Unraveling a Salt Marsh Mystery: Using Case-Based Learning for Teaching Science

Ecosystem regulation and trophic cascades — check. Teaching method: lecture and diagrams — check. Student engagement — no check! Over time, teachers develop a set of pedagogical tools to engage students. Selecting the best method for the topic and teaching scenario from that toolbox is part of the art of teaching. One teaching method I have enjoyed over the years is case-based learning.

Case-based teaching allows for flexibility and adaptability, and case-based learning leads to many positive educational outcomes, such as an upturn in engagement and an increase in abilities to apply scientific concepts. Stories are at the heart of case-based learning since stories include a plot and unfold as more information becomes available. Students immerse themselves in the stories to learn complex topics as problem-solvers, and in turn learn at a deeper level and develop proficiency in a variety of scientific practices (Kulak and Newton 2015).

In teaching about ecosystem regulation and trophic cascades, I opted to use a case study because the backstories of what we know and understand about these topics are quite compelling. The Serengeti Rules: The Quest to Discover How Life Works and Why It Matters by Sean B. Carroll provides a wonderful treatise on the topic. Just as Dr. Carroll masterfully tells the stories of how we have come to learn about ecosystem regulation, we can turn those stories into case studies for students to unravel while engaging in ready-made BioInteractive teaching resources. In this case study about salt marsh regulation and trophic cascades, I use several BioInteractive resources, which are described below.

Dr. Brian Silliman, a salt marsh ecologist from Duke University, is a skilled storyteller; his 10-minute video Trophic Cascades in Salt Marsh Ecosystems provides a perfect framework for the case study. I break the video into segments that lend themselves to student interaction. For example, at approximately 3:25, Dr. Silliman mentions that he and his team designed several experiments to test his hypothesis. I pause the video and ask students (partners or teams) to brainstorm how Dr. Silliman would go about answering his question. While students report out, I listen for their proficiency in the practices of science within their experimental design ideas, including whether they will be able to answer the research question based on how they set up their experiment. For example, students may suggest growing salt marsh grasses in the laboratory, adding snails to the grasses, and watching what happens. They may also suggest going into the field and sampling a plot of salt marsh grass by counting the numbers of snails on the unhealthy grasses versus the healthy grasses.

Next, we watch the video up to 4:32 and compare their ideas with the experiment Dr. Silliman set up to test his hypothesis. There are typically some commonalities, but students are always surprised by the simple equipment used in this fieldwork. I ask them to predict the results of the experiment, which had multiple treatments (snail and fertilizer treatments), and then watch the film up to 5:06 to view the results. We discuss the ecological principles governing the results and define terms like top-down and bottom-up control.

Like any good research question, the results beckon the generation of the next research question. Dr. Silliman generates a new question to figure out if the periwinkle snail and cordgrass relationship existed where the mudflat abuts the cordgrass, where he noticed a “snail front.” We watch the film up to 5:30 and then complete the “Snail Fronts and Salt Marsh Die-offs” Data Point. Here, students are introduced to a figure from one of Dr. Silliman’s research papers (Silliman et al. 2005) and analyze it relative to what they understand thus far about salt marsh ecosystems: the relationship between the snail and grass.

Afterward, we go back to the question and field experiments in the video (up to 6:08) to find out what would happen to the grasses if snails were excluded and to connect this with top-down regulation. I ask students to make a claim about the trophic relationships in the salt marsh and to provide evidence to support their claim. By now, students should recognize that if the snail population was not being regulated, the salt marsh grasses would be gone. Therefore, as abundant as the snails are, something is regulating them.

I challenge students to ask a testable question that might provide an answer to the question of how the snail population is being regulated. We watch the video up to 8:00 and then discuss whether Dr. Silliman has an answer to his question. Depending on students’ experience with data analysis, they may recognize that the results are only correlational. Another experiment is needed and, at this point, students should have an idea of how to design an experiment for this last question.

We finish watching the video; I ask students to create a model of the relationships in the salt marsh among the grass, snails, and crabs. Included in their model are the direct effects of one organism on another, as well as the indirect effect of the crab on the grass. Students share their models and are asked to identify another ecosystem with top-down regulation. Depending on when I use this case within my ecology unit, they may not be able to come up with additional examples.

Therefore, I use the “Modeling Trophic Cascades” activity to apply what they learned to a new ecosystem. Of course, I challenge them to figure out how, by way of fieldwork, an ecologist may have developed their model of ecosystem regulation for each ecosystem. The activity provides seven ecosystems, but I use only five to reduce the amount of time needed to complete the activity. I leave out the salt marsh ecosystem and one of the others, which I use as a formative assessment later in my ecology unit. For students who could benefit from extra assistance in this topic, I assign the Exploring Trophic Cascades Click & Learn. This Click & Learn provides additional examples of trophic cascades and shows the indirect or direct effects that organisms may have on other organisms in the ecosystem. 

I have been using this combination of resources for a few years with my AP Environmental Science students. When they engage in this compiled case study, they improve their ability to use the practices of science as they see the principles of ecology being authentically applied. Subsequently, when they view a new ecosystem, they don’t hesitate in asking questions to figure out what is regulating what and why. In addition, they understand that a good scientific question is testable and that a field experiment looks a lot different than a laboratory experiment.

This is one example of how BioInteractive materials may be threaded together to create a compelling case study. There are numerous videos and associated materials that can be used to do this for other topics. The key to developing a case study from these resources relies on the quality of the story as it connects with the scientific principles, teaching objectives for a course, and the intended audience (Herreid 2019). Inherent to its success in the classroom are the content and pedagogical content knowledge of the teacher. Fortunately, the BioInteractive resources assist with both.

References

Carroll, S. B. The Serengeti Rules: The Quest to Discover How Life Works and Why It Matters. Princeton, NJ: Princeton University Press, 2016.

Kulak, V., and G. Newton. “An Investigation of the Pedagogical Impact of Using Case-based Learning in an Undergraduate Biochemistry Course.” International Journal of Higher Education 4, 4 (2015): 13–14. https://doi.org/10.5430/ijhe.v4n4p13.

Herreid, C. F. “The Chef Returns: A Recipe for Writing Great Case Studies.” The Journal of College Science Teaching 48, 3 (2019): 38–42. https://doi.org/10.2505/4/jcst19_048_03_38.

Silliman, B. R., J. van de Koppel, M. D. Bertness, L. E. Stanton, and I. A. Mendelssohn. “Drought, Snails, and Large-Scale Die-Off of Southern U.S. Salt Marshes.” Science 310, 5755 (2005): 1803–1806. https://doi.org/10.1126/science.1118229.