Behavioural mechanisms underlying functional response of sea stars Asterias vulgaris preying on juvenile sea scallops Placopecten magellanicus

Abstract

The functional response characterises the relationship between prey density and the consumption rate of individual predators. Typically, it is studied by fitting a model to observations of predation rate at different prey densities. The behavioural mechanisms underlying a functional response are not well understood, and estimates of model parameters seldom conform to observations of behaviour. We have developed a mechanistic approach that directly incorporates behavioural observations into characterisation of the functional response. Laboratory experiments were used to record predation rates and observe foraging behaviour of sea stars Asterias vulgaris preying on juve- nile sea scallops Placopecten magellanicus at different densities. Experiments were conducted in tanks with no sediment and tanks with sediment. Behavioural data from the experiments were used to calculate parameters of functional response models Th (handling time per prey) and a (rate of suc- cessful search). On both substrates, Th remained constant across prey density, while a was density- dependent. An inverse quadratic was used to describe a and was incorporated into a functional response model. Estimates of a were also obtained by fitting the functional response model to the pre- dation rate data using regression analysis. These estimates of a were highly consistent with the esti- mates calculated from behavioural data. On both substrates, sea stars preying on scallops had a Type III sigmoid-shaped functional response; on sediment, predation rate decreased at high prey densities. Sea star ability to capture attacked prey was probably the mechanism underlying the observed responses. In general, behavioural information can lead to better understanding of observed func- tional responses.