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NetLogo Models Library:
Sample Models/System Dynamics/Unverified

Note: This model is unverified. It has not yet been tested and polished as thoroughly as our other models.

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Tabonuco Yagrumo

[screen shot]

If you download the NetLogo application, this model is included. (You can also run this model in your browser, but we don't recommend it; details here.)

WHAT IS IT?

This is a system dynamics model of a simple ecosystem. Two species of trees -- tabonuco and yagrumo -- compete for space in a forest canopy. This model illustrates the role of hurricane destruction in this ecosystem, as well as the resultant nitrogen and carbon produced by the ecosystem.

HOW IT WORKS

The relations between the tree species and the amount of space into which they can grow are defined in the System Dynamics Modeler. The TABONUCO and YAGRUMO stocks represent the population of the two tree species. They can only grow in the gaps of the forest canopy. These spaces are represented by the GAPS stock. The rate at which the trees grow or die is represented by the TABONUCO-GROWS and YAGRUMO-GROWS flows. The flows connect to the GAPS stock to the tree stocks. If more trees grow, there are fewer gaps in the forest. If trees die then there are more gaps.

Tabonuco trees grow slower than yagrumo trees, these rates are represented by the TABONUCO-GROWTH-RATE and YAGRUMO-GROWTH-RATE variables. However, tabonuco trees outlive yagrumo trees. This relationship is represented by the TABONUCO-OUTGROWS-YAGRUMO flow from the YAGRUMO stock to the TABONUCO stock.

The YAGRUMO-GROWS and TABONUCO-GROWS flows are also connected to the DISTURBANCE variable. DISTURBANCE represents the destruction of trees by hurricanes. The HURRICANE-STRENGTH and HURRICANE-INTERVAL variables determine amount of disturbance and the number of ticks between disturbances, which can be controlled by sliders.

The model also tracks the amount of carbon and nitrogen produced by the trees. The trees produce resources via photosynthesis. The rest of the living things in the forest use these resources. The CARBON and NITROGEN variables calculate how much of those chemicals are produced by the trees.

HOW TO USE IT

Press the SETUP button to initialize the model.

To run the model continuously, press GO.

To run the model for one time step, press STEP.

Alternatively, you can run only 250 steps by pressing the SETUP REPEAT 250 [ GO ] button.

Adjust the USER-HURRICANE-INTERVAL slider to determine how many time units occur between hurricanes.

Adjust the USER-HURRICANE-STRENGTH slider to determine how many trees are destroyed by the hurricanes

THINGS TO NOTICE

Hurricanes play an important role in the ecosystem. The relative amount of each tree is highly sensitive to both the rate and strength of hurricanes. Both species co-exist only within a certain range of hurricane frequency and strength.

THINGS TO TRY

Change the USER-HURRICANE-INTERVAL slider, observe what happens to the YAGRUMO population, and the productivity. Do the same with USER-HURRICANE-STRENGTH.

Use BehaviorSpace (on the Tools menu) to sweep the parameter space for the USER-HURRICANE-INTERVAL and USER-HURRICANE-STRENGTH. An experiment that does this is included.

Use the GLOBALS monitor to observe the value off stocks, constants, model time, and model dt.

EXTENDING THE MODEL

Add a third species of tree to the model.

Model the competition for the shared GAP stock in a different manner.

Vary the strength of the hurricanes, randomly, across an interval.

NETLOGO FEATURES

This model uses the System Dynamics Modeler. This model also shows how to simulate discrete events (like hurricanes) in system dynamics.

It includes a built-in BehaviorSpace experiment.

RELATED MODELS

Tabonuco Yagrumo Hybrid

CREDITS AND REFERENCES

This model is based on the work of The Learning Partnership in the Journey to El Yunque project (PI's include Steven McGee, Jess Zimmerman, and Steven Croft). To view the original materials or to learn more about that project, visit http://elyunque.net/journey.html.

HOW TO CITE

If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.

For the model itself:

Please cite the NetLogo software as:

COPYRIGHT AND LICENSE

Copyright 2006 Uri Wilensky.

CC BY-NC-SA 3.0

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.

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