ABSTRACT

In this one to two week lab, students examine hypotheses about the interactions between herbivores and plants; these interactions could be studied in the context of a course segment on symbioses or trophic relations. This lab involves the study of the relationship between tall goldenrod and a gall-forming herbivorous insect; the presence or absence of the gall formed by the insect is easy to assess and plant reproductive output can be measured after first frost, making this system useful as a field-lab well into the fall semester. The purpose of the lab is to: determine if plants with particular attributes differ in susceptibility to attack by herbivores, and if herbivores act as parasites, commensals, or possibly mutualists when they feed on plants by having a negative, neutral, or positive effect on plant reproduction. First, students measure plant traits to test two competing hypotheses to explain variation in abundance of herbivorous insects among plants. The Plant Stress Hypothesis and the Plant Vigor Hypothesis both argue that plant physiological status and growth will impact the success of herbivores, but one proposes that stressed plants will be vulnerable to attack because they provide better food with higher amino acid content for herbivores, while the other proposes that vigorously growing plants will provide better food. Second, students measure plant reproductive traits to test competing hypotheses about the effect of herbivores on plant reproduction. These hypotheses are the Negative Impact, the Plant Tolerance, and the Overcompensation hypotheses; the hypotheses suggest, respectively, that plants attacked by herbivores will produce a number and quality of seeds that is less than, similar to, or greater than the number and quality produced by plants not attacked by herbivores. In this lab, students will discuss the nature of the interaction of herbivores with plants, collect and share data, analyze data using statistical tests, examine two primary sources from the research literature to address ideas in the lab, and draw conclusions and communicate their findings by writing a Brief Communication.

AUTHOR

Christopher F. Sacchi

Kutztown University, Department of Biology, Kutztown University, Kutztown PA, 19530. sacchi@kutztown.edu

CLASS TIME

One week (3 hour lab) if system is chosen by instructor. Two weeks if students are required to find a plant-herbivore interaction to study that is different from the goldenrod-gall system.

OUTSIDE OF CLASS TIME

Five to six hours to analyze data, to collect and interpret two literature sources, and to write a Brief Communication.

STUDENT PRODUCTS

Formative Assessment of student understanding of concepts to be studied: Students will be asked to discuss, as a class, some of the "Questions for Further Thought and Discussion" at the beginning of lab. A one-minute paper can be assigned before going to the field in which students address 'misconceptions' they had about plant-herbivore interactions before the lab discussion.

Brief Communication: this is a short report such as the Brief Communications found in many journals, in which students present a short but complete report on the results of the two or three comparative tests that they will perform.

SETTING

This lab can be conducted in any field environment with plants of a single species that are attacked by one species of herbivore that leaves distinctive evidence of the damage it has caused. The plant-herbivore system that is most useful for this lab is the tall goldenrod (Solidago altissima) and one of the gall-forming herbivore species that attack this plant. Using the goldenrod-galling insect system, the lab can be conducted in late summer through late fall; plant attributes can be measured after the first frost in fall, since the goldenrod retains its inflorescences and indicators of seed production into November. All measurements can be recorded in the field.

Other plant-herbivore systems that can be used include many species of willows that are attacked by different species of stem galling insects and also produce evidence of their reproductive potential (enlarged reproductive buds) in the presence or absence of damage caused by a gall-forming herbivore (see TIEE lab by Ernest (2005) for one example of such a plant-herbivore system).

COURSE CONTEXT

This lab is taught during the fall semester in a general ecology laboratory class for biology majors. The class typically includes 24 students. Students work cooperatively in groups of three or four.

INSTITUTION

This course is taught to undergraduate Biology majors at a regional public four year university of 8000 students.

TRANSFERABILITY

This lab would work well with general ecology or upper division ecology courses at institutions of all sizes. The lab is simple in design and requires no special technical skills so could be transferred to non-majors general biology classes, but access to suitable field sites might be difficult for multiple-section non-majors classes. A suitable field site for the lab, as written, would include any area that includes a stand of goldenrod with more than a few hundred stems; such sites could include an old field, a prairie, an unmowed road edge, or perhaps an abandoned lot or an unmowed area in a park in an urban area.

ACKNOWLEDGMENTS

The ideas presented in this lab exercise were developed largely as a result of the work and inspiration of colleagues in the lab of Peter W. Price at Northern Arizona University (NAU), where I completed my doctoral studies and later through interactions with students and colleagues, particularly Ed Connor, at the University of Virginia’s Blandy Experimental Farm (UVa). Through discussions, seminars, work in the field, and critical review of the work of colleagues in the Price lab at NAU, my understanding of the rudiments of the Plant Vigor Hypothesis evolved. As part of my dissertation at NAU and later with input from Ed Connor at UVa, I studied the impact of herbivores on plant reproduction and developed familiarity with some of the ideas tested in this exercise. Colleagues who should be mentioned for their influence on my perspectives on the ideas in this lab include Tim Craig, Joanne Itami, Ken Paige, Mike Kearsley, Mike Wise, Tom Whitham, and both Peter Price and Ed Connor. Colleagues in the Biology Department at Kutztown University, including Carol Mapes, Ron Rhein, Wendy Ryan, Will Towne, and Anne Zayaitz have influenced my awareness of the pedagogical approaches employed in this lab exercise. This laboratory exercise was completed with support from the National Science Foundation’s Division of Undergraduate Education, Course Curriculum and Laboratory Improvement (CCLI) program (Award # 0126817) to C.F. Sacchi, C.C. Mapes, W.L. Ryan, and A.E. Zayaitz. I would also acknowledge the efforts of Charlene D’Avanzo, Bruce W. Grant, and Kathy Winnet-Murray for helping authors to develop TIEE lab exercises and two anonymous reviewers whose comments have helped in the development of this lab.

CITATION

Christopher F. Sacchi. April 2006, posting date. Testing Hypotheses about Herbivore Responses to Plant Vigor and Herbivore Impact on Plant Reproduction. Teaching Issues and Experiments in Ecology, Vol. 4: Experiment #1 [online]. http://tiee.ecoed.net/vol/v4/experiments/herbivore_responses/abstract.html