Summary: Functional response and prey defence level in an
experimental predatorprey system
Res Altwegg,* Margaret Eng, Stephanie Caspersen and Bradley R. Anholt
Department of Biology, University of Victoria, PO Box 3020, Victoria,
British Columbia V8W 3N5, Canada
Questions: How do defences expressed only in the presence of predators (inducible defences)
affect the relationship between the number of prey eaten and prey density (the predator's
functional response)? What is the relationship between prey defence level and vulnerability?
Do inducible defences show features that are likely to stabilize predatorprey dynamics on
Organisms: We conducted experiments in a laboratory system. The prey was the protist
Euplotes octocarinatus, exposing different levels of a morphological inducible defence. The
predator was the turbellarian Stenostomum virginianum.
Methods: We analysed the data using non-linear mixed effects models that combine
non-linear curve fitting with random effects.
Results: This predator's functional response was a sigmoid Holling-Type III. The induced
defence lowered the maximum number of prey that predators ate, suggesting that the defence
increased the predator's handling time. The level of defence expressed by Euplotes depended on
the level of predator cue. There was a negative exponential decline in the number of prey eaten