Challenges to Anticipate and Solve

Challenge 1. Goldenrod and Gall Identification: Students must learn to identify the goldenrod that they choose to study and the types of galls on plants in their study system; in SE Pennsylvania, I commonly find both the ball gall and the rosette gall, so I have asked classes to study the hypotheses related to plant vigor and herbivore impact in response to a different herbivore in different years, e.g. in year 1 study the ball gall and in year 2 study the rosette gall. In some years, you must choose a particular gall type due to its greater abundance while the second species may be less abundant in that same year. Carefully dissect a ball gall to show students the larvae that can be found within the gall (Figure 4).

Figure 4. A ball gall formed by Eurosta solidaginis that has been dissected to show the larva in the interior of the gall. A ruler is included to provide scale.
full size image

Challenge 2. Estimating Goldenrod Shoot Height: Students often ask exactly which part of the plant stem to measure. I encourage students to measure from the ground to the apical bud on the tallest stem of the plant. Frequently, there are many lateral stems on plants that are attacked by the rosette gall, which appears to affect the apical dominance of the main stem of the plant; I still encourage students to measure from the ground to the height of the tallest stem, even if it the tallest stem is a branch formed by release of an axillary bud.

An important assumption of this study is that the gall former does not appreciably alter the growth of the stem of the plant. This assumption is likely violated in this study system since we are measuring the stems long after herbivores have attacked the goldenrod stems, and you will probably find that growth of the stem is altered by the presence of the gall. To more carefully test the Plant Vigor Hypothesis and the Plant Stress Hypothesis, it would be best to measure plant size at, or near, the time when the female gall forming insect oviposits and the gall is initiated. In the goldenrod system, stem height on galled and ungalled stems should be measured in late June or early July. I ask students to consider factors that may have influenced the results they obtain. The time of year when we conduct this study and the effect of the gall on plant growth are among the complicating factors. The results of the test of the Plant Vigor and Plant Stress Hypotheses may be influenced profoundly by the effect of the gall on plant growth and the time of year when the students record plant height on galled and ungalled stems.

Challenge 3. Estimating Goldenrod Reproduction: Students often ask which part of the stems that bear inflorescences should be measured. I encourage students to measure the portion of the stem from the location of the first inflorescence to the end of the stem where the last inflorescence is located. Measurement of the number of seeds produced by goldenrod could be a real challenge; as a member of the plant family Asteraceae, each 'flower' is a composite inflorescence with many flowers and many seeds. Further, there can be many inflorescences on a single stem and there can be few to many stems with inflorescences on an individual plant. So, for expediency, we make the grand assumption that cumulative length of stems bearing inflorescences is a proxy for seed production and fitness. I am unaware of studies that have verified the assumption that length of stems bearing composite inflorescences is correlated with total seed production. However, the use of biomass data that is correlated with goldenrod seed production, rather than counts of seeds themselves, has been used in past studies of the effect of galls on goldenrod reproduction (Hartnett and Abrahamson 1979).

Students should be aware that measurement of length of stems bearing inflorescences may be a very good estimator of seed output or it may, in reality, be an inadequate predictor of seed number and ultimately plant fitness. In the discussion, students may speculate about the methods they might use to determine if length alone is a good indicator of seed output. One or two student groups may focus on specifically addressing this issue by measuring the relationship between length of stems bearing inflorescences with some other trait associated with reproduction such as biomass.

For their analysis of herbivore impact, students may ask how to deal with plants on which no inflorescences were found. I encourage students to count stems with 0 inflorescences, using a measure of 0 cm length, in making their estimates of plant reproductive output. In their spreadsheet, every galled plant and ungalled plant for which they recorded length of stems with inflorescences should be represented, even if the value is a zero when the plant has produced no inflorescences.

Challenge 4. Statistical Analyses: Students may need guidance about which statistic to use in comparing galled vs. ungalled plants for both the Plant Vigor/Plant Stress Hypothesis and for the Herbivore Impact Hypotheses. Students should be aware that they should use a paired t-test, since the ungalled and galled stems that were studied were selected due to their proximity to one another, so they should be treated as non-independent samples for purpose of statistical testing. Instructions on how to run a paired t-test are provided in the Statistics Appendix (57k *.doc) or (35k *.pdf).


Lab Description

Within the framework of a General Ecology course, this lab has been taught under the heading of: 1) Coevolution and Mutualism in Ricklef's, The Economy of Nature, Fifth Edition and 2) Predation in Smith and Smith, Elements of Ecology, Fifth Edition, and might also be taught in the context of Trophic Relations.

Introducing the Lab to Your Students: There are several published studies that demonstrate that in some insects, females oviposit or insects feed on the most vigorously growing plants or plant parts (Craig et al. 1988, Fritz et al. 2003, Hull-Sanders and Eubanks 2005). For other herbivorous insect species, there is support for the idea that herbivores attack stressed plants (White 1984, Hull-Sanders and Eubanks 2005). Finally, some authors report that they can support neither the Plant Vigor nor the Plant Stress Hypotheses (Williams and Cronin 2004). Huberty and Denno (2004) survey the literature on tests of the Plant Stress Hypothesis and they report that herbivore-feeding mode can play an important role in determining whether the herbivore responds to plant stress.

Plant Vigor: Types of herbivory and applying ideas to other systems

Different species of herbivores obtain nutrition by feeding on a variety of plant tissues, using mouthparts that are adapted to allow the insect to obtain food from the particular tissues on which they feed. Types of herbivorous insects include: 1) free–feeders which include a) foliage feeders such as Monarch caterpillar larvae or potato beetle larvae or b) sap feeders, such as aphids that feed on phloem fluids, 2) gall formers whose offspring feed on gall tissue or on plant sap obtained from a location within the gall, and 3) leaf miners where insects feed on plant tissue located between the epidermal layers of the leaf. Students could study how herbivores in different feeding guilds respond to plant vigor or plant stress by comparing the size of plant parts attacked by herbivores and the sizes of those same parts that are not attacked.

If the instructor would like to conduct this study using another type of herbivore, or if students are given the chance to independently choose a plant and herbivore for study, some or all ideas in this lab might be suitably tested using foliage feeders, leaf miners, or other herbivores whose presence on a plant or plant part, e.g. a mine or chewing damage, can be documented. The Plant Vigor and the Plant Stress Hypotheses can be tested once the presence of an herbivore (or signs of the damage an herbivore causes, e.g. chewing) is documented. By measuring the size of the plant part the herbivore has attacked, e.g. the area of a leaf attacked by a leaf miner or a foliage feeder, and comparing this size measure to similar measures taken from unattacked plant parts, one can test these hypotheses. In many parts of North America, galls on oak leaves would permit study of the Plant Vigor and Plant Stress Hypotheses.

The hypotheses about the reproductive response of plants to herbivores can only be tested if flowers or fruits are present on the plant at the time of year when the study is conducted. Not every plant species will bear indicators of plant reproduction during the fall, when this lab exercise is most often conducted.

Other gall formers that might be suitable for this lab exercise include the different types of galls formed on many species of willows, particularly stem galls. Stem galls are retained into the winter, simplifying identification and measurement of galled and ungalled stems after the growing season. Further, many (but not all) willows produce enlarged reproductive buds in fall that will produce catkins the next spring; these reproductive buds can be counted on galled and ungalled stems and are indicators of future reproduction (Sacchi et al. 1988). Since willows are dioecious, with separate male and female plants, one can only conclude that reproductive output is or is not affected by herbivores. With willows attacked by stem gall-forming insects, both the Plant Vigor and the Herbivore Impact Hypotheses can be studied. See the TIEE laboratory experiment by Ernest (2005) that includes a thorough description of leaf galls and provides mention of the stem galls formed on willows.

An additional study system in eastern North America, particularly in the mid-Atlantic and southeastern US, is the flowering dogwood, Cornus florida and the dogwood club gall, Reselliela clavula. Sacchi and Connor (1998) studied reproductive effects of the club gall on flowering dogwood; since the dogwood produces inflorescence buds in the fall, one can survey the length of stems attacked by the club gall to test the Plant Vigor Hypothesis and the presence or absence of inflorescence buds on galled and ungalled shoots allows one to test the effect of herbivores on plant reproduction. Thus, all questions discussed in this lab exercise could be successfully studied using the dogwood-club gall system.

Herbivore Impact

In coevolutionary theory relating to the evolution of herbivores and plant defenses, it was traditionally assumed that an herbivore would need to reduce plant fitness (Strong et al. 1984). In several study systems, the negative effects of herbivores on plant reproductive structures have been documented (Sacchi et al. 1988; Wise and Sacchi 1996).

Tolerance is a strategy whereby a plant can be damaged by herbivores but the plant exhibits little or no reduction in plant fitness (Strauss and Zangerl 2002, pp. 88-89). Levels of plant tolerance to herbivory can range from little loss in fitness, to no loss, to overcompensation in fitness (Strauss and Zangerl 2002, p. 89).

The case for overcompensation following plant damage by an herbivore has been an area that has been controversial. Paige and Whitham (1987) studied a particular plant-herbivore interaction, between elk and the monocarpic Ipomopsis aggregata, and they documented an instance of Overcompensation, in which plants fed on by elk produced more seeds than plants that were not fed on by elk. The work by Paige and Whitham has stimulated a vigorous debate (see Crawley 1997, p. 443) about overcompensation and whether this could be an infrequent or a common phenomenon.

Constructivist Approach and Inquiry

Some faculty members may prefer to lead students to study the hypotheses posed in this lab exercise in the context of a constructivist and basic inquiry approach to the exercise. In this case, students could be told that they will study plants and herbivores, to list all factors that they think might influence plants and herbivores, and students could visit field sites where they would look for plant-herbivore systems to study. Students would be guided to measure any attributes of plants or herbivores that they consider to be important to explain the abundance of herbivores on plants. Students would analyze data upon return to the lab, they would be asked to look for patterns in the data, and they would then be asked to define questions and hypotheses they would like to study to account for the patterns they have observed. The students would then plan a study they would conduct to test their hypotheses and would return to the field to conduct this study. The instructor would discuss with students their findings along the way and could guide students to appropriate citations in the plant-herbivore literature.


Activities in the Lab

The information in this lab should be fairly straightforward to present to students. The non-technical nature of the data collection should allow the data collection portion of the lab to proceed relatively quickly.

A blank data sheet is provided to help students to structure their data collection; you may choose to give the blank data sheet appended to this exercise to your students.

This lab can be done after the first frost in the fall, since the goldenrod seems to retain its inflorescences and seeds into November. This lab works well when you are looking to conduct a lab that does not involve study of actively growing plants or animals.

Faculty members interested in the relationship between the Plant Stress Hypothesis and herbivores in different feeding guilds, e.g. phloem feeders, foliage feeders, gall formers, are encouraged to read the careful review and analysis of the many studies on insects in diverse feeding guilds by Huberty and Denno (2004).

See the section on Introducing the Lab to Your Students for ideas about other study systems that can be used to test the ideas in this lab exercise.


Questions for Further Thought

The questions for further thought are useful in several contexts. First, there are Formative Assessment questions that should be discussed in the lab with students before they conduct the exercise to assure that students have a basic understanding of the concepts that will be studied in this lab. Second, there are a variety of questions that would be helpful to students in interpreting the data they have analyzed. Further, the questions also provide students information that they might consider in writing the discussion of their Brief Communication; all questions provide students with a link to the literature so that they may use not only their personal experience and impressions in addressing the questions but also a primary literature source. As this lab exercise is written, there are probably more Questions for Further Thought than any one student would have time to research and consider in a one week lab assignment. The instructor for the course will probably want to choose one or two of the Questions for Further Thought for students to consider as they write their Brief Communication.

Students are often surprised to learn that plants differ, both between and within plant species, in their suitability as food for herbivores. The idea that plants can actively defend themselves is a concept that many students are only vaguely familiar with, but they can easily grasp this concept with some discussion about structural and chemical defenses in the pre-lab introduction. Further discussion will also clarify the idea that plant physiological status may affect plant vulnerability to herbivores leading to the contrasting ideas represented by the Plant Vigor and Plant Stress Hypotheses. Many students assume that plants will be damaged if fed upon by herbivores based on the common perception that in gardens and on farms herbivores reduce yield of crop plants. Many students are often surprised to learn that native plants may be tolerant of damage caused by herbivores and that in some cases plants may overcompensate in their reproductive output following reproduction compared with plants that have not been attacked by herbivores.


Assessment of Student Learning Outcomes

Student learning outcomes are assessed based on: 1) the pre-lab formative evaluation represented by the one minute paper about misconceptions students had before the pre-lab discussion, and 2) the Brief Communication. A detailed rubric is provided for instructor use in assessment of the Brief Communication the students have written. Lab objectives are assessed through the combination of the formative evaluation and the Brief Communication. The expectation that students will use higher level thinking in completing this exercise is addressed by student responses to Questions for Further Thought either submitted separately or embedded in their Brief Communication.


Evaluation of the Lab Activity

Three different formats for a post-lab formative evaluation will be suggested. Each is intended to provide the instructor with information about student perceptions of the lab and about student learning so that the lab might be improved in the future. It is assumed that you will choose one of these three formative evaluation formats that best suits your needs.

A simple type of formative evaluation of the lab activity that may be given to students when the lab has been completed could consist of two simple questions: what was the part of the lab you liked the best, and what part or parts of the lab should we change for the future to make things work better? By asking these questions, we get an idea of whether students have identified the goals that we’ve established for the exercise and with appropriate feedback, we can plan to adapt the lab in the future. This is not a highly structured formative evaluation but it provides rapid feedback from students about the lab exercise.

A second format for a formative evaluation of the lab activity is to ask students what they thought of the lab as suggested by the author. Questions in this survey might include: 1) What aspect of the lab helped you learn the most, and 2) what part of the lab was most confusing? Responses to these questions could provide some indication of how to improve student learning.

A third format for a formative evaluation of the lab activity would be to use a directed evaluation in which you ask students to rate each activity on a 1-5 scale on the degree to which the activity helped with student understanding of the topics studied. Students could rate: introductory lecture, discussion questions, data collection, data analysis, Brief Communication assignment, use of scientific literature, discussions with lab partners, etc. The information provided in such a survey could provide some indication of how to improve the lab exercise in the future.

Extensive notes on how to conduct formative evaluation are in the Teaching Resources sector of TIEE in an ESSAY ON EVALUATION.


Translating the Activity to Other Institutional Scales or Locations

This lab involves non-destructive study of plants in old fields using simple supplies such as rulers and tape meters. Multiple lab groups in a larger multi-section ecology course at a larger university could do this lab on separate days or in separate areas of the same field, as long as there is a large enough stand of goldenrod available.

In the section, Introducing the Lab to Your Students, there are ideas for several alternative study systems for conducting this lab including gall formers on willows (different willow species and gall formers can be found throughout North America), dogwood and the dogwood club gall found in eastern North America, and oaks and their gall formers (cynipid wasps cause galls on leaves and other plant parts in oaks). In urban areas, the Plant Vigor and Plant Stress Hypotheses may be studied on oaks in parks or in botanical gardens. Further, the lab may be conducted in abandoned lots or unmowed areas in parks that harbor goldenrod or weeds such as lamb's quarters (Chenopodium album); lamb's quarters are likely to have leaf miners that would allow testing of the Plant Vigor and Plant Stress Hypotheses. Community gardens may hold plants that have insects on them that would allow tests of the Plant Vigor or Plant Stress Hypotheses, e.g. Brassica including cabbage, broccoli, or brussel sprouts and the larvae of Pieris rapae.

Additionally, there are ideas in Introducing the Lab to Your Students that describe how the instructor might guide students to be more actively involved in selecting the study systems that they choose or that would allow students to use a constructivist and inquiry approach in formulating the questions that they would study.

This lab could be easily used in the pre-college setting; t-statistics might be eliminated for younger students, but interpretation of effects could be based on plots of averages for data collected to test both of the major hypotheses presented in the exercise.