with particular emphasis on the thermodynamic interpretation of this ecosystem theory. It does not mean that the network interpretation by Ulanowitz and Patten or the green accounting using emergy by H.T. Odum are less important. They are just other angles to, in principle, the same ecosystem theory, as the quantum mechanic theory has been approached differently by Heisenberg's uncertainty relationships and by Schrodinger's wave functions.
The thermodynamic interpretation of an ecosystem theory by use of the concept of exergy has been chosen as the main focus of this volume. Exergy may be applied as a core concept in a thermodynamic edition of an ecosystem theory, as will be shown many times throughout the volume. The various approaches have, however, different advantages in different situations. When an ecosystem problem is best solved by use of an approach
based on energy and exergy, these concepts should be applied, but when the processes and reactions concern the network, the use of a network theoretical approach may give clear advantages. The relationship between the different approaches will therefore be mentioned to emphasise the importance of a pluralistic view to describe an ecosystem. When a simple physical phenomenon such as light needs two descriptions, it is not a surprise that a very complex ecosystem needs many different complementary descriptions (J0rgensen, 1992c, 2001b, 2002b).
It is a very important step forward in ecology and system ecology that we now have a theory that several system ecologists can agree upon, as this is the prerequisite for further progress in system ecology. Furthermore, it makes it feasible to construct a network of laws, rules and observations as we know from physics, where a few fundamental laws can be applied to derive other laws which can be used to explain, if not all, then almost all physical observations (see Fig. 1.1). We do not know yet to what extent this is possible in ecology, but, assuming that it is the right time to start to build such a theoretical network in ecology, it should be possible at least to propose a promising direction for our thought and create some fragments of the network.
Our book demonstrates that it is possible with the present ecosystem theory in hand to start to build such a theoretical network in ecology and shows to what extent such a theoretical network has been established today. It may be concluded that we do have sufficient knowledge about the behaviour of ecosystems to be able to explain many observations, rules and regressions on the basis of an ecosystem theory.
The advantages of having an ecosystem theory is, of course, that it allows us to understand nature better, including the behaviour of ecosystems and their reactions to different perturbations. An ecosystem theory is, however, also applicable in environmental management, because it allows us to predict how ecosystems will react to various sets of man-controlled forcing functions.
Finally, in addition to our own book, we strongly recommend "Entropy for Biologists: an introduction to thermodynamics" by H.J. Morowitz (1970).
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