Supplementary Material for Chapter 13

Exploring Trophic Cascades in Lake Food Webs with a Spreadsheet Model

This chapter is published as:

Emery KA, Gephart JA, Wilkinson GM, Besterman AF, Pace ML.2016. Exploring trophic cascades in lake food webs with a spreadsheet model. In: Byrne L (ed) Learner-Centered Teaching Activities for Environmental and Sustainability Studies. Springer, New York. DOI 10.1007/978-3-319-28543-6_13

Kyle A. Emery, Jessica A. Gephart, Grace M. Wilkinson, Alice F. Besterman, Michael L. Pace

Department of Environmental Science, University of Virginia, Charlottesville, VA USA

Corresponding Author Email:

This file contains the following supplementary material:

  • A: Instructor Guide…beginning on p. 1

This chapter also has the following supplementary material, available on the chapter’s website:

  • B:Student worksheet
  • C: Glossary
  • D: Food web spreadsheet model (Excel file)
  • E: Introductory presentation slides

Supplementary Material A: Trophic Cascade Simulation Instructor Guide

Instructor Preparation:

  • Download and review the slide presentation (Electronic Supplementary Material (ESM) E) which provides background information, an example scenario for the model, and student instructions for completing the modeling exercise. The presentation can be edited or augmented to meet the instructor’s needs.
  • The instructor should become familiar with the model by exploring the four different scenarios (more information on the model is provided below). For each scenario, enter numbers into the yellow boxes at the low, medium and high end of the possible range (provided in the file to the right of the yellow box) and observe the effects of these changes on the output graphs.
  • Instructors should understand what drives the changes in the output graphs:
  • The first two scenarios represent top-down control. These activities in these two scenarios cause cascading effects down the food chain, changing resource availability for lower consumers and altering the biomass of each lower consumer. It is important to note the alternating effect on subsequent trophic level biomass in the top-down control scenarios.
  • Scenario 1 is the removal of the top predator due to overfishing.
  • Scenario 2 is the addition of a top predator to promote recreational fishing.
  • The last two scenarios represent bottom-up control. These two activities cause effects that reverberate up the food chain by altering resource availability for higher consumers.
  • Scenario 3 is eutrophication due to farm nutrient runoff in the watershed. In this scenario, students should observe the threshold upon which top predator biomass declines (fish kill) as the eutrophication event leads to reduced oxygen in the water.
  • Scenario 4 is removal of phytoplankton due to the introduction of an invasive species such as a zebra mussel.
  • The model being manipulated in the scenarios described above is based on the biomass relationships in Figure 1 of Carpenter et al. (1985), which are modeled from a food web manipulation to induce a trophic cascade in a lake. Note that the equations used to produce the graphs are hidden in a table behind the graphs in the Excel file.
  • The unit of measure for all trophic levels is biomass (e.g. kilograms of piscivores in the lake).
  • As an example, the system of equations for each trophic level in scenario 1 are:

Trophic Level / Equation
Piscivore biomass / (user input value from cell H26/H27)
Planktivore biomass / = (˗ Piscivore biomass +100) × 5
Zooplankton biomass / = Piscivore biomass × 25
Phytoplankton biomass / = (˗ Piscivore biomass +100) × 100
  • The scaling factors (×5 for planktivores, ×25 for zooplankton, and ×100 for phytoplankton) create the trophic pyramid shape.
  • The negative sign for planktivores and phytoplankton create the alternating biomass response seen in Figure 1 of Carpenter et al. (1985).

Example of how to manipulate the Excel file to run the model:

1)The scenario 1 graph (outlined in red) and initial piscivore biomass (outlined in blue) should look like the screenshot copied below.

  1. Note that the initial value (outlined in green) is indicated in the scenario to help the user return to starting conditions if needed.
  2. The range of potential input values (outlined in orange) are also provided. These are the values that should be entered in the piscivore biomass input box (blueoutline around yellow box, cells H26/H27).

2)The scenario as described in the worksheet instructs the user to enter a piscivore biomass value into cells H26/H27 (blue box in figure above and below) that reflects the removal of piscivores from the system due to over fishing.

  1. In the example below, the user input the value 20 in the piscivore biomass box (blueoutline) and hit the Enter key.
  2. The graph changes to reflect the new piscivore biomass (red arrows). The blue bars on the figure are the initial biomass (when piscivore biomass= 50) and the yellow bars have shifted to the new biomass of each trophic level resulting from the removal of the top predator.

In Class Instruction:

  • Present the background information (see slides 1-12 and the glossary in ESM-D and E). The slides can be modified to match the students’ background and achieve the instructional goals for the course.
  • Introduce the activity and present the four simulation scenarios. A demonstration of how to use the model is also included (slides 13-14).
  • There are three possible formats for guiding the students through the activity.
  • 1. The class completes the activity together.
  • The instructor should manipulate the values in the Excel spreadsheet and walk through the scenarios with the students.
  • In this format, we recommend providing the students with the worksheet to record their hypotheses and observations.
  • 2. Students complete the activity in class individually or in small groups.
  • After introducing the activity and scenarios in the lecture, the students should break into small groups to complete the simulation activity.
  • Have students download the model file ESM-D) and save it to the desktop or a working directory.
  • We recommend supplying the worksheet for students to individually record their hypotheses and observations, possibly to be turned in for credit. Instructors maydelete or ignore the “Observations” and “Hypotheses” boxes in the Excel file or choose to have the students use them and remove these sections from the worksheet.
  • Consider deleting the Follow Up question from the presentation (slides 16,18,20, 22) from the power point presentation and just present the scenarios (slides 15, 17, 19, 21).
  • 3. The students complete the activity outside of class
  • After introducing the activity and scenarios in the lecture, the students can be assigned the activity and follow up questions to complete on their own time or in class.
  • Have students download the model file (ESM-D) and save it to the desktop or a working directory.
  • In this format students can either complete the assignment within the Excel document by typing hypotheses and observations or be provided with the worksheet to complete.
  • Consider deleting the Follow Up question from the presentation (slides 16, 18, 20, 22) from the power point presentation and just present the scenarios (slides 15, 17, 19, 21).

Synthesis and Follow Up:

  • The follow up questions listed after each scenario in the worksheet (and on slides 16, 18, 20, 22) should be answered as a part of the activity.
  • The activity synthesis questions and extension synthesis questions listed on the worksheet can be assigned as homework or used as exam questions in the course.
  • Real world examples of the four scenarios (Journal articles given below. News articles may be found online):
  • Removal of top-predator: Wolves removed from Yellowstone National Park (Ripple and Beschta 2012), Sea otters and kelp forests (Wilmers et al. 2012)
  • Addition of top-predator: Wolves reintroduced in Yellowstone National Park (Ripple and Beschta 2012),Nile perch in Lake Victoria (Goldschmidt et al. 1993)
  • Bottom-up: Gulf of Mexico dead zone (Dodds 2006), Eutrophication in Lake Erie, USA (Kane et al. 2014) and Lake Taihu, China (Xu et al. 2010)
  • Invasive Species: Zebra mussels in the Hudson River (Caraco et al. 1997), Burmese python in the Florida Everglades (Dorcas et al. 2012)

Additional References

Caraco, N.F. et al. 1997. Zebra mussel invasion in a large, turbid river: phytoplankton response to increased grazing. Ecology. 78: 588-602.

Dodds, W.K. 2006. Nutrients and the “dead zone”: the link between nutrient ratios and dissolved oxygen in the northern Gulf of Mexico. Frontiers in Ecology and the Environment. 4: 211-217.

Dorcas, M.E. et al. 2012. Severe mammal declines coincide with proliferation of invasive Burmese pythons in Everglades National Park. Proceedings of the National Academy of Sciences of the United States of America. 109: 2418-2422.

Goldschmidt, T. et al. 1993. Cascading effects of the introduced Nile perch on the detritivorous/phytoplanktivorous species in the sublittoral areas of Lake Victoria. Conservation Biology. 7: 686-700.

Kane, D.D. et al. 2014. Re-eutrophication of Lake Erie: correlations between tributary nutrient loads and phytoplankton biomass. Journal of Great Lakes Research. 40: 496-501.

Ripple, W.J. and Beschta, R.L. 2012. Trophic cascades in Yellowstone: the first 15 years after wolf reintroduction. Biological Conservation 145: 205-213.

Wilmers, C.C. et al. 2012. Do trophic cascades affect the storage and flux of atmospheric carbon? An analysis of sea otters and kelp forests. Frontiers in Ecology and the Environment. 10: 409-415.

Xu, H. et al. 2010. Nitrogen and phosphorus inputs control phytoplankton growth in eutrophic Lake Taihu, China. Limnology and Oceanography. 55: 420-432.

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