ABM/I(C?)BM

[Doug Donalson;]

Ah, you have all jumped into the topic closest
to my heart (well maby not the closest, but close).

I am working on IBM's in ecology and am going through the wars
of justification of a new technique.

1.)  To Chris (ABM/IBM) You can call a gas station attendent
a petroleum transfer engineer but he still smells like benzene! ;-)
(Sorry Chris, I couldn't resist.)  By the way, IBM in ecology
doesn't just imply individual agents.  It can also refer to differential
equation based biomass change models where detailed physiology is
included.  See DeAngelis and Gross Individual Based Models.


WARNING: PERSONAL OPINION NOW RUNS AMUCK!!!!

2.)  To understand the attitude of mainstream theoretical ecology
you have to start with it's emergence in the early '70s.  Many of the
builders were transfers from physics and were hoping to bring
the idea of a nice clean set of paradigms (sorry for all the mis-spells)
to ecology.  There is a hugh litrature of stability boundries and
equlibrium boundries for large groups of differential equation based models.

Those who have put in so much hard work probably don't take kindly to
being told that the assumptions behind their models are broken by almost
every real ecological system!  (The truth does hurt doesn't it?)

The major arguments against complex models is that including
parameters that are not precisly known can lead to large error
propigation and misleading results.  In particular, movement
behavior is considered especially suspect.  Unfortunatly for the detractors
of complex models (see how easily I got out of the ABM/IBM discussion!)
is that their argument points a lot of fingers at the simple models
as well.  If a model is highly sensitive to a single parameter
or a group of parameters then what does that say about the results
of a model that solves this problem by ignoring them!  In a
discussion with Alan Hastings (UC Davis) he stated that we should probably
not yet study more than two or three species interactions because we
(after 20+ years?) don't yet know enough about them.  Maby this is because
there is no such thing as a two or three species interaction in the real 
world.

My work to date (pre-swarm) with a model I call the Heuristic Acynchronous
Discrete Event Simulation or HADES, has shown that the assumptions of
simple, differential equation based models can compleatly change
the outcomes of a simulation.  We used the original Lotka-Volterra
predator-prey models as a base line and then successivly
removed two of the main assumptions, that demographic stochasticity
can be ignored and that the populations can be modeled as "well-mixed"
at all times.  The results showed that the equlibrium resullts
predicted by the differential equation based model held fairly true
over a large range of system sizes but that the time to extinction
varied greatly with space playing both a stabilizing and destabilizing
role at different system sizes.

Bottom line.  There is a lot of work that needs to go into understanding
this new realm of complex models.  One of the key areas is that of
error propigation due to a combination of multiple parameters and
inaccuracies in their measurements.  Right or wrong, we have to prove
the validity of the models to the main stream theoretical ecological
community.  Be of strong heart, remember, the young doctor who
first proposed sterile surgical techniques was just about driven
out of the profession by the greatest medical minds of his time!

Doug Donalson

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* Doug Donalson                          * Office: (805) 893-2962         *
* Ecology, Evolution, and Marine Biology * Home:   (805) 961-4447         *
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*                                                                         *
*   The most exciting phrase to hear in science, the one that             *
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*   "That's funny ...?"                                                   *
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*       Isaac Asimov                                                      *
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