Resource gradients create energy trade‐offs in the inducible defense response of <i>Paramecium aurelia</i>

Abstract Ecological communities are structured by inter‐ and intraspecific interactions. The stability of communities is governed by the strength and number of species interactions. Therefore, mechanisms that act to decrease the interaction strength between species are thought to stabilize communities. One proposed stabilizing mechanism is the inducible defenses of prey. Inducible defenses are morphological and behavioral traits only expressed in the presence of a predator or under perceived predation threat. By creating a population of prey that is less susceptible to predation, inducible defenses reduce interaction strength between predators and prey. In this study, we evaluated the expression of an inducible defense in the protozoan, Paramecium aurelia , along two environmental gradients (perceived predation threat and basal nutrients) and for three clonal populations of P. aurelia . The three clonal populations display a range in expression of one morphological defense (i.e., increase in body width), with one population expected to display an inducible defense, one expected to remain in a permanently defended state, and one expected to remain in a permanently undefended state. We found that for the clone that displays an inducible defense, there was a trade‐off in energy expenditure that occurred at around 30 predators in which protists in high nutrient concentrations were narrower than protists in low nutrient conditions despite additional resources. These results indicate either that there are potential costs to inducible defenses or that energy gained from increased nutrients was being diverted elsewhere, such as for reproduction or maintenance. Additionally, for all three clonal populations, protist width approached an asymptote of max width at the highest nutrient concentrations, suggesting a diminishing or maximal benefit of protist width even if a trade‐off did not occur. In all, the results from our study provide information on the potential limits to the benefits of inducible defenses along two environmental gradients, and that both bottom‐up and top‐down pressures interact to significantly affect the response of prey in predator–prey interactions. With an increased understanding of how bottom‐up and top‐down pressures are affecting species' interactions, we can better predict changes in community composition and stability under changing environmental conditions.