ANALYSIS O F PROCESSES I N CONTROL O F INSECTS
9
Regulation of a population may be influenced by more than one
density-dependent process simultaneously. It may be exerted by
different processes at different times and places. It may be continuous
or intermittent, or, presumably, even absent in the case of marginal
populations which are ephemeral offshoots of more permanent ones.
B. MODIFICATION OF THE REGULATORY PROCESSES
Density-dependent regulation is of course subject to many modifying
influences bearing upon the population and its environment. Influents
such as weather may greatly modify a population’s reproductive and
survival rates, its food supply, and its competitors and natural enemies.
Such influences often play a prominent part in determining which
regulatory process(es) shall, for the time being, be the decisive one(s),
and at what level of density further increase would be prevented.
Nicholson (1954b) refers to modifying influences as “legislative”.
The legislative or modifying factors include many and perhaps most
genetic changes. Only when such changes are geared to the level of
density in some way can they play a direct part in regulation. While
examples are available of genetic change which seems to act as a modifying influence, leading to increases in abundance, the density-related
action of genetic change is still almost entirely a matter of theory and
speculation (cf. Franz, 1949 ; Pimentel, 196lb).
c. IMPOSITION OF CHANGES I N ABUNDANCE
INDEPENDENTLY OF DENSITY
It is generally agreed that influents such as weather, not responsive
to population density, commonly have a density-independent action
on a population. (Of course, severe weather may give rise to a situation
in which competition occurs for a small residue of favourable sites;
then the situation may be expressed as “density-independent influent
+ limited space +density-dependent competition +regulation” ; this
is the same class of situation as that described under B - weather
determining which regulatory process is to be prominent and also
modifying its intensity.) It has occasionally been argued that the
effects of influents that are apparently density-independent may in
fact often depend in some small degree on population density
(Andrewartha and Birch, 1954; Chitty, 1960). However that may be, if
their action cannot be shown to be substantially dependent on density,
it is realistic to treat them as density-independent.
There is little point in discussing in a general way the relative contributions of regulatory factors and density-independent factors to
natural control. A more constructive approach to this question is to
find means of identifying and measuring the action of both types of
9
Regulation of a population may be influenced by more than one
density-dependent process simultaneously. It may be exerted by
different processes at different times and places. It may be continuous
or intermittent, or, presumably, even absent in the case of marginal
populations which are ephemeral offshoots of more permanent ones.
B. MODIFICATION OF THE REGULATORY PROCESSES
Density-dependent regulation is of course subject to many modifying
influences bearing upon the population and its environment. Influents
such as weather may greatly modify a population’s reproductive and
survival rates, its food supply, and its competitors and natural enemies.
Such influences often play a prominent part in determining which
regulatory process(es) shall, for the time being, be the decisive one(s),
and at what level of density further increase would be prevented.
Nicholson (1954b) refers to modifying influences as “legislative”.
The legislative or modifying factors include many and perhaps most
genetic changes. Only when such changes are geared to the level of
density in some way can they play a direct part in regulation. While
examples are available of genetic change which seems to act as a modifying influence, leading to increases in abundance, the density-related
action of genetic change is still almost entirely a matter of theory and
speculation (cf. Franz, 1949 ; Pimentel, 196lb).
c. IMPOSITION OF CHANGES I N ABUNDANCE
INDEPENDENTLY OF DENSITY
It is generally agreed that influents such as weather, not responsive
to population density, commonly have a density-independent action
on a population. (Of course, severe weather may give rise to a situation
in which competition occurs for a small residue of favourable sites;
then the situation may be expressed as “density-independent influent
+ limited space +density-dependent competition +regulation” ; this
is the same class of situation as that described under B - weather
determining which regulatory process is to be prominent and also
modifying its intensity.) It has occasionally been argued that the
effects of influents that are apparently density-independent may in
fact often depend in some small degree on population density
(Andrewartha and Birch, 1954; Chitty, 1960). However that may be, if
their action cannot be shown to be substantially dependent on density,
it is realistic to treat them as density-independent.
There is little point in discussing in a general way the relative contributions of regulatory factors and density-independent factors to
natural control. A more constructive approach to this question is to
find means of identifying and measuring the action of both types of