4
M. E. SOLOMON
of repPoduction &hat many species can achieve under favourable
conditions. Whatever processes are responsible for this restriction are
referred to collectively as natural control. Franz (1962) has suggested
the alternative term limitation. If a more formal statement is necessary,
natural control can be d e h e d as the process(es) keeping the numbers
of animals, in a population not controlled by man, within the limits
of fluctuation observed over a sufficiently representative period (cf.
Solomon, 1957, p. 132, also Stern et al., 1959, p. 87).
80
b 4 0 80 120 160 200 240 280
L
1
I
I
I
I
Large larvae per IOsq. ft. of folioge
FIG. 1. A density-dependent relationship in the spruce budworm, after Miller (1963a).
Each point is the mean of ten values. The effect can be explained in terms of food supply
or starvation.
Among the processes involved in natural control some can be distinguished as density-dependent ; their action (measured proportionately, as percent mortality or as mean effect per individual of the
population) becomes increasingly adverse when density rises, and
decreasingly so when density falls (Fig. 1, and cf. Figs. 9 ~ ,
14, 15, 16).
This relationship between adverse action and density may show itself
promptly, as in some forms of competition, or in a lagging reaction,
as in the case of an increase of parasites or predators following an
increase in the hosts or prey (Fig. 3).
Because the proportionate adverse action of density-dependent
processes declines when density falls, as well as intensifying when
density rises, such processes tend to curtail fluctuations, whether
upwards or downwards, that go beyond the average or normal levels
of abundance. Nicholson (1933) regarded them as acting in a compensatory way against any departure from an equilibrium level which
continually changes. The restriction of increases in density in this way
is an active process ; the curtailment of downward fluctuations is not -
M. E. SOLOMON
of repPoduction &hat many species can achieve under favourable
conditions. Whatever processes are responsible for this restriction are
referred to collectively as natural control. Franz (1962) has suggested
the alternative term limitation. If a more formal statement is necessary,
natural control can be d e h e d as the process(es) keeping the numbers
of animals, in a population not controlled by man, within the limits
of fluctuation observed over a sufficiently representative period (cf.
Solomon, 1957, p. 132, also Stern et al., 1959, p. 87).
80
b 4 0 80 120 160 200 240 280
L
1
I
I
I
I
Large larvae per IOsq. ft. of folioge
FIG. 1. A density-dependent relationship in the spruce budworm, after Miller (1963a).
Each point is the mean of ten values. The effect can be explained in terms of food supply
or starvation.
Among the processes involved in natural control some can be distinguished as density-dependent ; their action (measured proportionately, as percent mortality or as mean effect per individual of the
population) becomes increasingly adverse when density rises, and
decreasingly so when density falls (Fig. 1, and cf. Figs. 9 ~ ,
14, 15, 16).
This relationship between adverse action and density may show itself
promptly, as in some forms of competition, or in a lagging reaction,
as in the case of an increase of parasites or predators following an
increase in the hosts or prey (Fig. 3).
Because the proportionate adverse action of density-dependent
processes declines when density falls, as well as intensifying when
density rises, such processes tend to curtail fluctuations, whether
upwards or downwards, that go beyond the average or normal levels
of abundance. Nicholson (1933) regarded them as acting in a compensatory way against any departure from an equilibrium level which
continually changes. The restriction of increases in density in this way
is an active process ; the curtailment of downward fluctuations is not -