26
M. E. SOLOMON
to be largely determined by the weather of the preceeding autumn, but
sometimes modified by conditions during early spring. Andrewartha
and Birch (1954) discussed the work in full and concluded that since
they had shown most of the variation was due to weather, there was
no room for density-dependent regulation. They claimed that the
abundance each year was determined by the previous density and the
influence of density-independent factors, favourable and unfavourable.
The conclusions of Andrewartha and Birch (Zoc. cit.) have been
subjected to a series of adverse criticisms on the grounds that they
are extremely improbable or that density-dependence can be demonstrated in the data, or both, by Solomon (1957), Kuenen (1958),
Nicholson (1958), Smith (1961; see also Andrewartha, 1963 and Smith,
1963), Klomp (1962) and Varley (1963). The question I raised was how,
without regulation, did the population fluctuate about the same level
over the years, instead of drifting upwards or downwards persistently?
Nicholson (Zoc. cit.) went into this matter more fully and argued that
if the “peak numbers” were highly correlated with weather conditions,
then the implication was “that the weather-induced changes in numbers
of thrips took place each year from some approximately constant
number”. Therefore, some factor “must have adjusted the intensity
of its action in relation to the numbers coming under its influence, for
these varied greatly, from year to year. I n short, a density-governing
factor is required to operate in this situation”. Klomp (1962) elaborated
the same point to good effect with the aid of diagrams.
In simple general terms, the argument may be summarized as follows :
if a persistently good correlation is found between a seasonal weather
index and density, this implies that the weather effects operated each
year on a population starting from some fairly constant base level of
density, a feature that can be maintained only by some form of regulation.
c. ASSESSING’ THE ROLE OF PARTICULAR FACTORS
1. By Direct Inspection of the Relationship between Effect and Density
I have already referred to the detection of density-relationships in
rates of mortality, reproduction and net increase (e.g. as in Fig. 1).
The rate of loss by dispersal or gain by immigration can be examined
in the same way when such movements occur, since it cannot be taken
for granted that they are density-independent.
The next step may be to examine the density-relationship of the effect
of particular factors on one or more of the above-mentioned processes.
For example, we may enquire into the effect of a particular predator,
or of several species together, upon the species we are studying, to
M. E. SOLOMON
to be largely determined by the weather of the preceeding autumn, but
sometimes modified by conditions during early spring. Andrewartha
and Birch (1954) discussed the work in full and concluded that since
they had shown most of the variation was due to weather, there was
no room for density-dependent regulation. They claimed that the
abundance each year was determined by the previous density and the
influence of density-independent factors, favourable and unfavourable.
The conclusions of Andrewartha and Birch (Zoc. cit.) have been
subjected to a series of adverse criticisms on the grounds that they
are extremely improbable or that density-dependence can be demonstrated in the data, or both, by Solomon (1957), Kuenen (1958),
Nicholson (1958), Smith (1961; see also Andrewartha, 1963 and Smith,
1963), Klomp (1962) and Varley (1963). The question I raised was how,
without regulation, did the population fluctuate about the same level
over the years, instead of drifting upwards or downwards persistently?
Nicholson (Zoc. cit.) went into this matter more fully and argued that
if the “peak numbers” were highly correlated with weather conditions,
then the implication was “that the weather-induced changes in numbers
of thrips took place each year from some approximately constant
number”. Therefore, some factor “must have adjusted the intensity
of its action in relation to the numbers coming under its influence, for
these varied greatly, from year to year. I n short, a density-governing
factor is required to operate in this situation”. Klomp (1962) elaborated
the same point to good effect with the aid of diagrams.
In simple general terms, the argument may be summarized as follows :
if a persistently good correlation is found between a seasonal weather
index and density, this implies that the weather effects operated each
year on a population starting from some fairly constant base level of
density, a feature that can be maintained only by some form of regulation.
c. ASSESSING’ THE ROLE OF PARTICULAR FACTORS
1. By Direct Inspection of the Relationship between Effect and Density
I have already referred to the detection of density-relationships in
rates of mortality, reproduction and net increase (e.g. as in Fig. 1).
The rate of loss by dispersal or gain by immigration can be examined
in the same way when such movements occur, since it cannot be taken
for granted that they are density-independent.
The next step may be to examine the density-relationship of the effect
of particular factors on one or more of the above-mentioned processes.
For example, we may enquire into the effect of a particular predator,
or of several species together, upon the species we are studying, to