1.1. 4: Equilibria

December 15th, 2009

Open systems tend to exist in a state of balance: Equilibrium. Equilibrium avoids sudden changes in a system, though this does not mean that all systems are none changing. If change exists it tends to exist between limits. We can therefore think of equilibrium states in two ways STATIC and “STEADY STATE”.

Static Equilibrium is where the components of a system remain constant over a long period of time.

Possibly the best example of static equilibrium in the environmental system in which we ourselves have to survive is the oxygen content of the atmosphere.

Around 4 billion years ago there was very little oxygen in the atmosphere. Why? Our planet was void of life. Then life appeared and importantly photosynthesizing life, first cyanobacteria (bacteria with chlorophyll) and later plants. Both of which produce molecular oxygen a a waste product.

As the oxygen levels rose so a new type of organism appeared that could use the external oxygen in respiration - animals - and so the Oxygen cycle was born. Eventually over time a balance was achieved in the level of atmospheric oxygen and for the last 2 billion years, plants and animals have held the oxygen level stable at 21% of the atmosphere.

Snowshoe hare LynxSteady State equilibria: this is a much harder concept to define and there are still arguments for what a dynamic equilibrium really is. The best way to think about it is that a system is in a steady state because the inputs and outputs that affect it approximately balance over a long period of time.

An example of this can be seen in a classic study of the populations of Snowshoe Hares and Lynx in Canada. As the population of the Lynx rises the Hare population falls this is then followed by a fall in the Lynx population which in itself is followed by a rise in the Hare population etc. etc.

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