ANALYSIS O F PROCESSES I N CONTROL O F INSECTS
3
of populations, and the elementary biological thinking that should
guide us in these matters, are of prime importance. The questions
involved include the following, which I propose to discuss in later
pages. How can the roles played by different factors and sorts of factors
in natural control be estimated? How can the presence of regulation
be detected, and how measured? How is the effect of a mortality factor
changed when it is preceded or followed by other mortalities of various
types? I n what ways is the action of predators and insect parasites
related to the density of the prey? Can different aspects of this action
be considered separately?
Methods of attacking these questions will be illustrated as far as
possible by use of published data from field investigations, but in some
cases by means of hypothetical examples. My aim will be to deal with
the methods and examples in their simplest forms. Simple procedures
based on elementary ideas are not only easily assimilated; their
implications are relatively clear, and they are amenable to development in various directions to meet the needs of particular investigations.
I shall not deal with some of the more sophisticated methods and models
which forego some or all of these advantages in the interests of specialization for a particular set of circumstances. This does not imply any
depreciation of the making of mathematical models, an important
aspect of population dynamics which has recently undergone vigorous
development, as may be seen from the papers of Watt (1961, 1962),
Holling (1962) and others. I agree with Watt’s view that in the study
of insect populations, as already in fisheries research, this sort of theory
is likely soon to become a major means of advance, the more so to the
extent that the models are kept in touch with field data, and field
investigations are organized in such a way as to use and test the models.
But the present article will deal rather with the elements from which
complex models may be constructed.
Theories of natural control involve assumptions that must be tested
by observation or experiment if the theories are to be seriously employed.
This aspect of the relationship between theory and practice emerges
explicitly in Section VI, in connection with the influence of predators
and parasites upon insect populations.
Thus the article concentrates on a few aspects of a wide subject,
and refers to other aspects only briefly or not at all. Within the chosen
topics, selection has often been necessary, and sometimes inescapably
arbitrary .
11. CONCEPTS AND TERMS
We must begin with the truism that the numbers of animals in
natural populations are limited - strikingly so in view of the high rates
3
of populations, and the elementary biological thinking that should
guide us in these matters, are of prime importance. The questions
involved include the following, which I propose to discuss in later
pages. How can the roles played by different factors and sorts of factors
in natural control be estimated? How can the presence of regulation
be detected, and how measured? How is the effect of a mortality factor
changed when it is preceded or followed by other mortalities of various
types? I n what ways is the action of predators and insect parasites
related to the density of the prey? Can different aspects of this action
be considered separately?
Methods of attacking these questions will be illustrated as far as
possible by use of published data from field investigations, but in some
cases by means of hypothetical examples. My aim will be to deal with
the methods and examples in their simplest forms. Simple procedures
based on elementary ideas are not only easily assimilated; their
implications are relatively clear, and they are amenable to development in various directions to meet the needs of particular investigations.
I shall not deal with some of the more sophisticated methods and models
which forego some or all of these advantages in the interests of specialization for a particular set of circumstances. This does not imply any
depreciation of the making of mathematical models, an important
aspect of population dynamics which has recently undergone vigorous
development, as may be seen from the papers of Watt (1961, 1962),
Holling (1962) and others. I agree with Watt’s view that in the study
of insect populations, as already in fisheries research, this sort of theory
is likely soon to become a major means of advance, the more so to the
extent that the models are kept in touch with field data, and field
investigations are organized in such a way as to use and test the models.
But the present article will deal rather with the elements from which
complex models may be constructed.
Theories of natural control involve assumptions that must be tested
by observation or experiment if the theories are to be seriously employed.
This aspect of the relationship between theory and practice emerges
explicitly in Section VI, in connection with the influence of predators
and parasites upon insect populations.
Thus the article concentrates on a few aspects of a wide subject,
and refers to other aspects only briefly or not at all. Within the chosen
topics, selection has often been necessary, and sometimes inescapably
arbitrary .
11. CONCEPTS AND TERMS
We must begin with the truism that the numbers of animals in
natural populations are limited - strikingly so in view of the high rates