Re: ABM/I(C?)BM and mathematical nonsense

[gross]

Hi folks,
   I've been reluctant to jump into the recent discussions, and I'm not
certain I've read all of them, but thought I might at least now state
a couple of opinions. These are based on my experiences with individual-
based models over the last 10 years or so.

1. There are different purposes for modeling, and different approaches 
appropriate based upon these purposes. Although I have long felt, as
an educator, that there is vastly too much emphasis in our current
curricula on analytic methods (e.g. ODE, PDE) relative to computational
approaches (e.g. rule-based approaches, stochastic simulation), analytic
methods allow us to address issues in a different way. The real difficulty,
and one that I have the hardest time with in both the formal modeling 
courses I teach as well as in mentoring graduate students, is deciding
which approach is most appropriate for the problems you wish to address.
A group of us working on models for Everglades restoration (the ATLSS
project - home page at 
http://www.tiem.utk.edu/~gross/atlss_www/atlss_frame.html)
have dealt with these issues for at least one major environmental project.
We have concluded that multiple approaches are required (a multimodel, in the
sense that Paul Fishwick has defined it), based on the varying spatial 
and temporal resolutions and associated organismal detail needed to answer
the questions of interest in restoration. So we mix ODE models, structured
matrix population models, and individual-based models. 

2. There are indeed very competent folks who disagree with my belief that
individual-based approaches are the appropriate method to address many 
problems in ecology arising from practical concerns. For one opinion,
see Levin et al. (1997) Science 275, 334. The authors of that paper and I 
disagree on several points, as in particular I cannot concur with their
characterization that individual-based models as producing "cartoons that 
may look like nature but represent no real systems". These differences of
opinion are based upon different views of what we can hope to attain by
modeling, as well as more philosophical considerations regarding what
can arise from reductionist approaches.

3. On nomenclature, an ecologist knows what an individual is (OK - so 
clonal organisms produce problems here, but that's another topic), and
therefore intuitively understands what individual-based approaches
mean with very little explanation. I have no desire to try to explain
what an "agent" is - I will continue to use the term individual-based 
models when dealing with ecologists. Names are important. Hal and 
Meredith's definition of the i- and p-state and configuration models 
is highly useful, but these have not really entered the ecology 
literature - people find them too confusing. I have given up trying to 
explain them to audiences. 

Cheers,
      Lou Gross
Professor of Ecology and Evolutionary Biology
    and Mathematics
The Institute for Environmental Modeling
University of Tennessee - Knoxville
address@hidden
http://www.tiem.utk.edu/~gross/
http://archives.math.utk.edu/mathbio/ (Math Archives for Life Sciences)



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