Interactions are at the core of many ecological and evolutionary forces in nature. Plant–soil interactions provide a rich example of the interconnectedness of living systems, but they are hidden from everyday view and overshadowed in the classroom by more popular teaching examples involving animals, reptiles, or invertebrates. To highlight the importance and relevance of plant–soil relationships, we devised a simple role-playing activity suitable for college students. Specifically, the activity simulates how feedbacks between plants and soil environments influence plant species abundance and community richness. With this activity, students will gain a better understanding of these prolific, but overlooked, forms of biological interactions that impact the diversity and functioning of ecosystems.
Biological interactions are a predominant way in which students learn about the ecological and evolutionary processes that influence biodiversity. However, most general biology textbooks primarily use animals, reptiles, or invertebrates as case studies to demonstrate the importance of interactions in nature (Uno, 1994; Link-Perez et al., 2010; Schussler et al., 2010). This contrasts with the facts that (1) plants are ubiquitous, and students encounter them regularly in their daily lives; and (2) most interactions that plants rely on happen belowground. Since it can be difficult to present plants and (especially) soils in exciting ways, many students unintentionally cultivate a fauna-centric view of the natural world (Wandersee & Schussler, 2001). To highlight the importance and relevance of plant–soil relationships, we devised a simple role-playing activity suitable for college students.
Research on plant–soil interactions and their importance in ecology and evolution has blossomed in recent decades. Specifically, feedbacks occur when plants condition soil properties and, in return, are affected by the conditioned soils (Bever, 1994). Negative feedbacks reduce the performance of individuals of the same species in relation to other species, resulting in negative frequency-dependent selection (Packer & Clay, 2000; Mangan et al., 2010). Positive feedbacks encourage conspecifics to thrive in their respective soils more than heterospecifics, leading to the monodominance of single species (e.g., invasive species). These reciprocal interactions can shape the nonrandom assembly of plant communities (Bever, 1994; van der Putten, 2013).
The activity has two main purposes: (1) to engage students in a more active interpretation and discussion of the interactions between plants and soils, and (2) to connect these interactions to larger concepts of drivers of biodiversity and ecosystem function. This aligns with a core concept in biology (sensuAAAS, 2011): Living systems are interconnected and interacting. Plant–soil interactions provide a rich example of the interconnectedness of living systems, but they are hidden from everyday view and overshadowed by more popular teaching examples. By actively role-playing plants and soils, students can see how these interactions operate in nature.
The primary learning objective of this activity is for students to recognize how plant–soil interactions alter patterns of plant community diversity. This activity simulates how plant–soil feedbacks influence species abundance and richness over time. It is recommended that the game first be played in Negative Mode. A Positive Mode variation is introduced at the end of this description.
Negative Mode demonstrates how negative plant–soil feedbacks promote and maintain diversity. When the same plant species and soil properties are matched, the plant dies. Oppositely, plants survive when species and conditioned soils are mismatched. For example, if a spruce interacts with soil that has been previously conditioned by spruce, that plant will die. But if the soil has been conditioned by pine, ash, or Ailanthus species, the spruce will survive.
The directions for setup are as follows:
Split the class so that there are equal numbers of plant and soil players (Figure 1A). Clear enough space in the room so that plant and soil players can stand in two opposing lines with no obstacles between the groups (Figure 1B). Soil players each receive one blank notecard and paperclip. Include a pile of paper cutouts of the different plant species (3× the total amount of plant players for each species) to serve as the species pool, as well as a separate discard pile for when plants die and are not returned to the species pool.
Each plant player randomly draws one species from the species pool and returns to the line of opposing soil players. Record the abundance of each plant species in the community at this initial time point (T0). These abundances are recorded on multiple graphs (one graph for each time point; Figure 2A). Note: the game can be purposefully set at different levels of diversity at T0.
After setup, the gameplay begins:
Plant players approach a random soil player to begin the conditioning phase that determines how plants change the physical, chemical, and biotic components of their soils on the basis of traits related to their identity. For example, ash leaves have lower ratios of carbon (C) to nitrogen (N) than spruce needles, which affects the quality of litter inputs to the soil and structures decomposer communities. To simulate conditioning, plant players hand their species to soil players, and soil players paperclip the species behind their soil card to hide the species that conditioned them.
Plant players then randomly draw another species from the species pool, form a new line opposing the now conditioned soil players, and interact with a random soil player. It is important that plant players do not know the conditioned status of soil players before interacting with them (just as tree seedlings cannot preferentially choose more hospitable soil locations in a forest). In this interaction, plants approach soils and show their species identity; in response, soils reveal to the plants what species they have been conditioned with. If they match, the plant dies.
Plants that survive remain standing next to their respective soil player (no other plant can interact with soils that have a surviving plant). Plants that die discard their species in a separate pile (i.e., their genes do not get returned to the gene pool), before randomly redrawing species from the species pool to interact with the remaining unmatched soil players. This gameplay continues until all plants are surviving and matched with a soil player. At this point, pause the game to record diversity with species' abundances (Figure 1B).
Once diversity has been recorded, plants recondition soils with their current species identity, making soil players clip the new species over their previous species before the next round (repeating Step 3). Again, the new conditioned statuses of soils should be hidden. Repeat steps 4–6 until you have measured diversity for multiple generations (T0–T4, or longer).
Positive Mode is a variation of the game with rules to show how positive plant–soil feedbacks are unstable and reduce diversity as one species becomes monodominant (e.g., invasion success through allelopathy). To play in Positive Mode, one plant species must be designated to have positive soil interactions, while all other plant species continue playing in Negative Mode. This designated species survives in all soil types conditioned by all species (including its own), and no other plant species can survive in soils conditioned by the designated species (Figure 1B). For example, Ailanthus is an invasive tree species that produces toxic chemicals that inhibit the growth of nearby plants (Heisey, 1990). Therefore, if Ailanthus were designated to have positive interactions, they would survive in any soil no matter what plant species conditioned that soil. In addition, pine, spruce, and ash species would die if they interact with Ailanthus-conditioned soil or their respective soils.
Assessment, Feedback & Suggestions
For a follow-up activity, we asked students to relate the direction of plant–soil feedbacks (positive or negative) to the abundance of specific plant species and total species richness (using results from the primary literature: Packer & Clay, 2000; Klironomos, 2002; Bais et al., 2003; Mangan et al., 2010; Bennett et al., 2017). Students successfully predicted that species with negative feedbacks would be rarer in communities that could sustain a greater total number of species, and that species with positive plant–soil interactions would be more abundant in less diverse communities.
We also asked the following question before and after the activity: “What do plant–soil feedbacks make you think of?” Responses that included the words “fungi” or “mycorrhizae” increased by 75%, and the word “diversity” appeared only in post-activity responses (7 out of 26 responses) (Figure 2B). This suggests that students began to recognize how plant–soil relationships relate to biodiversity patterns.
The activity takes ~30 minutes to complete and preceded a brief lecture and small-group work in an upper-level ecology course (24 students, 18–25 years old). Depending on student level and module topic, instructors using this activity could discuss a range of mechanisms, such as soil nutrient depletion by the plant, mutualistic benefits from mycorrhizal fungi, or buildup of soilborne pathogens. Since plant–soil interactions have been explored in a variety of areas (van der Putten et al., 2013), the activity can be uniquely paired with different biology topics. For introductory students, shapes could be used in place of species to focus on the mechanics of feedback loops in nature.
This activity may be most applicable for small class sizes (20–40 students). In larger classes, it could be implemented as a demonstration with student volunteers or during discussion/laboratory sections. We found it best to use paper cutouts and notecards to drive home the role-playing aspects of the game. We have provided resources for teachers to print the species used in the current example (https://github.com/mvannuland/Species_supplies), but the activity is amenable to any suite of species (four or five plant species is the appropriate number for a small class). With an inexpensive, time-efficient, and engaging activity, we hope to enable teachers to encourage student understanding of prolific, but overlooked, forms of biological interactions that impact the diversity and functioning of ecosystems.