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Disappearing Marine Iguanas

A Case of Population Collapse


Conrad Toepfer
Division of Mathematics & Natural Sciences
Brescia University


In this interrupted case study, students apply the scientific method to probe possible reasons behind declining marine iguana populations in the Galapagos Islands. Initially students are given rudimentary information and encouraged to generate wide-ranging hypotheses. Students are then given further information to help them refine their initial hypotheses into single, testable statements. As the case progresses, students shift to an examination of experimental methods and data interpretation. Over the course of the case, they utilize both inductive and deductive reasoning in developing their conclusions about the factors influencing marine iguana populations.  The case is appropriate for an introductory course for majors or non-majors in general biology, ecology, or environmental science.


  • Refine hypotheses to single, preferably testable, statements.
  • Suggest possible experiments.
  • Develop sensitivity to issues of data interpretation such as the need to consider variation and the influence of how data are presented.
  • Appreciate inductive reasoning by seeing that the solution requires consideration and integration of multiple studies.
  • Illustrate the tentative nature of our conclusions by pointing out potential gaps in information or problematic experimental methods.


Population biology; Galapagos Islands; El Nino / Southern Oscillation; ENSO; global warming; data interpretation; inductive reasoning; deductive reasoning; marine iguana; Amblyrhynchus cristatus

Topical Areas

Scientific method

Educational Level

High school, Undergraduate lower division, Undergraduate upper division


PDF, PowerPoint

Type / Methods




Subject Headings

Biology (General)  |   Ecology  |   Environmental Science  |   Earth Science  |  

Date Posted


Teaching Notes

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Supplemental Materials

A complete set of figures used in the case is also available in two PowerPoint files according to figure orientation: iguanas1.ppt for the landscape figures (Figures 1, 2, 3, 4, 8, and 9) and iguanas2.ppt for the portrait figures (Figures 5, 6, and 7 and Table 1):

  Landscape-oriented figures for marine iguana case
  Portrait-oriented figures for marine iguana case

Answer Key

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