Biology Reference
In-Depth Information
a view supported by studies on immune responses of human and domestic
hosts to parasites. However, the latter systems are not natural in the sense of
representing naturally occurring parasite population sizes. Furthermore, the
important point made by authors most critical of the importance of equili-
bria and competitive effects is that most habitats are unfavourable, and that
our views have been distorted because there is a tendency to select systems
for study that occur under favourable conditions (e.g., snail populations that
are infected with a diverse trematode fauna, and not those that contain
none or few trematodes and are too boring to be studied (see pp. 131-134).
More generally, species, populations, and sites studied are not selected
at random, but results from these studies are nevertheless treated as
representative.
Nonequilibrium in populations and metapopulations: some
empirical studies
In the previous section I discussed experimental evidence for equilibrium
conditions, in this section I discuss some examples that suggest non-
equilibrium.
As mentioned earlier, Cappucino (
1995
) defined nonequilibrium in
a very narrow way, claiming that only randomly walking populations
should be considered unambiguously nonequilibrial. Time to extinction
of random-walking populations may be very long (Nisbet and Gurney
1982
; Murdoch
1994
), or almost infinite (Middleton
1993
; Murdoch
1994
). But even in populations that are not random-walking, i.e., that
reach equilibrium when not disturbed, apparent nonequilibrium after
disturbance may last for long periods. Alternatively, disturbances may
be so frequent that even in fast-recovering populations equilibrium may
seldom be obtained. For many species, nonequilibrium conditions resulting
from disturbances are perhaps so long-lasting that equilibrium will never
be established (Murdoch
1994
). I will now discuss some examples; some
further, more detailed, examples are discussed in Chapter
7
.
A very well documented and analysed example of persistent instability in
an animal population is that of the Soay sheep, Ovis aries, on Hirta in the
St. Kilda archipelago, discussed by Clutton-Brock et al.(
1991
; see also
Clutton-Brock and Pemberton
2004
). The sheep were originally intro-
duced into the archipelago from Europe. After evacuation of the human
population, 107 sheep were introduced into Hirta, the largest of the islands
(638 ha), in 1932. Numbers increased rapidly (to about 650-700 by 1948).
Between 1955 and 1990, sheep were counted each summer, but the
