Biology Reference
In-Depth Information
The wagtail study illustrates two general points. Firstly, it is an example of how
apparently different kinds of costs and benefits (defence and feeding) can sometimes be
reduced to a single currency - feeding rate in this case. Secondly, it shows that one
advantage of group living is shared resource defence. The wagtail groups were never
larger than two, but the same argument could be generalized to larger groups (Brown,
1982). In Chapter 6 we will come back to the costs and benefits of group living.
Pied wagtails gain
from territory
sharing when
food renewal is
rapid and
intrusion rate is
high
Producers and scroungers
Competition for scarce resources often leads to variable competitive behaviour within a
population. We will consider two hypotheses for how such variation can come about.
Imagine, for example, that there are two foraging alternatives: producers make food
available by digging or otherwise exposing prey, while scroungers steal the food found
by the producers. How could a mixture of producers and scroungers be maintained?
One possibility is that producers are the better quality competitors and scroungers
have to 'make the best of a bad job', settling for a technique with a poorer pay-off
because of their inferior competitive ability. As the proportion of scroungers in the
population increases, the fitness of a producer will decline (more of its food is stolen)
and the fitness of a scrounger will also decline (more competition for stealing). However,
producers always do better than scroungers (Fig. 5.9a).
(b)
(a)
producer
producer
scrounger
scrounger
x
Proportion of scroungers
Fig. 5.9
Two models for how a mixture of producers and scroungers may be
maintained in a population. (a) As the proportion of scroungers increases, both
producers and scroungers have declining fitness, but producers always do best.
Scroungers are poorer quality competitors. (b) There may be no difference in competitive
ability. Each behaviour does best when rare. The stable equilibrium frequency of the two
is at x, where producers and scroungers have equal fitness. Model (b) after Barnard and
Sibly (1981). With permission from Elsevier.
