Teach the Earth > Environmental Geology > Teaching Activities > The Non-linearity of Environmental Change: A coral reef model

The Non-linearity of Environmental Change: A coral reef model

Dennis Hubbard, Oberlin College
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This activity was selected for the On the Cutting Edge Exemplary Teaching Collection

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This page first made public: May 16, 2012


This is an exercise that is used in an undergraduate, non-major course titled "Coral Reefs: Biology, Geology & Policy".

The course uses this popular environment as a proxy for environmental decline in general and has two broad goals beyond the course content:
1) to encourage science majors to think about complex environmental problems outside the context of their individual major, and
2) to help non-majors understand the scientific thought process in the context of their own personal interests and opinions.

The computer model described here was built to provide a user-friendly interface that is visually stimulating but non-"threatening" to math-phobic students. It runs on FREE software that can be run on any computer. It can be run and modified by an instructor or student with no modeling skills.

For this exercise, it demonstrates how losses of grazing fish and/or the addition of nutrients to the reef system will reduce the relative abundance of corals and algae on the reef - leading to eventual decline. The main lessons for the students are:

1) If you increase a particular stress, there is often little or no change until suddenly the system rapidly declines.
2) If multiple stresses are added, the pattern is more complicated but basically the same.
3) Once the system collapses, simply returning to the "safe" side of the collapse threshold has no result.

The Big Picture: All of this is referred to as "non-linearity" and demonstrates that on the reef (as in most natural systems), it's a LOT easier (and cheaper) to not "break it" in the first place than it is to "fit it" once it crashes.

Coral Reefs
Computer Model
Environmental change
Reef Decline
Anthropogenic Stress



This exercise was set up for an introductory course with a mix of science and non-science majors with an interest in the environment. The materials provided below should allow a geology teacher to set the stage for their students without a broad background in either the biology or geology of coral reefs. Background materials include:
1) a Powerpoint describing the balance between corals, grazing fish, nutrients and algae on a "healthy" reef.
2) a detailed set of instructions for running the model that can be used by both the instructor and the students
3) a detailed set of scenarios and questions (including answers - remember to remove them before giving them to your students) that helps the teacher understand what the model is showing. Once you go through the provided exercise, you should be ready to decide whether to run it as is or to modifiy it for your needs. To do the latter, you need no programming skills.

Skills and concepts that students must have mastered

The only requirements are 1) a reasonable ability to read graphs and 2) a willingness to do multiple runs (the "reef" does not behave identically given the same start-up conditions; this is one of the "lessons".). and draw inferences from the output. What is NOT REQUIRED is programming or modeling skill.

How the activity is situated in the course

I use this in the third part of my course where we discuss reef decline and the relationships between science and policy. It has been set up here so it can be run with a short introduction (included) followed by running the model and discussing the results. I do it in two 75-minute lecture periods with a homework exercise in between (my course has no lab). However, it could be easily completed in a 3-hour lab. In any format, the main components are:

1) a short background lecture (Powerpoint provided)
2) instructions and hands-on demo of the model
3) homework assignment or lab exercise to use the model
4) discussion of model outcomes


Content/concepts goals for this activity

The underlying goal is to demonstrate fairly complex and important environmental principles in a way that is inviting and non-threatening to students who self declare as "non-science types". The focus is on understanding how natural systems work and how this information can be used to inform important environmental policy decisions using cross-disciplinary principles. At a more basic level, it helps science majors to think at a larger scale while helping non-majors understand how observation and inference are situated in "thinking scientifically". The outcome provides an ideal opportunity to integrate science and policy.

Higher order thinking skills goals for this activity

At the most basic level, students will learn to 1) run and manipulate the model, 2) use graphic output to generate data from multiple runs and then 3) use those data to understand specific changes in coral reefs in response to the most common anthropogenic stresses operating at local or regional scales. At a higher level, the exercise fosters "deductive reasoning" based on observations - the heart of the scientific thinking. Hopefully, students will realize that "modeling" need not involve complex mathematical equations and that they effectively model every time the see something and come to a conclusion. The focus is to get them thinking about what constitutes a "good" model.

Other skills goals for this activity

You could ask each student to prepare an "op ed" piece to relate the outcome to the general public. In this scenario, non majors will need to make sound policy and economic arguments based on the actual model data. Conversely, science majors will have to translate their observations into policy in a way that will make sense to the public.

Description and Teaching Materials

All materials on the SERC site can be downloaded and used without limitation for non-commercial use as long as the author is properly attributed.
  • The NetLogo model can be downloaded for free and works on any computer. It can be downloaded from: NetLogo. If the link is broken, simply Google "NetLogo" and go to their download page.
  • The CoralReefs model can be downloaded from: Coral Reef Exercise by Denny Hubbard
Accompanying teaching materials can be directly downloaded from the following list:

Teaching Notes and Tips

Teaching notes and a background lecture are provided in the materials on the SERC site. A detailed manual that describes specifically how to set up and run the model can also be downloaded below.


Handout 2 includes a model-specific set of questions that lead students through the logic process. The instructor can use this to assess a student's level of comprehension. It can also be used to help the instructor master the concepts and the model before trying it in class. I would estimate that you can download the materials, run the model and be ready to either go with the exercise as is or modify it in 4-6 hours.

We created a detailed assessment that examined both what and where students learned in this exercise. While the model reinforced the basic principles, much of that information was effectively transmitted in the lecture. However, the depth of thinking about the natural system and the appreciation of a) the non-linearity, and b) the process of scientific thinking are learned came primarily from the hands-on exercise and the following discussion. A copy of this survey can be requested through the Geology Department at Oberlin College.

References and Resources

The following links were chosen because they lead to sites that are likely to be maintained for at least the next decade. However, simply searching for "reefs", "NOAA" or "NASA reefs" will get you to a variety of great sites that you can use to gather useful background for class.
Introduction to Ocean Sciences by Douglas Segar