"Physics in Ecology"
December 16, 2005
EURANDOM, Eindhoven, The Netherlands
Thermodynamics and ecology: two fields in a state of confusion
The link between thermodynamics and ecology is at present rather vague and hazy. Undoubtedly, ecologists are partially to blame. A common practice in ecology is to use the standard concepts of thermodynamics miles away from the context in which they were originally defined. Moreover, ecologists don’t seem to like the term entropy. Instead of using entropy, the functioning of ecosystems is explained in terms of a multitude of obscure y-ending-quantities (emergy, anergy, exergy, ascendancy, etc). Typically, the precise meaning and the interrelation of these properties remain rather mystical. However, one can ask: are ecologists the only one to blame? Upon closer inspection, thermodynamics looks less self-consistent than appears at first sight. A recent review counts about 30 different versions of the second law, and a similar number of definitions for entropy. Moreover, thermodynamics seems to exist in many flavours (equilibrium, finite-time, rational, linear, non-equilibrium, irreversible, extended, statistical), and these different thermodynamic theories are not necessary compatible. Given the rather inconsistent “internal” state of thermodynamics and ecology, how can we find a consistent link between them?
Dissipative structures and goal functions for ecosystems
I will talk about the correspondence between a chemostat model, where a chemical reaction system is set up in order to model a predator-prey ecosystem, and Rayleigh-Benard convection cells as a prime example of a dissipative structure. I will focus on the behaviour of the entropy production, and the possibility of other ecological goal functions.
Quasi Gaussian Entropy Theory and its Applications in Ecology
The ecosystem exergy concept: theory and applications
The ecosystem exergy concept has its roots in the second law and in ecosystem succession theory. It hypothesizes that during succession and evolution ecosystems tend to maximize the dissipation of exogenic exergy fluxes by increasing their exergy content in terms of biomass, structure and information. The causal relationship between exergy content of a system and its performance to do dissipation work is rather obvious and can – in closed systems – be described by the Carnot cycle. More enigmatic is why ecosystems would tend to maximize exergy dissipation. According to Kay living systems which can speed up the entropization of the universe have an evolutionary advantage (‘entropy machines’). This is in agreement with Dewar, which states that for large complex systems the state of maximum entropy production is the most probable sum of its microscopic parts. Nevertheless the theory remains exposed for many criticism from mainstream science, but mainly because of pitfalls and misconceptions. Some pitfalls are:
- Maximum exergy content and exergy dissipation level is site specific and dependent on the regional species pool.
- In the ecosystem exergy concept biodiversity is considered order and exergy, while the Shannon diversity index suggests that it is entropy. Information seems to be contextual.
- The calculation of exergy content of information as proposed by Joergensen et al. is incorrect. One solution is the definition of information exergy, as corrected by Joergensen, another is the definition of potential exergy, which refers to a context in which information can potentially become (real thermodynamic) exergy.
Although theoretical aspects need to be further cleared out we are currently developing a series of promising applications:
- A better understanding of the function of biodiversity in ecosystems
- A more operational definition of sustainable development
- A land use impact assessment method useful for Life Cycle Analysis of products
- The development of thermal indicators of ecosystem state and functioning.
We illustrate these applications with field data and results.
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