Biology Reference
In-Depth Information
(a)
180
Leaf size
(cm
2
)
18.9
160
16.6
140
14.6
120
(b)
100
12.3
80
10.2
60
8.1
1
2
3
Number of stem mothers per leaf
Fig. 5.6
(a) The thin lines are a family of fitness curves for habitats of varying quality
(leaf size) and competitor density (no. stem mothers per leaf ) in the aphid
Pemphigus
betae
. The pale blue horizontal line is the average success for one, two and three stem
mothers per leaf. See text for explanation. From Whitham (1980). (b) Stem mother
aphids fight for prime positions on a leaf by kicking and pushing. The winner will settle
at the base of the mid-rib where food is richest. From Whitham (1979). Reprinted with
permission from the Nature Publishing Group.
The economics of resource defence
Some animals, as we have seen, compete for resources by exploitation, others by defence.
Is it possible to predict when the latter form of competition should be adopted instead of
the former?
Economic defendability
Jerram Brown (1964) first introduced the idea of economic defendability. He pointed
out that defence of a resource has costs (energy expenditure, risk of injury and so on) as
well as the benefits of priority of access to the resource. Territorial behaviour should be
favoured by selection whenever the benefits are greater than the costs (Fig. 5.7). This
idea led field workers to look in more detail at the time budgets of territorial animals.
Frank Gill and Larry Wolf (1975), measured the nectar content of territories of the
golden-winged sunbird (
Nectarinia reichenowi
) in East Africa, where it defends patches
of
Leonotis
flowers outside the breeding season. They also calculated from time budget
studies and laboratory measurements of the energetic costs of different activities, such
as flight, sitting and fighting, how much energy a sunbird expends in a day. When the
daily costs were compared with the extra nectar gained by defending a territory and
excluding competitors, it turned out that the territorial birds were making a net
energetic profit. Therefore, the resource was economically defendable (Box 5.1).
Costs and benefits
of territory
defence
