Biology Reference
In-Depth Information
regulated and not nonequilibrial. Moreover, if all subpopulations of a
metapopulation are randomly walking, the metapopulation as a whole
must also randomly walk to extinction (Chesson
1981
).
An important discussion of equilibrium in nature is provided by
Cooper (
2001
), who looks critically at the various arguments given by
different authors in favour of equilibrium. He distinguishes two ''balance of
nature'' arguments in population/community ecology. The first argument
is based on the assumption that there is a strong tendency towards
constancy in population size. But how much variability is allowed before
the population is no longer considered to conform to this alleged
constancy? The second argument has been developed to overcome this
difficulty. It simply assumes that the balance is represented by populations
tending to persist. But this persistence must be the result of some kind of
regulation. The reason is that random-walking populations must become
extinct over time; therefore, those that do persist must be regulated in
some way, in the sense that they must display statistical stability over time.
Cooper refers to several authors who have proposed explanations of
stability (i.e., May
1973
; Chesson
1981
,
1982
; Murdoch
1994
; Dennis
and Taper
1994
;Turchin
1995
). The last two papers discuss the problem in
terms of stationary probability distributions (SPD) in population density-
time series. Population densities are assumed to fluctuate around a mean
density level, and the variance of fluctuations is bounded in the long
term. Turchin (
1995
) defines equilibrium (or being regulated) as exist-
ence of an SPD. Since most populations persist, i.e. have an SDP, they
must be in equilibrium. In other words, they must be regulated. Also,
according to this argument, regulation implies density dependence,
because populations would either become extinct or increase ad infinitum,
if density-dependent factors did not operate (May
1986
). Most likely,
density-dependent factors are competitive ones, at least if predation and
other such influences do not limit population density to a level below
which competitive effects can become operative. However, as pointed
out by Cooper (
2001
) and others to whom he refers, all populations
become extinct sooner or later, and one would have to demonstrate that
populations persist longer than expected if they simply executed random
walks. Furthermore, a tendency to return may only be a necessary but not
a sufficient condition for equilibrium (or regulation), i.e., empirical
evidence for the occurrence of regulation has still to be given. In other
words, Cooper argues that the inclination of many (if not most authors) to
use a-priori arguments for a balance in nature (equilibrium, regulation)
is ill advised.
