ANALYSIS O F PROCESSES I N CONTROL O F INSECTS
19
problem. As with Lucilia, the productivity of the beetle cultures
increased with the rate of exploitation, sometimes up to an exploitation
rate of about 90%, depending partly on the age-distribution of the
residual population.
(A1
(B)
3. From the Form of the Population Curve at Different Levels of Density
If we are to go beyond measuring the range of fluctuation of a
population, and try to interpret the fluctuations, we need to be able to
distinguish, for example, between reverses in increase that are due to
the more or less direct effects of weather, and reverses or decelerations
that are imposed through density-dependent processes. Whether or not
the action of these latter may have been modified by the weather may
also be an important consideration, but it is a secondary one. Some
evidence about these alternatives can often be gained by inspecting
the population curves. If the reversal is sudden, it is likely to be caused
by a change in some environmental factor, such as weather. If it is
approached by a gradual deceleration of increase, it may be the result
of regulation, although one cannot be sure, without other evidence, that
it is not due to a gradual change in weather, for example.
Evidently, methods of inspecting population curves need to be
formalized and made objective. One way of doing this has been described and used by Morris (19638, 1963b). He finds in his work with
forest insect populations that if the logarithm of population density in
I
I
I
I
I
+
P
I
I
I
I
.
4 -
m
-
\ - I
.. ..
FIG. 8a. Annual population density of spruce budworm larvae in one plot at Green
River, New Brunswick, 1944-60. The two points below the curve are values corrected
for immigration.
B. Graph of log density in each generation against log density in the following
generation; values from two plots. (From Morris, 1963a.)
19
problem. As with Lucilia, the productivity of the beetle cultures
increased with the rate of exploitation, sometimes up to an exploitation
rate of about 90%, depending partly on the age-distribution of the
residual population.
(A1
(B)
3. From the Form of the Population Curve at Different Levels of Density
If we are to go beyond measuring the range of fluctuation of a
population, and try to interpret the fluctuations, we need to be able to
distinguish, for example, between reverses in increase that are due to
the more or less direct effects of weather, and reverses or decelerations
that are imposed through density-dependent processes. Whether or not
the action of these latter may have been modified by the weather may
also be an important consideration, but it is a secondary one. Some
evidence about these alternatives can often be gained by inspecting
the population curves. If the reversal is sudden, it is likely to be caused
by a change in some environmental factor, such as weather. If it is
approached by a gradual deceleration of increase, it may be the result
of regulation, although one cannot be sure, without other evidence, that
it is not due to a gradual change in weather, for example.
Evidently, methods of inspecting population curves need to be
formalized and made objective. One way of doing this has been described and used by Morris (19638, 1963b). He finds in his work with
forest insect populations that if the logarithm of population density in
I
I
I
I
I
+
P
I
I
I
I
.
4 -
m
-
\ - I
.. ..
FIG. 8a. Annual population density of spruce budworm larvae in one plot at Green
River, New Brunswick, 1944-60. The two points below the curve are values corrected
for immigration.
B. Graph of log density in each generation against log density in the following
generation; values from two plots. (From Morris, 1963a.)