Modelling as a Discipline

Emeritus Research Scientist

Bedford Institute of Oceanography

Dartmouth, N. S., Canada B2Y 4A2

E-mail: bill@silvert.org

William Silvert

Two Major Points

• Modelling is a universal activity. We are all modellers, not just those who are participants in this particular workshop.

Consider this lovely young child, with his nice red teddy bear. What does he know about models?

Let’s see how he develops his modelling ...

He has some other nice things that are bright red, so that seems to be a good feature to use in predicting what will be fun to play with.

But some objects that are a nice pretty shade of red are not quite as nice to the touch,

How we Start to Develop Our Modelling Skills

And some are downright dangerous!

Model Testing and Validation

• So the original model, that red objects make good toys, works for teddy bears and wagons, but not for pots, kettles, and matches.

• Which is probably a reasonably good model for a child to use ...

Hey, who just came in to say good night? It’s my mother -- in a RED COAT!

And she is going to kiss me with those RED LIPS!

Good Night!

Modelling is Like Breathing• We all breath,

and we all model.

• Some people breath better than others because they have learned special skills.

Model Mimicry

Two Major Points

• Modelling is science, not mathematics. It deals with reality. Good mathematics cannot save a model based on bad science.

A Little Math can be Dangerous!

• Is this a linear relationship?

• If you test it for the significance of linear regression, it seems to be!

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We Need Powerful Computers!

There are some researchers who are convinced that it has been the hardware limitations [of computers] that have obstructed progress and that advances in modelling are now possible because of larger computer capacity. There is no basis for this belief; bigger computers simply permit bigger mistakes.

D. H. Lee (1973)

Uptake Kinetics

Many important processes in ecology can be modelled with the uptake-clearance equation

dC/dt = aX – bC

This is one of the most universal and reliable equations we have.

Aliasing

Fish Catches, China & Others

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Landings of Trachurus japonicus in millions of tonnes

Modelling Food Chains

Phytoplank ton

Zooplank ton

F ish Linear Model dZ/dt = aP - bZ

Lotka-Volterra Model dZ/dt = aPZ -bFZ

Two Major Points

• Modelling is a universal activity. We are all modellers, not just those who are participants in this particular workshop.

• Modelling is science, not mathematics. It deals with reality. Good mathematics cannot save a model based on bad science.