PATTERN AND PROCESS IN COMPETITION
43
reductions in realized fecundity and fertility after the first few days of
cultures, so that the conditions suitable for a high rate of production
are restricted to a very short period (Robertson and Sang, 1944).
Natural breeding sites also deteriorate rapidly in relation to their
suitability as oviposition surfaces (Carson and Stalker, 1951) and seldom
contain more than a few eggs or larvae (cf. Gordon, 1942; Birch and
Battaglia, 1957; Sokoloff, 1957). Harrison (1964) studied the factors
affecting the abundance of four species of Lepidoptera that spend their
life cycles on banana plants, and concluded that they are not affected
by intra- or interspecies competition because they normally lay so few
eggs per unit area that their population levels never reach critical densities in relation to the food that is available. Adult female Ceramidia
butleri will lay far more eggs on banana plants in cages in an insectary
than they will under natural conditions in plantations, and the larval
densities required to produce a significant decrease in pupal weight and
an increase in mortality in the laboratory are much higher than densities recorded in the field. Population control through limitations on
realized fecundity is so effective that competitive exploitation does not
occur (Harrison, 1964).
An absolute shortage of food is probably a rare event in the lives of
animals such as insects and other invertebrates that would normally
compete through exploitation. In spite of the tremendous increase
potential these animals possess, their populations appear to be regulated ‘
quite strongly by climatic factors that tend to reduce fecundity, fertility
and adult longevity t o values that are only fractions of those reached
under optimum conditions (Birch, 1948). Birch (1945, 1963) has shown,
for example, that the beetles Rhizopertha dominica and Calandra oryzae
are extremely sensitive to temperature and moisture in their rates of
development and in population characteristics affecting their intrinsic
rates of increase. Data for other poikilotherms are less precise than
those provided by Birch’s elegant studies, but it is well known that
their population characteristics are affected in much the same ways by
relatively slight changes in physical factors. If competition for food
occurs in such populations, it is likely to be a transient phenomenon
that occurs only infrequently. The fact that population irruptions or
“outbreaks” are especially characteristic of certain insect populations
and are seldom observed in homiotherms lends support to the view
that the populations of most terrestrial insects are normally under
climatic control. They only express rates of increase approaching their
full potential when an unusual combination of optimum conditions
occurs. In species such as dung and carrion insects, on the other hand,
in which the oviposition site and larval environment are less subject to
the effects of physical factors and over-population is more liable to
43
reductions in realized fecundity and fertility after the first few days of
cultures, so that the conditions suitable for a high rate of production
are restricted to a very short period (Robertson and Sang, 1944).
Natural breeding sites also deteriorate rapidly in relation to their
suitability as oviposition surfaces (Carson and Stalker, 1951) and seldom
contain more than a few eggs or larvae (cf. Gordon, 1942; Birch and
Battaglia, 1957; Sokoloff, 1957). Harrison (1964) studied the factors
affecting the abundance of four species of Lepidoptera that spend their
life cycles on banana plants, and concluded that they are not affected
by intra- or interspecies competition because they normally lay so few
eggs per unit area that their population levels never reach critical densities in relation to the food that is available. Adult female Ceramidia
butleri will lay far more eggs on banana plants in cages in an insectary
than they will under natural conditions in plantations, and the larval
densities required to produce a significant decrease in pupal weight and
an increase in mortality in the laboratory are much higher than densities recorded in the field. Population control through limitations on
realized fecundity is so effective that competitive exploitation does not
occur (Harrison, 1964).
An absolute shortage of food is probably a rare event in the lives of
animals such as insects and other invertebrates that would normally
compete through exploitation. In spite of the tremendous increase
potential these animals possess, their populations appear to be regulated ‘
quite strongly by climatic factors that tend to reduce fecundity, fertility
and adult longevity t o values that are only fractions of those reached
under optimum conditions (Birch, 1948). Birch (1945, 1963) has shown,
for example, that the beetles Rhizopertha dominica and Calandra oryzae
are extremely sensitive to temperature and moisture in their rates of
development and in population characteristics affecting their intrinsic
rates of increase. Data for other poikilotherms are less precise than
those provided by Birch’s elegant studies, but it is well known that
their population characteristics are affected in much the same ways by
relatively slight changes in physical factors. If competition for food
occurs in such populations, it is likely to be a transient phenomenon
that occurs only infrequently. The fact that population irruptions or
“outbreaks” are especially characteristic of certain insect populations
and are seldom observed in homiotherms lends support to the view
that the populations of most terrestrial insects are normally under
climatic control. They only express rates of increase approaching their
full potential when an unusual combination of optimum conditions
occurs. In species such as dung and carrion insects, on the other hand,
in which the oviposition site and larval environment are less subject to
the effects of physical factors and over-population is more liable to