8
M. E. SOLOMON
be distinguished as a density relationship. Thus, if we find that a
population suffers a density-dependent mortality, and identify a
predatory population as the cause of this, the predators or their
predation constitute a density-dependent factor, the predation is a
density-dependent process, and the process can be regarded as the
expression of a density-dependent relationship. The use of these
different terms need imply no more than choosing the words appropriate
to the context.
The term population, as I use it in this paper, simply means any
group of animals, usually of one species, that can conveniently be
considered as a unit. For the present purposes we may assume that all
individuals of the same species and of a particular stage of development
are equivalent, although in practice it is desirable, when possible, to
take account of differences in age and genetics. In practice, males and
females may sometimes be differently involved in density relationships.
When different developmental stages occur together, one should take
account of the numbers of each stage separately.
111. THREE TYPES OF PROCESSES INFLUENCING ABUNDANCE
On an elementary and fundamental level one can make a three-fold
classification of the processes involved in natural control. The three
categories are (a) regulation by density-dependent processes, ( 6 ) modification of the regulatory processes, and (c) imposition of changes in
abundance independently of population density. The use of these
simple distinctions is convenient in thinking about population dynamics
on the theoretical level and in analysing field observations. The following paragraphs are intended to establish this classification for the
purposes of the succeeding discussions.
A. REGULATION BY DENSITY-DEPENDENT PROCESSES
The simpler aspects of regulatory action are the immediate effects of
increases or decreases in density, e.g. in intensifying or alleviating
competition.
When there are significant lagging effects the picture is more complicated, for lagging or persistent effects tend to give rise to oscillations.
For example, it sometimes happens that predators, parasites and
phytophagous animals do not immediately relax their attack on the
supply of food if this becomes over-taxed, but rather exploit it more
intensively; then their numbers are belatedly reduced by shortage of
food; the consequent relaxation of attack allows the food to increase
again; the consumers, owing to such delays as the time required for
gfrowth and reproduction, at first increase rather slowly, and only later
come again to dominate the food supply.
M. E. SOLOMON
be distinguished as a density relationship. Thus, if we find that a
population suffers a density-dependent mortality, and identify a
predatory population as the cause of this, the predators or their
predation constitute a density-dependent factor, the predation is a
density-dependent process, and the process can be regarded as the
expression of a density-dependent relationship. The use of these
different terms need imply no more than choosing the words appropriate
to the context.
The term population, as I use it in this paper, simply means any
group of animals, usually of one species, that can conveniently be
considered as a unit. For the present purposes we may assume that all
individuals of the same species and of a particular stage of development
are equivalent, although in practice it is desirable, when possible, to
take account of differences in age and genetics. In practice, males and
females may sometimes be differently involved in density relationships.
When different developmental stages occur together, one should take
account of the numbers of each stage separately.
111. THREE TYPES OF PROCESSES INFLUENCING ABUNDANCE
On an elementary and fundamental level one can make a three-fold
classification of the processes involved in natural control. The three
categories are (a) regulation by density-dependent processes, ( 6 ) modification of the regulatory processes, and (c) imposition of changes in
abundance independently of population density. The use of these
simple distinctions is convenient in thinking about population dynamics
on the theoretical level and in analysing field observations. The following paragraphs are intended to establish this classification for the
purposes of the succeeding discussions.
A. REGULATION BY DENSITY-DEPENDENT PROCESSES
The simpler aspects of regulatory action are the immediate effects of
increases or decreases in density, e.g. in intensifying or alleviating
competition.
When there are significant lagging effects the picture is more complicated, for lagging or persistent effects tend to give rise to oscillations.
For example, it sometimes happens that predators, parasites and
phytophagous animals do not immediately relax their attack on the
supply of food if this becomes over-taxed, but rather exploit it more
intensively; then their numbers are belatedly reduced by shortage of
food; the consequent relaxation of attack allows the food to increase
again; the consumers, owing to such delays as the time required for
gfrowth and reproduction, at first increase rather slowly, and only later
come again to dominate the food supply.