What is the difference between a 'complicated' and a 'complex' system?

Discussing the difference between 'complicated' and 'complex' may seem rather pedantic to many, but it is an essential differentiation in systems management.

Complicated system - A system with many interacting components where the interrelationship between those components is, within 'normal' conditions, predictable.  A Boeing 747 would be considered a 'complicated system'.

Complex system - A system with many interacting components where the interrelationship between those components is, even within 'normal' conditions, unpredictable.  An ecosystem would be considered a 'complex' system.

Problems also fall into the categories of 'complicated' and 'complex' for the same reason.  A more classical English term for a 'complex problem' is a 'wicked problem'.

Human IQ has often been described as a measurement of an individual's ability to manage complexity.  It is useful to consider this alternative; 'Human IQ is a measurement of an individual's intellectual ability to turn a 'complex' problem or system into a 'complicated' problem or system.  What is viewed as a 'complex' or unpredictable system to someone with an average IQ may be a 'complicated' and predictable system to someone with genius level IQ.

This Rolls Royce Trent turbofan engine is a 'complicated' system.  The interrelationship between it's multitude of components is robustly modeled and very predictable under normal conditions.
This is a picture of 'Pride Rock' at Borana Ranch in northern Kenya.  Borana and their sister conservancy Lewa Downs, are Black Rhino sanctuaries. With a combined 94,000 acres, they cover a 'complex' ecosystem in which changes in one animal population, geographical area or plant species have large and often unpredictable knock-on effects on all the rest.
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This is also a 'complex' ecosystem, part of the 'north' pasture herd at MFL Bison Ranch Ltd.  With a billion or more microorganisms in every cup of soil, fed by plant root exudates, dung, urine and trace sediment minerals, etc., a change in grazing patterns of the bison herd can have substantial and often surprising effects on the overall system.  A simple dose of certain types of de-wormer to the animals can render the dung toxic to dung beetles that bury the dung deep in the topsoil where it can be accessed and processed by other organisms.  If the dung beetles die off, the efficiency of the four key ecological processes is reduced, with unpredictable consequences.