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Animals Can Run Away, but Plants Must Stay

Responses to Herbivory


Nicole D. Tunbridge
Department of Biology
Kwantlen Polytechnic University
Carol Pollock
Department of Zoology
University of British Columbia
Joan Sharp
Department of Biological Sciences
Simon Fraser University


In this PowerPoint-driven case study, students consider the many challenges faced by plants and discuss which of these might induce a morphological response. Examples of phenotypic variation within a plant species are presented and students discuss in small groups how to determine whether the observed variations have a genetic basis. The concept of a "common garden" experiment, in which plants are grown in a common environment and variation is measured, is elucidated and discussed. A specific example of phenotypic variation is introduced: Plectritis congesta from islands without herbivores (deer) are taller than the same species from islands with deer. An experiment with simulated herbivory is described and students are asked to predict results, assessing whether modification of pattern of growth is an induced response or a constitutive defense. A field experiment involving deer predation is introduced. The morphological response of Plectritis to predation is presented as an evolutionary tradeoff. The case is appropriate for introductory general biology (majors and non-majors), ecology, and plant biology courses and, with some modification, introductory evolution courses.


  • Explain how biotic and abiotic factors can act as agents of selection.
  • Distinguish between constitutive defenses and induced responses and explain the conditions under which each may be adaptive.
  • Explain how a common garden experiment can distinguish between genetic and environmental causes of phenotypic variation in plants.
  • Plan common garden and field experiments, and identify dependent and independent variables from these experiments.
  • Interpret data from common garden and field experiments.


Natural selection; adaptation; evolution; evolutionary fitness; phenotypic variation; common garden experiment; experimental design; plant defenses; constitutive defenses; induced responses; herbivory; plasticity

Topical Areas

Scientific method

Educational Level

Undergraduate lower division


PDF, PowerPoint

Type / Methods

Clicker, Discussion, Interrupted



Subject Headings

Biology (General)  |   Ecology  |   Genetics / Heredity  |   Evolutionary Biology  |   Botany / Plant Science  |   Science Education  |  

Date Posted


Teaching Notes

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Supplemental Materials

The PDF document below is a pre-case assignment on Plectritis congesta. Students will also need access to the journal article: Mondor and Addicott (2003) “Conspicuous extra‐floral nectaries are inducible in Vicia faba,” Ecology Letters 6 (495–497), Please note that this article, for copyright reasons, is not provided with the case; instructors wishing to use it will need to acquire it through their library.


Answer Key

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