The effects of inducible defenses on population stability in Paramecium aurelia.

Predator-prey dynamics have been studied across many different systems over the past 80 years. The outcomes of this past research have yielded useful theoretical and empirical models of predator-prey systems. However, what stabilizes predator-prey dynamics is often debated and not well understood. One proposed stabilizing mechanism is that the inducible defenses of prey decrease predation risk by creating a portion of the prey population that is invulnerable to predation, leading to a reduction in trophic interaction strength. We investigated the potential stabilizing effects of inducible morphological defenses in the protozoan, Paramecium aurelia, across a range of nutrient concentrations to better understand a potential stabilizing mechanism of systems under nutrient enrichment (Paradox of Enrichment). Using P. aurelia clones that differ in their ability to induce defenses, we found that the most susceptible clone that does not express any known inducible defense showed reduced survival along a gradient of increasing nutrient concentrations. Clones expressing either inducible or permanent morphological defenses (increasing body width in response to predation threat) were not significantly affected by increasing nutrients demonstrating a potential benefit of these defenses. However, when evaluating population stability (coefficient of variation) rather than survival, we found a stabilizing effect of increasing nutrients on all P. aurelia populations. Our results demonstrate varied effects of increasing nutrients on population stability depending on the level of defense expression and stability metric used. Our results reinforce that choice of stability metric can alter conclusions about population stability and persistence, highlighting the need to adopt multiple metrics and approaches.