6
M . E. SOLOMON
density-dependent processes tend to regulate abundance or population
density, and I restrict the use of the term regulation to this process
(cf. Nicholson, 1954b, density regulating factors). If an increase of
population is stopped by some process other than a density-dependent
one, this may for the time being constitute an aspect of natural control
as defined above, but I do not call it regulation.
In an earlier paper (Solomon, 1949) I equated natural control and
regulation, but the general tendency has been away from this strict
interpretation of natural control, and I have since used the more
inclusive definition. In practice, regulation cannot be studied adequately
without reference to the wider aspects of population dynamics, for
anything that happens to a population, and anything that it does,
may have an effect on its regulation.
Regulation can be imposed by all types of density-dependent
processes : by the action of predators, parasites, or pathogens, by
intra-specific competition for various requisites including food, shelters
and nesting-sites, and by mutual interference or agression which can
also be interpreted as an aspect of competition for space or resources.
Competition may lead to losses by emigration. In animals that have a
social organization, regulation through competition may be mediated
by restrictions imposed by the population upon its members.
Density-dependent processes are distinguished from inverse processes,
which operate in the opposite sense, i.e. their adverse action becomes
proportionately weaker as density rises, or intensifies as density falls.
For example, in a sparse population reproduction may be hindered
by the infrequency of encounters between the sexes; or, as density
increases the proportion parasitized may decline (Fig. 4, and 'cf. Figs.
9~ and 17). Many natural enemies behave as inverse factors under
certain environmental conditions, or when the ratio of enemies to prey
is low. This is a consequence of their limited capacity for attack. The
significance of this feature was first emphasized by Thompson (1939,
and earlier). Examples will be cited in Section V.
The action of density-independent processes is not significantly
dependent upon population density.
A little more should be said about the differences between prompt
and lagging density-dependence. Intra-specific competition generally
seems to be promptly density-dependent, and so, at times, does the
influence of predators. But in a common type of parasite-host interaction, part of the response of the parasites to an increase in host
density is to increase in abundance, which cannot be done promptly.
The parasites commonly fail to increase for a time even after the host
increase from a low density has been resumed. The result of this delay,
as Varley (1953) has pointed out, is that in parts of the parasite-host
M . E. SOLOMON
density-dependent processes tend to regulate abundance or population
density, and I restrict the use of the term regulation to this process
(cf. Nicholson, 1954b, density regulating factors). If an increase of
population is stopped by some process other than a density-dependent
one, this may for the time being constitute an aspect of natural control
as defined above, but I do not call it regulation.
In an earlier paper (Solomon, 1949) I equated natural control and
regulation, but the general tendency has been away from this strict
interpretation of natural control, and I have since used the more
inclusive definition. In practice, regulation cannot be studied adequately
without reference to the wider aspects of population dynamics, for
anything that happens to a population, and anything that it does,
may have an effect on its regulation.
Regulation can be imposed by all types of density-dependent
processes : by the action of predators, parasites, or pathogens, by
intra-specific competition for various requisites including food, shelters
and nesting-sites, and by mutual interference or agression which can
also be interpreted as an aspect of competition for space or resources.
Competition may lead to losses by emigration. In animals that have a
social organization, regulation through competition may be mediated
by restrictions imposed by the population upon its members.
Density-dependent processes are distinguished from inverse processes,
which operate in the opposite sense, i.e. their adverse action becomes
proportionately weaker as density rises, or intensifies as density falls.
For example, in a sparse population reproduction may be hindered
by the infrequency of encounters between the sexes; or, as density
increases the proportion parasitized may decline (Fig. 4, and 'cf. Figs.
9~ and 17). Many natural enemies behave as inverse factors under
certain environmental conditions, or when the ratio of enemies to prey
is low. This is a consequence of their limited capacity for attack. The
significance of this feature was first emphasized by Thompson (1939,
and earlier). Examples will be cited in Section V.
The action of density-independent processes is not significantly
dependent upon population density.
A little more should be said about the differences between prompt
and lagging density-dependence. Intra-specific competition generally
seems to be promptly density-dependent, and so, at times, does the
influence of predators. But in a common type of parasite-host interaction, part of the response of the parasites to an increase in host
density is to increase in abundance, which cannot be done promptly.
The parasites commonly fail to increase for a time even after the host
increase from a low density has been resumed. The result of this delay,
as Varley (1953) has pointed out, is that in parts of the parasite-host