Biology Reference
In-Depth Information
BOX 3.2
A MODEL OF CHOICE BETWEEN BIG AND SMALL PREY
(CHARNOV, 1976B; KREBS
ET AL
., 1977)
Consider a predator which encounters two prey types, big prey
1
with energy
value
E
1
and handling time
h
1
, and small prey
2
with energy value
E
2
and
handling time
h
2
. The profitability of each prey (energy gain per unit handling
time) is
E/h
. Imagine that the big prey are more profitable, so:
EE
hh
1
>
2
1 2
How should the predator choose prey so as to maximize its overall rate of gain? Let
us assume that the predator has encountered a prey - should it eat it or ignore it?
(a) If it encounters prey
1
, it should obviously always eat it. Therefore, choice of
the more profitable prey
1
does not depend on the abundance of prey
2
.
(b) If it encounters prey
2
, it should eat it provided that:
Gain from eating > gain from rejection and searching for a more profitable prey
1
that is, if:
E E
hSh
2
>
1
(B3.2.1)
+
2
1
1
where
S
1
is the search time for prey
1
.
Re-arranging, the predator should eat prey
2
if:
Eh
S
>
12
−
h
(B3.2.2)
1
1
E
2
Thus the choice of the less profitable prey, prey
2
, does depend on the
abundance of the more profitable prey, prey
1
.
This model makes three predictions. Firstly, the predator should either just eat
prey
1
(specialize) or eat both prey
1
and prey
2
(generalize). Secondly, the decision
to specialize depends on
S
1
, not
S
2
. Thirdly, the switch from specializing on prey
1
to eating both prey should be sudden and should occur when
S
1
increases such
that Equation B3.2.2 is true. Only when the two sides of the equation are
exactly equal will it make no difference to the predator whether it eats one or
both types of prey.
An experiment which tested these predictions is illustrated in Fig. 3.5. The predators
were great tits and the prey were large and small pieces of mealworm. In order to control
precisely the predator's encounter rate with the large and small worms the experiment
involved the unusual step of making the prey move past the predator rather than vice
versa (Fig. 3.5a). The big worms in the experiment were twice as large as the small ones
(
E
1
/
E
2
= 2) and
h
1
and
h
2
could be accurately measured as the time needed for the bird
to pick up a worm and eat it. During the experiment the bird's encounter rate with large
