Atlantic Forest

Exercise 7: Food Web Modeling
Module 7: Consumers

Your Question

1. How does the abundance and diversity of species that are present affect the stability of the ecosystem? We will test this by introducing a systems modeling software called Stella.

Background

Those who support the view that more diverse ecosystems are also more stable point to the commonly accepted perspective that the tropical forests are more stable because they have very few introduced species that take over entire forests. In contrast, simple ecosystems with smaller food webs like the desert and northern forests have many more introduced pest species that displace native species. These scientists point out that tropical forests tend to recover more quickly from natural disturbances such as hurricanes and land clearings than do less diverse habitats. A cleared tract of northern tiaga coniferous forests takes a very long time to fully regenerate. This viewpoint is more strongly favored by most scientists and conservation biologists.

Those who dissent from the above perspective argue that tropical forests may have fewer invasive species and may recover quicker due to other forces. These forests may be more stable than less diverse sites only because they have a rapid growth rate. This, they argue, better explains the data than that they are more diverse. These scientists either suggest that the idea has not been adequately tested, that more simple ecosystems may be just as stable as more complex ecosystems, or that other forces (such as climate or soil type) better explain these data. It may be that we conflate several independent factors when we try to create a facile connection between biodiversity and stability.

Given that the Atlantic Forest ecosystem around IPÊ is such an imperiled and fragile ecosystem, this is a very important question. This problem will only become worse in the near future. The southern coast of Brazil in particular, has one of the most rapidly growing human populations as well as one of the most active home-construction industries in South America. As populations swell, cities sprawl and intact habitats become disturbed more strongly and with greater regularity.

How will the Atlantic Forest respond?

Your Assignment

1. Let's first become familiar with how Stella works, what a finished modeling project looks like, and how to construct one from scratch using the Getting Started with STELLA 6.0 Tutorial. 
1. Stella is a systems modeling software package from High Performance Systems, Inc. that we have installed on the network.
2. After you have started up Stella6, open the file entitled "deerA.stm" in the "Stella6 > Getting Started with STELLA 6.0" folder. 
3. Then click through the parts of the model that you see and can interact with. Essentially it is laid out and functions like an interactive web page. Quickly work through the file, without really trying to understand the content. Your goal at this point is to become familiar with what one portion of the program feels like. It is a very large program and will take you many uses to become familiar with all the excellent functions that are available. 
4. Now, let's open and complete the tutorial for the program. If you do not already have a print out of the "Getting Started with STELLA 6.0" PDF tutorial, switch out of Stella to the desktop. Now open the PDF entitled "Getting Started with STELLA 6.0" in the "Stella6 > Getting Started with STELLA 6.0" folder. 
5. If you would prefer to work through a printed version, use one that is available.
6. Read and work through the 43 pages of the tutorial. This should take about 3 hours to complete. 
7. Then we break for lunch. 

 
2. After lunch, we will create an editable food web model using the skills that we acquired by working through the Stella Tutorial. These models will allow us to try to answer the above research question. To start this, confer with your lab mate(s) to achieve the following tasks: 
1. Based on your experience thus far in the forests surrounding the IPÊ campus, write down some subset of all the organisms that you know that live nearby. 
2. It may be preferable (and easier) if you include only the plant species that provide the greatest quantity of biomass such as shrubs and trees. 
3. Be certain to include species from each of the main ecological functional groups - producers, primary consumers (herbivores), secondary and higher consumers (predators and parasites), and decomposers. 
4. Next, connect all the species to each other - consumers to consumed, and don't forget about how different species of decomposers relate to all other species when they die. 
5. Include slider bars for modifying the growth rates or consumption rates of each key species in your model. 
6. Experiment with modifying the model, keep track of the amount of change that you induce (how much you augment population growth of the herbivore, etc.). Graph out the population size or growth rates of four key species (one at each level of the food web). Print out graphs with three sets of value alterations. Look for species extinction, the time to extinction, or for any near extinctions from the local ecosystem. 
7. Next, remove several of the other species from the model, and run the models repeatedly by modifying the slider bars. In the process, you are simplifying the local ecosystem. Print out graphs with the same three sets of value alterations as you did with the full ecosystem. 
8. Compare the results of the full community with those of one of the simplified communities you created. Which is more prone to have species go extinct? Why? Were certain trophic levels more prone to extinction than others? 
9. Make some generalizations concerning the relationship between local diversity and ecosystem stability, based on your models.

General Comments

1. Be able to orally summarize your experimental results and conclusions to each other and the instructors. 
2. Participate in the group discussion of all the models created today. Based on your analyses of your models, do you agree or have additional insights with the conclusions that others have drawn from their models?

Objectives

1. Grasp of the influence that species diversity has on stability relative to natural and unnatural disturbances.
2. Understanding that a consensus on these issues does not exist.
3. Better knowledge of basic ecology of the local flora and fauna.
4. Ability to predict the differential impact of disturbances of intensities on forested ecosystems.
5. Understanding of the multifarious ways in which species can interact.

Key Skills

1. Introduction to Stella systems software.
2. Ability to come up with novel applications of preexisting models.

Timetable

1. Total elapsed time to perform the experiment : One day 
1. Morning
1. Learn how to use Stella by working through the tutorial (3 hrs)
2. Afternoon
1. Create and then modify model (1.5 hrs)
2. Discuss how model changes affected stability and persistence (0.5 hrs)

Procedural Notes

1. This experiment is the first time that we introduce the Stella program, although we will revisit it a few times through the course of the semester. This program is tremendously flexible and thus powerful and is an excellent way to deepen your understanding of a great many fields, including fluid dynamics, economics, ecology, engineering, and many others.
2. The estimate of 5 hours is likely undergenerous. It is possible that the experiment could take as many as 7 hours, if the tutorial or construction of the activity takes longer than anticipated. Be ready for a longer day than scheduled at present.

Materials Needed

1. Computer lab, with Stella installed on all machines
2. Printouts of the tutorial (7)
3. Printers and paper