A N A L Y S I S O F P R O C E S S E S I N CONTROL O F I N S E C T S
7
Pupal density (no.per IOsq. ft. of foliage)
FIG. 4. A predominantly inverse density relationship in the influence of parasites upon
the spruce budworm. (From Miller, 196310.)
oscillation the parasite acts like an inverse factor, at other times like
a promptly density-dependent one. While the differences between
lagging and prompt density-dependence are often important, they both
tend in practice towards regulation, oscillatory in the one case, plain
in the other ; and both are likely to be rather irregular in most natural
environments. The more rapidly the parasite can develop and reproduce
compared with the host, the more closely is its action likely to approximate to that of a promptly density-dependent factor. Also some
predators and insect parasites can react at once to a rise in prey density
by attacking at a proportionately higher rate, in which case this part
of their response is promptly density-dependent (Solomon, 1949).
Some lagging density-dependent processes do not arise from natural
enemies, but from damage to the environment, or, as in one recorded
instance (Wallace, 1962), from a toxic effect of feeding on dead bodies.
Taken as a group, natural enemies cannot be classified as all promptly
density-dependent, all lagging density-dependent, nor as inverse factors,
nor as density-independent. Their action is varied, and affords examples
of all of these relationships except perhaps the last. A classification of
types of action or types of density relationship is one thing, a descriptive classification of factors (weather factors, parasites, etc.) is
another, and the two cannot be fitted neatly together. Certain correspondences occur, not in accordance with firm rules but rather as
tendencies, subject to various qualifications and exceptions.
In discussing the influences acting upon a population we may refer
to any element in the situation, e.g. predators, competitors, predation
or competition, as a factor. At the same time, predation and competition are processes, and may be studied as such. If we are concerned
with the way in which the effect of a process or factor varies with the
population density, we are studying a relationship, of a type that may
7
Pupal density (no.per IOsq. ft. of foliage)
FIG. 4. A predominantly inverse density relationship in the influence of parasites upon
the spruce budworm. (From Miller, 196310.)
oscillation the parasite acts like an inverse factor, at other times like
a promptly density-dependent one. While the differences between
lagging and prompt density-dependence are often important, they both
tend in practice towards regulation, oscillatory in the one case, plain
in the other ; and both are likely to be rather irregular in most natural
environments. The more rapidly the parasite can develop and reproduce
compared with the host, the more closely is its action likely to approximate to that of a promptly density-dependent factor. Also some
predators and insect parasites can react at once to a rise in prey density
by attacking at a proportionately higher rate, in which case this part
of their response is promptly density-dependent (Solomon, 1949).
Some lagging density-dependent processes do not arise from natural
enemies, but from damage to the environment, or, as in one recorded
instance (Wallace, 1962), from a toxic effect of feeding on dead bodies.
Taken as a group, natural enemies cannot be classified as all promptly
density-dependent, all lagging density-dependent, nor as inverse factors,
nor as density-independent. Their action is varied, and affords examples
of all of these relationships except perhaps the last. A classification of
types of action or types of density relationship is one thing, a descriptive classification of factors (weather factors, parasites, etc.) is
another, and the two cannot be fitted neatly together. Certain correspondences occur, not in accordance with firm rules but rather as
tendencies, subject to various qualifications and exceptions.
In discussing the influences acting upon a population we may refer
to any element in the situation, e.g. predators, competitors, predation
or competition, as a factor. At the same time, predation and competition are processes, and may be studied as such. If we are concerned
with the way in which the effect of a process or factor varies with the
population density, we are studying a relationship, of a type that may