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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 Hybrid

[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 model of a simple ecosystem of two species of trees that compete for space in a forest canopy. Tabonuco Yagrumo Hybrid is different from Tabonuco Yagrumo in that the competition for space is explicitly modeled using turtles and patches, as opposed to being modeled as a relationship between stocks in the System Dynamics Modeler. It is intended to be a simple example of how the two approaches can be combined into a hybrid model, and its behavior is similar to the non-hybrid version.

## HOW IT WORKS

The TABONUCO and YAGRUMO stocks represent the population of the two tree species. The rate at which the stocks of both trees grow or die is represented by the TABONUCO-GROWS and YAGRUMO-GROWS flows. These values come from the agent-based part of the model, where the trees compete for open pace in the forest. The forest is modeled using turtles and patches. Each tree is a turtle and lives on a patch. It can only grow into an open patch immediately next to it. 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. In each tick of the model, the two tree species attempt to grow into the available spaces. However, tabonuco trees outlive yagrumo trees. This relationship is represented by the TABONUCO-OUTGROWS-YAGRUMO flow from the YAGRUMO stock to the TABONUCO stock. To represent this in the agent-based portion of the model, yagrumo trees are turned into tabonuco trees in the forest.

The YAGRUMO-DIES and TABONUCO-DIES flows are connected to the DISTURBANCE variable. This variable represents the destruction of trees by hurricanes. The HURRICANE-STRENGTH and HURRICANE-FREQUENCY variables determine the timing and amount of disturbance, 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-FREQUENCY 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.

The results of the model is different from Tabonuco Yagrumo in several ways. First, the curves are not smooth, because they are not generated only by a function, but have have random events generating the results. Second, the modeling of Tabonuco outgrowing Yagrumo in this model causes the Yagrumo to grow less, because the available spaces for it to grow into are reduced.

## THINGS TO TRY

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

Use the BehaviorSpace Tool to sweep the parameter space for the USER-HURRICANE-FREQUENCY and USER-HURRICANE-STRENGTH.

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

## EXTENDING THE MODEL

Experiment with trying different rules for competition for space.

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

## NETLOGO FEATURES

This model provides a simple example of combining agent-based and system dynamics modeling.

## RELATED MODELS

Tabonuco Yagrumo

## CREDITS AND REFERENCES

The following 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 in a publication, we ask that you include these citations for the model itself and for the NetLogo software:

* Wilensky, U. (2006). NetLogo Tabonuco Yagrumo Hybrid model. http://ccl.northwestern.edu/netlogo/models/TabonucoYagrumoHybrid. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.
* Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

## COPYRIGHT AND LICENSE

Copyright 2006 Uri Wilensky.

![CC BY-NC-SA 3.0](http://i.creativecommons.org/l/by-nc-sa/3.0/88x31.png)

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit http://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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