Biology Reference
In-Depth Information
We can see an example of this in action when people stand in line at the counters of a
supermarket. If all the serving clerks are equally efficient and all the customers are equal
in the time they require for service, then the lengths of all the lines should end up being
equal. If one line gets shorter, then customers would profit by joining it until its length
becomes the same as the others. Because everyone is free to join whichever line they like,
each person goes to the best place at the time, and the lines fill up in an ideal free way
with the result that every customer should have the same waiting time for service.
A simple model of
how competitors
should distribute
between habitats
Competing for food: sticklebacks and ducks
An equivalent in animals is Manfred Milinski's (1979) experiment with sticklebacks.
Six fish were put in a tank, and prey ( Daphnia ) were dropped into the water from a pipette
at either end. At one end prey were dropped into the tank at twice the rate of the other
end. The best place for one fish to go clearly depends on where all the others go. There
was no resource defence and Milinski found that the fish distributed themselves in the
ratio of the patch profitabilities, with four fish at the fast-rate end and two at the slow-
rate end. When the feeding regimes were reversed, the fish quickly redistributed
themselves so that four were again at the fast end (Fig. 5.2a). This is the only stable
distribution under ideal free conditions. With any other distribution it would pay an
individual to move. For example, if there were three fish at each end then one fish would
profit by moving from the slow to the fast-rate end. Once it had done so, it would then
not pay any of the other fish to move. In our supermarket analogy, this experiment is
equivalent to what should happen if one clerk is twice as efficient at serving customers
as another; the stable distribution would be for this line to be twice as long.
(a)
(b)
6
27
(i)
(ii)
24
5
21
4
18
15
3
Predicted
12
2
9
6
Observed
1
3
x
y
2
4
6
8
10
12
14
16
1
2
3
4
5
1
2
3
4
5
Time (min)
Time after start of experiment (min)
Fig. 5.2 (a) Milinski's (1979) feeding experiment with six sticklebacks. At time x , end B of the tank had twice
the amount of food as end A. At time y the profitabilities were reversed. The pale blue lines indicate the number
of fish predicted at end A according to ideal free theory, and the points and dark blue line are the observed
numbers (mean of several experiments). (b) Harper's (1982) feeding experiment with a flock of 33 mallard
ducks. In (i) food was thrown into the pond at twice the rate at site A compared to site B. In (ii) the profitabilities
were reversed. Pale blue lines indicate the predicted numbers at site A according to ideal free theory. The points
and dark blue line are the observed numbers (means of many experiments).
 
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