| I. Introduction | VI. Meeting the NSF Standards |
| II. Objectives | VII. Remarks |
| III. Methods | VIII. Aknowledgements |
| IV. The Models | IX. Bibliography |
| V. Modeling in the Classroom | X. About the Author |
I.
Introduction back
to top
This project focuses on the process of building
two new STELLA
models one to study forest recovery rates, the other to study global carbon
cycles and associated environmental issues. The models presented
are examples of models that could be developed in a high school classroom
studying environmental issues. They are not finished products, but works-in-progress
requiring further definition and testing. The central idea is that model
building is itself the more valuable teaching tool rather than using a
finished model.
The systems approach and the attempt to build
formal models encourage disciplined analysis and theory development.
Modeling forces us to dissect a system into its smallest details and to
evaluate the significance of components. Modeling helps us find patterns
in seemingly random events and make predictions for outcomes. A well-conceived
model must behave plausibly under extreme conditions and should be tested
against what we can observe in the real world. This leads to asking questions
about what the limiting conditions are to any particular trend or behavior.
II.
Objectives back
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My long-term goal is to coach my students in
building their own STELLA models about environmental problems. I have been
inspired by the degree of inquiry and critical thinking that are necessary
for the construction of a good model. I hope to inspire my students to
question current scientific beliefs and even bad modeling with their own
reasoning and research. Sound modeling should help them develop the self-confidence
that is needed to create and find original solutions to old problems.
III.
Methods back
to top
A. Brainstorming
The modeling process began with several brainstorming
sessions to define a specific problem, select important variables and suggest
theories for system structure. The critical thinking process required at
the conceptualization stage is the most demanding and the most revealing
for the modeler, especially as a novice. Some of the many questions raised
for me during brainstorming for these models are as follows:
On tropical reforestation:
On the global carbon cycle:
IV. The Models
back
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a) A STELLA model
of Tropical Forest Regeneration After Deforestation
Guiding question:
What will happen to a small patch of tropical rainforest after selective
logging and burn?
The reforestation model deals with the forces
controlling re-frowth of a small area of tropical forest (1km2)
to climax vegetation after selective logging and burning.
The model reflects patterns of temperature and rainfall and their impact
on plant growth, litter decay and ecological succession. Early pioneer
plants do reestablish under bad conditions and eventually lead to climax
growth. Over a few decades biomass recovers to climax levels. Light and
genetics turn out to be the limiting factors to plant growth, not
nutrient soil depletion, due to intensive recycling in even semi-mature
forests. The model does not include the potential
effects of increased concentration of atmospheric carbon dioxide on photosynthesis
and growth rates.
Preliminary results:
Re-growth is predicted under conditions of temperature
at 250C and 300 mm rain per year. Decay time for organic material
is defined at 1 year. Plant lifetime is defined as a function of biomass:
the larger the plant biomass, the longer the lifetime. Nutrient availability
is defined as a ratio between decomposition and unlimited growth. The limiting
factors on unconstrained growth in this ecosystem are light and genetics.
Plants need light to grow and are genetically programmed to reach only
a certain height. Initial amounts of plant biomass and forest litter are
both set at 1 ton per km2
b) A STELLA
Model of the Global Carbon Cycle
Guiding question:
What happens to CO2 after it is dumped into the atmosphere?
The model of the global carbon cycle deals with
the forces that act on carbon as it circulates between reservoirs.
The reservoirs of carbon are the earth, the biosphere, the atmosphere and
the ocean. The model should show how what happens to the carbon dioxide
temissions in the atmospher from fossil fuel burning and deforestation.
Carbon dioxide emissions are never localized because they are gases
and immediately diffuse into the atmosphere. The interaction of carbon
with the ocean surface is critical to understanding the carbon cycle because
the ocean covers approximately 75% of the earth's surface. Research indicates
that of the 5 x 1015 g of C yr-1 released in 1985
by burning of fossil fuels, only 58% remains as what is called the "airborne
fraction". No one knows for certain where the remaining 42% are stored.
Oceanographers hypothesize that this missing fraction of CO2
is taken in by the oceans, used by aquatic producers for photosynthesis
and buffered by the precipitation of marine carbonates.
The key to the "mystery of the missing carbon
sink" is linked to the emission and absorption rates of carbon dioxide
at various points in the cycle. Remote sensing and GIS data on the vegetative
cover on land and in the ocean would be useful to evaluate these processes
further. The model below reflects only an initial attempt to describe the
carbon cycle; further work is needed.
The Global Carbon Cycle: A Model Sketch
V.
Modeling in the classroom.
back
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A. Introduction to STELLA
modeling
After a general introduction to STELLA modeling,
students should start by looking at some of the classic simple models that
are available for study. Models on ozone depletion, predator-prey relationships,
disease epidemics and others should highlight the utility of modeling for
high school students.
B. Are all models good
models?
Students should be aware that modeling is merely
a tool that should be used carefully, with sound thinking as a foundation.
Like statistics and speeches, models can be deceiving. Assumptions and
structures need to be questioned for consistency and reasonableness. I
have adapted from Vanclay a possible checklist to help students evaluate
models as follows:
VI.
Meeting the NSF
Standards: back
to top
Some of the NSF standards that were met
in completing this project were
A. 9- 12 content
standards on:
a) Biological Evolution.
b) The Interdependence of Organisms.
c) Matter, Energy, and Organization in Living
Systems.
d) Bio-geo-chemical Cycles (especially that of
the element carbon).
e) Origin and Evolution of the earth system.
B. 9-12 Science
as Inquiry content standards on :
a) Formulation of testable (or researchable)
hypothesis and demonstration of logical connections.
b) The use of mathematics and technology to improve
investigations and communications.
c) The formulation and revision of scientific
explanations and models using logic and evidence.
d) Recognition and analysis of alternate explanations
and models.
e) Communication and defense of a scientific
argument.
VII.
Remarks back
to top
The process of building models has made me rush
to the library for more information on tropical rain forests, plant physiology,
terrestrial ecology, systems modeling, the bio-geo-chemical cycle of carbon
and even oceanography. I emerged from the libraries with even more questions,
but with a sense that the modeling had given a purpose and direction to
my research. As a result, I have begun a growing list
of books and web sites
for anyone who wishes to pursue these topics further. I plan to continue
working on the models presented here, and others, over the course of the
next year. I hope to be able to introduce my students to aspects of the
modeling process .
Presenting finished models to my students would
probably be less risky for the teacher but less powerful as a teaching
tool. The model building experience presents challenges that bring new
relevance and motivation to learning about environmental problems.
I would greatly appreciate any comments or suggestions on how to continue my modeling work and specially on how to transfer it to a high school classroom. Please e-mail me at
fduarte@lovett.org
or click on the envelope for a direct link. 
VIII.
Acknowledgments back
to top
A special thanks Dr.
Nathan Forrester for his assistance in in my modeling efforts.
A special thanks to Ms. Louise Shaffer and Mr.
David Goodman at the Biology
Library at Guyot Hall, Princeton University for their assistance in
my research.
A special thanks to Mary
Sherrill Forrester for her editorial assistance.
IX.Bibliography
back
to top
Odum, Eugene P.1971. Fundamentals of Ecology.
3rd ed.W.B. Saunders Company
Odum, Howard T. 1994. Ecological and General Systems: An Introduction to Systems Ecology. Newot, Colorado: University Press of Colorado
Edwards, Gerry & Walker, David. 1983. C3, C4: Mechanisms and Cellular and Environmental Regulations of Photosynthesis. Berkeley; University of California Press
Koch, George W. & Mooney, Harold A. 1996. edited by, Carbon Dioxide and Terrestrial Ecosystems. Academic Press
Vanclay, Jerome K. 1994. Modeling Forest Growth and Yield Applications to Mixed Tropical Forests. CAB International
Yunus, Mohammad & Iqbal, Mohammad. 1996. edited by, Plant Responses to Air Pollution. John Wiley & Sons
Nepstad, Daniel C. 1989. Forest Regrowth in Abandon Pastures of Eastern Amazonia: Limitations to Tree Seedling Survival and Growth; A Dissertation Presented to the Faculty of the Graduate School of Yale University in Candidacy for the Degree of Doctor of Philosophy. UMI - Dissertation Services
Edwards, DEW., Booth, WE. and Coy, S.C. 1996. edited by, Tropical Rain forest Research - Current Issues, Kluwer Academic Publishers
Hurtt, George C. 1997. Ocean Ecosystem Models for Use in Studies of the Air-Sea Balance of Carbon Dioxide; A Dissertation Presented to the Faculty of Princeton University in Candidacy for the Degree of Doctor of Philosophy; Princeton University
Schlesinger, William H. 1991. Biogeochemistry An Analysis of Global Change. Academic Press
Butcher, Samuel S., Charlson Robert J., Orians
Gordon H. & Wolfe, Gordon V. 1992. edited by, Global Biogeochemical
Cycles, Academic Press
X.
About the Author back
to top
Florence Sherrill Duarte
High School Biology Teacher
The Lovett School in Atlanta
GA
4075 Paces Ferry Road NW
Atlanta GA 30327
www.lovett.org
fduarte@lovett.org