Teaching about regulation of species composition I: An interactive classroom simulation of recruitment limitation and the role of generalist predators

Undergraduate ecology courses usually include a discussion of factors that regulate community structure, especially those that affect species diversity and composition.  Factors that limit species diversity (e.g., interspecific competition) are contrasted with those that promote it (e.g., primary productivity), and equilibrium factors like competition and predation are contrasted with nonequilibrium ones like physical disturbance. Many undergraduates struggle with the complexity of how these factors might interact.  Fewer than 30% of our students initially understand the effect of generalist predators on species diversity, for example. In addition, most introductory texts include no discussion of spatially-explicit factors like recruitment limitation (failure of superior competitors to reach all colonization sites, e.g. due to limited dispersal), even though theory predicts that it can promote the coexistence of virtually infinite numbers of competitors. To help students in our sophomore-level Ecology and Evolutionary Biology course (ca. 60-80 students) understand the roles of limited dispersal and generalist predators in mediating coexistence of competing species, we developed an interactive exercise, to be demonstrated during this presentation, that models stochastic effects of seed dispersal and seed predation. We use our own research on the pioneer plant guild in a Costa Rican cloud forest as a background example, and the simulation uses M&M’s as seeds and the students as seed predators. Results of the simulation show that both dispersal and random predation increase spatial heterogeneity among potential colonists of new treefall gaps, and hence facilitate coexistence among pioneer species even where a strict competitive hierarchy exists.