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Building a More Intricate Web

A Reexamination of Trophic Levels


Sarah A. Orlofske
Biology Department
University of Wisconsin–Stevens Point


This case study follows two beginning undergraduate researchers on a quest for a summer research project related to food web ecology. While writing their research proposal, the students receive feedback indicating that they have neglected an entire group of organisms from their food web—parasites, which leads them to a scientific research paper discussing how these organisms have been overlooked by scientists studying food webs, the challenges involved with including parasites in food webs, and the contributions parasites ultimately make to food webs. An integrated activity in which students visualize the food web using images of organisms and answer questions about species interactions provides opportunities to examine key concepts such as omnivory, ontogenetic diet changes, trophic levels, complex life cycles, and taxonomic aggregation. This case study was originally written for an undergraduate general ecology course, but could easily be adapted to undergraduate general biology courses covering community ecology or specific courses in invertebrate zoology, parasitology, or disease ecology. The teaching notes also discuss how the included code for R statistical computing software can be used to extend the case study in a more quantitative direction if desired.


  • Analyze the types of relationships represented by food webs, including predator-prey interactions as well as those previously overlooked such as parasitism.
  • Explore various research-based methodologies for examining species interactions and incorporating them into food web construction.
  • Apply typical food web terminology (primary producer, primary consumer, secondary consumer, tertiary consumer, carnivore, herbivore, omnivore) to a partial food web based on empirical scientific research.
  • Examine the role of complex life cycles, omnivory, and cannibalism in food webs.


Food web; community ecology; parasite; aquatic ecology; species interactions; network models

Topical Areas


Educational Level

Undergraduate lower division, Undergraduate upper division



Type / Methods




Subject Headings

Biology (General)  |   Ecology  |   Limnology  |   Zoology  |  

Date Posted


Teaching Notes

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Teaching notes are intended to help teachers select and adopt a case. They typically include a summary of the case, teaching objectives, information about the intended audience, details about how the case may be taught, and a list of references and resources.

Supplemental Materials

This case includes an optional extension for R statistical computing software that makes use of the following files (see teaching notes for further details).


  Free_Community Zip file
  Total_Community Zip file
  Freeliving.csv Spreadsheet file
  Parasite.csv Spreadsheet file
  MissingTrophicLevel_V3.R Script file

Answer Key

Answer keys for the cases in our collection are password-protected and access to them is limited to paid subscribed instructors. To become a paid subscriber, begin the process by registering.


Brigette Brinton
Georgia Southern University
Savannah, Georgia
I've used this case study for two years, and love it. I give it to my parasitology class early in the semester, and it's shocking how much it's an eye opener for them. They generally still need a lot of guidance on analyzing the food web - the table headings can be interpreted many different ways. Regardless of the numbers they get, the message usually comes through loud and clear - Remember the Parasites!