10
M. E . SOLOMON
factors in actual situations, and of assessing their impacts on populations. This matter is taken up in the following Section.
IV. ESTIMATINQ THE ROLES OF DIFFERENT FACTORS
AND PROCESSES IN NATURAL CONTROL
In Section 111, natural control was represented as a three-fold process
- regulation, modification of this, and variation not related to density.
Density-independent variations in abundance may be imposed by many
Werent types of factors, but, particularly with invertebrates that are
exposed to its influence, weather is the predominant cause of such
variations. Weather influences the state of affairs in the environment
in many ways, and to a great extent “sets the stage” for the processes
of population dynamics, including regulation ; it partly determines the
capacity of the environment with respect to the animals that live in it
(Solomon, 1949). It often influences abundance directly by periodically
killing a fraction of the population, and often determines the favourable
conditions that enable a population to increase rapidly in abundance.
We may examine the correlations between weather conditions and population increase, decrease and abundance; this has been a major preoccupation of field entomologists, and is obviously necessary and
important. Yet, from the point of view of an ecologist wishing to understand natural control, it is only a preliminary stage. He must go on to
discover how abundance is regulated, If he succeeds in this, he will be
able to explain how it is that, in spite of great powers of increase most
animals can display under favourable conditions, and in spite of the
fact that the action of the weather often bears no significant relationship
to the level of abundance at the time, yet variation remains within
definite limits, and populations are maintained in a loose state of
equilibrium: most of them neither die out nor maintain a net upward
trend of abundance. Since I have set out this argument at sufficient
length elsewhere (Solomon, 1957, 1962a), and others have done likewise,
it seems unnecessary to dwell upon it here. A more elaborate study of the
matter, using namerical examples, has been published by Klomp (1962).
A. VARIATION A N D ITS CAUSES
1. Amplitude of Fluctuation
Violent fluctuations in abundance are a characteristic of many insect
populations. The red locust, Nomadacris septemfasciata (Serville), in a
self-contained, controlled outbreak in Africa has been shown t o vary
in annual abundance by a factor of over 750 times (Gunn and Symmons,
1959); the increase over two breeding seasons, 1929 and 1930, in the
outbreak areas has been estimated at x 140; this was followed by an
M. E . SOLOMON
factors in actual situations, and of assessing their impacts on populations. This matter is taken up in the following Section.
IV. ESTIMATINQ THE ROLES OF DIFFERENT FACTORS
AND PROCESSES IN NATURAL CONTROL
In Section 111, natural control was represented as a three-fold process
- regulation, modification of this, and variation not related to density.
Density-independent variations in abundance may be imposed by many
Werent types of factors, but, particularly with invertebrates that are
exposed to its influence, weather is the predominant cause of such
variations. Weather influences the state of affairs in the environment
in many ways, and to a great extent “sets the stage” for the processes
of population dynamics, including regulation ; it partly determines the
capacity of the environment with respect to the animals that live in it
(Solomon, 1949). It often influences abundance directly by periodically
killing a fraction of the population, and often determines the favourable
conditions that enable a population to increase rapidly in abundance.
We may examine the correlations between weather conditions and population increase, decrease and abundance; this has been a major preoccupation of field entomologists, and is obviously necessary and
important. Yet, from the point of view of an ecologist wishing to understand natural control, it is only a preliminary stage. He must go on to
discover how abundance is regulated, If he succeeds in this, he will be
able to explain how it is that, in spite of great powers of increase most
animals can display under favourable conditions, and in spite of the
fact that the action of the weather often bears no significant relationship
to the level of abundance at the time, yet variation remains within
definite limits, and populations are maintained in a loose state of
equilibrium: most of them neither die out nor maintain a net upward
trend of abundance. Since I have set out this argument at sufficient
length elsewhere (Solomon, 1957, 1962a), and others have done likewise,
it seems unnecessary to dwell upon it here. A more elaborate study of the
matter, using namerical examples, has been published by Klomp (1962).
A. VARIATION A N D ITS CAUSES
1. Amplitude of Fluctuation
Violent fluctuations in abundance are a characteristic of many insect
populations. The red locust, Nomadacris septemfasciata (Serville), in a
self-contained, controlled outbreak in Africa has been shown t o vary
in annual abundance by a factor of over 750 times (Gunn and Symmons,
1959); the increase over two breeding seasons, 1929 and 1930, in the
outbreak areas has been estimated at x 140; this was followed by an