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Zusammenfassung:
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.
