CASE TEACHING NOTES
This case, suggested by Ommundsen (2000), presents data from Henke and Bryant (1999) on the effect of coyote removal in Texas. It is designed to help students in introductory level biology courses understand trophic level relationships and the role of keystone species. Students are required to interpret data that are presented graphically and to predict how changes at one trophic level may affect populations and communities at other trophic levels.
I use this case in the lecture portion of an introductory biology course for non-majors. This course generally has about 72 students, many of whom have poor math skills and difficulty graphing and interpreting data. They are often quite science phobic.
The use of case studies in this course reinforces basic concepts from lecture and helps the students develop reasoning skills. The students in Fall 2000 reported (on a scale of 1= strongly disagree to 5 = strongly agree) that case studies helped them to understand the scientific process (mean = 4.1, N=39) and to see how material from the course could be applied (mean = 4.3, N = 39). Students feel case studies are a useful addition to the course (mean = 4.3, N = 63 on a scale of 1-5, with 5 being very useful).
I use the coyote removal case study after introducing basic ecological principles. Students rate this case as very useful (mean = 4.4, N=64); it helps to solidify their understanding of ecological principles and the application of those principles to conservation.
I run this case as an in-class discussion that generally takes two 50-minute class periods. The case is presented with PowerPoint®, as described in the "Classroom Management" section below.
Questions 1-3 from Part I -- Students will propose a variety of possible outcomes. As I call on students to answer questions I build a table on the board with "Question," "Treatment," "Control" and "Variable" as columns. This allows us to discuss whether the proposed experiment actually addresses the question the students sought to answer. Student experiments invariably present a good opportunity to emphasize the importance of controls and replication in experiments. I usually begin this by asking, "How will you know that any changes are the result of coyote removal?"
Question 4 from Part II -- Students will again suggest a variety of possible outcomes. It is important to have them explain their reasoning. This is a good opportunity to introduce top down vs. bottom up control in communities and challenge the students to determine what assumptions they have made.
Questions 5 and 6 from Part III -- This graph clearly shows that the mesopredator population responds quickly and positively to the removal of coyotes. The students will assume that predation by coyotes is the major factor controlling mesopredator population size and further that the increase in mesopredators will result in a decline in rodent populations. Some will predict an ultimate decline in mesopredators as food supply dwindles.
Questions 7 and 8 from Part IV -- Coyotes are a major factor in controlling the total size of the rodent community. The students realize that coyotes feed at more than one trophic level. I emphasize this when I initially present the food web, but the students generally assume that this is relatively unimportant. These data may suggest that mesopredators do not eat rodents or that the reduced predation by coyote outweighs the effects of the increased mesopredator population. The students will predict an ultimate decline in the rodent population as food becomes limiting. Someone is likely to predict an increase in competition and, with leading, they may predict a decline in diversity.
Questions 9-13 from Part V -- With the removal of the coyote, the diverse rodent community is reduced to a single dominant species. The coyote is the keystone predator in this system. Removal of the coyote allows the Ord's kangaroo rat to be competitively dominant, driving other rodent species to local extinction. Question 12 should generate a list of variables that contribute to the difficulties associated with field experiments. It will provide an opportunity to stress again the importance of controls. The final question generates a good deal of discussion on the loss of biodiversity and the difficulty of predicting the consequences of environmental manipulation.
I present cases in this course with PowerPoint® using the progressive disclosure method as described by Colyer (2000). The progressive disclosure method, often referred to as the interrupted case method, is also described in detail elsewhere on this website. See the case study, "Mom Always Liked You Best: Examining the Hypothesis of Parental Favoritism."
The slides present details of the case in outline form. I made a simplified food web using the organization chart feature of PowerPoint® to present the trophic relationships in the community. It is useful to have this chart on an overhead that can be left up for the students to refer to throughout the exercise. In the original paper, the authors discuss coyote control, and to avoid confusion they use the term "comparison area." When I scanned the graphs I changed the terminology to coyote removal and control areas. I made these changes because I wanted to discuss the importance of experimental controls without confusing the students. The modified graphs are presented with relevant questions directly on the slide.
I have prepared response forms for the students on which I reprint the questions from the case. The students work in groups of four or five; each group is required to fill out a response form. The students are given approximately 10 to 15 minutes to answer each set of questions; we then discuss them as a group. I make a list on the board of all student responses; it is useful to list responses in table form in order to link related material, as in the first three questions.
Ensuring that all students participate is difficult in a class this size. It is essential that the students realize that all groups will be called on for answers, even those that do not volunteer. I try to start with a different group for each set of questions, then ask others to add new information. I require that the students hand in the response form with all of the team members listed at the end of the case. Students who miss one day of a two-day case, or who arrive substantially late, can earn no more than half credit for the case.
Some instructors may want to include follow-up exercises on predator populations in their local areas. Predator populations in many suburban areas are increasing as is the frequency of human encounters with these animals. Students could investigate local problem predators and their preferred prey species. They could be required to write an essay (or a letter to a legislator) concerning the possible impacts of the increasing predator population and its proposed removal.
The case study of Isle Royale trophic interactions (Fortier 2000) would be a good partner for this case. This pairing would allow students to see that communities are unique in their response to environmental manipulation. These two cases are written at different levels; therefore, one would have to be adjusted depending on the intended audience.
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Acknowledgements: Publication of this case study on the National Center for Case Study Teaching in Science website was made possible with support from The Pew Charitable Trusts.
Image Credit: Photograph provided courtesy of the Clay Hill Memorial Forest web site: http://cygnus.campbellsvil.edu/~chmf/.
Date Posted: 2/25/02 nas
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