Students observe galls on willow leaves, and begin their investigations in teams by collecting data to test the instructor-posed hypothesis that the number of galls per leaf varies among willow trees. This step gives students more guidance, allows them to practice sampling on a question they will discuss in class but won’t include in their graded assignment, and gives them a chance to view a number of galls and leaves to get a better sense of the study system and the typical pattern that herbivory varies among leaves and among plants (due to differences in plant chemistry, physical traits, environmental traits, etc.). Student teams then choose among several instructor-directed questions (such as whether female sawflies oviposit independently of other oviposition events, whether galls on leaves with other galls are less successful than single galls on leaves, and whether leaf-chewing herbivores select leaves independently of galls), or pose their own hypothesis. Instructors may assign these randomly to ensure that each hypothesis is tested by at least one team, or briefly discuss why each hypothesis might be interesting to test. More motivated teams might be challenged to formulate their own question based on their preliminary observations at the site. For example, they may notice that trees vary in size/age, or in distance to surface water, or that not all galls are the same size. Instructors can capitalize on these observations by encouraging students to ask how these variations might influence gall distribution or success. Once teams select a hypothesis, they then collect data to test the hypothesis, analyze their data, and prepare a formal oral report on their investigation.
At the conclusion of this lab, students will be able to:
- discuss in what ways and why herbivory varies among plants and among units (e.g., leaves) within plants,
- articulate several ecological and possible evolutionary consequences of this plant-herbivore relationship to each of the organisms involved,
- recognize sawfly galls on willows,
- use common statistical tests to analyze data on the distribution of galls on leaves,
- work collaboratively to collect and analyze data, find appropriate scientific literature, and organize a formal oral report using PowerPoint.
- data sheets
- random number table
- pocket knives to cut open galls
- hand lens or portable dissecting scope to view sawfly larvae
- digital camera (optional, but nice) to take photos for oral reports
- finding a site with willow trees that have sawfly galls, where leaves are easily accessible to students
- arranging transportation, if necessary
Summary of What is Due
- provide summarized data for hypothesis 1 (on the board in lab);
- deliver a 10-minute formal oral report on the additional hypothesis tested by their team
- hand in data set for the additional hypothesis (raw data table, data summaries, and statistical analysis)
- hand in an evaluation sheet for oral presentations by all other teams
- Principal Ecological Question Addressed: What are the ecological and possible evolutionary consequences of this plant-herbivore relationship to each of the organisms involved?
- Does herbivory vary among individual plants and among leaves within plants?
- Do oviposition choices affect larval success?
- Ecological Topic Keywords: herbivory, galls, plant-animal interactions, parasite-host interactions, willow, Salix, sawfly, Pontania, hypothesis testing, statistical tests, field studies
- Science Methodological Skills Developed: field observation skills, hypothesis testing, random sampling, sample size, statistical tests, graphing data, oral presentation
- Pedagogical Methods Used: guided inquiry, group data collection, cooperative learning, peer evaluation