waders and their estuarine food supplies - Vlaams Instituut voor de ...

OPTIMAL FORAGING AND THE FUNCTIONAL RESPONSE
DOES AN OPTIMALLY FORAGING OYSTERCATCHER OBEY
THE FUNCTIONAL RESPONSE?
We describe an experimental test of the optimal diet model. An Oystercatcher was offered bivalves Scrobicularia
plana which were buried in the substrate at different depths. All prey were of equal lengih and
thus of equal energy content. Since handling time increased with depth, deep-lying prey were less profitable.
From the known increase of handling and search time with depth we were able to predict for each
prey density which depth classes should be ignored by the bird so as to maximize its intake rate. The observed
depth selection and intake rate were as predicted for the lower prey densities. At high prey densities
the observed number of prey taken rose above the predicted intake rate. We suggest that the bird
started to ignore closed bivalves with long handling times. The observed relationship between prey density
and intake rate could not be described by Holling's type-2 functional response, because the handling
time and the rate of discovery were density-related. A multi-species fractional response is necessary to
describe the intake rate of an optimally feeding predator.
Introduction
The functional response, the predation tale of an individual
predator related to differences in prey density,
has been much studied during Ihe last decades. In the
simple one-prey situation, the intake rate of the predator
often rises decelerating to a plateau (type-2 functional
response: Holling 1959). The curve has been described
by the disc equation:
•y
/
aD
1 + aDh
(Holling 1959, «l)
where N = number of prey taken, T= time predator and
prey are exposed to one another, a = instantaneous rate
of discovery, D = prey density and h = handling time.
Equation (I) can be simplified to:
T \+Xli
(2)
where X = encounter rate: the inverse of searching time
per prey item (X = aD).
This one-prey model is based on the assumption
thai a predator takes every prey it encounters. However,
predators are usually faced with a number of prey
139
lypes from which they select their diet. When there are
/ prey types. Eq (2) can be rewritten as:
\+lXhP
(Charnov 1976) (3)
where E = energy intake. E. = energy from one item of
prey type i. P( - probability the predator takes a prey
item of type i after it is encountered, X( = encounter
rate with prey type / and ht = handling time of prey
type i. It can be shown that to maximize the energy intake
rate. P = I if E/T < E/h, and P = 0 if E/T > E/lxi
(Charnov 1976). So a predator should ignore a certain
prey type when its profitability is below E/T and
should take it if F./hi is above that level. This prediction
of the optimal diet model has now been confirmed
in a lot of studies i see review by Krebs et al. 1983).
If the intake rate increases (because of rising prey
densit] for instance), the model predicts exactly when
a prey type should be dropped from the optimal set of
prey types. This has been found in some studies (e.g.
Davidson 1978. Pulliam 1980. Zwarts & Drent 1981).
However, the functional response cannot yet be predicted
quantitatively from the optimal set of prey
types, since the encounter rate with the different prey
types must also be known before Eq (3) can be solved.