title: Density-dependent adjustment of inducible defenses.
abstract: Predation is a major factor driving evolution, and organisms have evolved adaptations increasing their survival chances. However, most defenses incur trade-offs between benefits and costs. Many organisms save costs by employing inducible defenses as responses to fluctuating predation risk. The level of defense often increases with predator densities. However, individual predation risk should not only depend on predator density but also on the density of conspecifics. If the predator has a saturating functional response one would predict a negative correlation between prey density and individual predation risk and hence defense expression. Here, we tested this hypothesis using six model systems, covering a taxonomic range from protozoa to rotifers and crustaceans. In all six systems, we found that the level of defense expression increased with predator density but decreased with prey density. In one of our systems, i.e. in Daphnia, we further show that the response to prey density is triggered by a chemical cue released by conspecifics and congeners. Our results indicate that organisms adjust the degree of defense to the acute predation risk, rather than merely to predators' densities. Our study suggests that density-dependent defense expression reflects accurate predation-risk assessment and is a general principle in many inducible-defense systems.
authors.affiliation: Department of Animal Ecology, Evolution and Biodiversity, Ruhr-University Bochum, Germany.
authors.forename: Ralph
authors.initials: R
authors.lastname: Tollrian
authors.affiliation: Section of Evolutionary Ecology, Department Biology II, Ludwig-Maximilian-University Munich, Germany.
authors.forename: Sonja
authors.initials: S
authors.lastname: Duggen
authors.affiliation: Department of Animal Ecology, Evolution and Biodiversity, Ruhr-University Bochum, Germany.
authors.forename: Linda C
authors.initials: LC
authors.lastname: Weiss
authors.affiliation: Animal Ecology I and BayCEER, University of Bayreuth, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Laforsch
authors.affiliation: Department of Physiological Ecology, Max-Planck-Institute of Limnology, Plön, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Kopp
mesh_terms: D000818:Animals; D016798:Ciliophora / Q000502:physiology; D003463:Cues; D003621:Daphnia / Q000502:physiology; D008954:Models, Biological; D011156:Population Density; D011235:Predatory Behavior; D012403:Rotifera / Q000502:physiology; D013045:Species Specificity