Biology Reference
In-Depth Information
Some individuals
may be better
competitors
Geoff Parker and Bill Sutherland (1986) pointed out that it might be difficult from the
numerical prediction alone to distinguish between the simple ideal free distribution
with equal competitors and one with unequal competitors, which they call the
'competitive unit' model, because it hypothesizes that the number of 'competitive units',
rather than the number of individuals, is equalized across patches. If one individual can
consume resources twice as rapidly as another it scores twice the number of competitive
units. The difficulty in distinguishing the two versions of the ideal free distribution
arises from the fact that, by chance alone, the competitive unit distribution will often
tend to look like the simple ideal free distribution (Fig. 5.5). This may be why many
studies appear to support the numerical prediction of the ideal free distribution, even
though competitors in these studies were unequal.
A good example which shows features of both the resource defence and ideal free
models is the study by Thomas Whitham (1978, 1979, 1980) of habitat selection in the
aphid (
Pemphigus betae
). In the spring, females known as 'stem mothers', settle on leaves
of narrowleaf cottonwood (
Populus angustifolia
) to feed and they become entombed by
expanding leaf tissue, so forming a gall. A stem mother reproduces parthenogenetically
and the number of progeny she produces depends on the quantity and quality of the
juices she can tap from the leaf. The largest leaves provide the richest supplies of vascular
sap and result in the greatest reproductive success, with up to seven times the number
of progeny that are produced by settling on a small leaf. As we would expect, all the
large leaves are quickly occupied, so additional settlers have the problem of whether to
settle on large leaves and share the resources or occupy smaller leaves alone.
Whitham made measurements of reproductive success which enabled him to
plot a family of fitness curves for habitats of varying quality (leaves of different
sizes) and with different densities of competitors (number of females (
The 'competitive
unit' model
Gall aphids: a test
of the competitive
unit model
galls) per
leaf). Figure 5.6a shows the results, which enable us to draw three conclusions. Firstly,
for any competitor density, the average reproductive success increases with habitat
quality. Secondly, within a habitat of a certain quality, success decreases as the
number of competitors increases. This shows that stem mothers settling on the same
leaf must compete with each other for resources. Thirdly, if the
average
reproductive
success is calculated for aphids which are alone on a leaf, those who share a leaf
with one other and those who share with two others, no significant differences are
found. There was also no significant difference in average success on leaves with
different numbers of competitors when other fitness measures were used, such as
body weight of the stem mother, abortion rate, development rate or predation.
The results support the predictions of the ideal free model. The conclusion, therefore,
is that the stem mothers settle on leaves of different sizes such that the average success
in good habitats with a high density of competitors is the same as in poor habitats
with fewer competitors.
However, although the results of average success on different sized leaves are in
accord with ideal free predictions, within a habitat not all individuals get equal rewards.
This is because a leaf is not a homogeneous habitat. The best place to be is on the mid-
rib at the base of the leaf blade because everything translocated into and out of the leaf
must flow past this point. Basal galls on a leaf give rise to more young than distal galls
and the stem mothers spar with each other, like boxers in a ring, for occupancy of these
prime positions (Fig. 5.6b). As we would predict from the defence model, if a basal
individual is removed her place is quickly occupied by another aphid from a distal site.
=
Average success is
equal on leaves of
different quality,
but individuals
near the leaf base
do better
