DYNAMICS O F FIELD POPULATION O F P I N E LOOPER
283
respective contributions to the gradual increase of the correlation coefficient. (The regression coefficient, which is an index of density dependence, will be discussed in the next chapter.)
The size of r,, indicates that there is hardly any relation between the
egg densities of successive generations (Fig. 33-A). Indeed, as shown in
Fig. 27, the egg density can change generation by generation from an
extremely high (1951) to an intermediate level (191i2), and in the opposite direction from a low (1953) to a high level (1954), thus giving quite
a different picture from most of the forest insects studied by Morris
(1959, 1963a, 1963b).
The addition of the effect of egg mortality to the influence of egg
density increases the correlation coe%cient to 0-3!1, and this improvement shows that this component supplies a clear, albeit not very pronounced contribution to egg density (Fig. 33-B).
The correlation is much more improved when larval survival is
plotted against egg density (Fig. 33-C), giving a coefficient of 0.85. This
shows that the density of the next generation is m,%inly determined by
the fate of this stage.
Further improvement of the coefficient is due to pupal mortality,
whereas moth mortality, and especially sex ratio and reduced fecundity
have hardly any effect (Fig. 33-D-F).
It will be evident that the addition of the final component, the reduction of fecundity, results in a correlation coefficienl; equal to unity, because of the fact that the values of log(Et . s1 . . . sI2) all equal log
Et+l/ log F = log Ettl/log 216 (Fig. 33-G).
To sum up it is shown that the results of this quantitative analysis
are in agreement with the conclusions reached by visual consideration
of Fig. 31. Egg density is mainly determined by the numbers of insects
surviving the larval stage in the previous generation. However, larval
mortality is due to a complex system of factors oporating on the population in succession over a long period of time. WCI have, therefore, to
concentrate on a more detailed analysis of this complex.
3. Analysis of Larval Mortality
As shown in Table XXIV, larval mortality is composed of five submortalities (k2-kE), which have been discussed on p. 271. From the life
tables it appears that the densities needed to compute the k-values of
the sub-mortalities have not been estimated completely annually.
Therefore we joined k, and k, as juvenile mortality (including all of the
larval mortality up to the end of August), and k, and k, as mortality of
advanced larvae. Prior to 1955 k4-5 could not be tieparated from prepupal mortality (kE), because of the fact that nymphal density was
inadequately measured (1954) or not measured at all.
283
respective contributions to the gradual increase of the correlation coefficient. (The regression coefficient, which is an index of density dependence, will be discussed in the next chapter.)
The size of r,, indicates that there is hardly any relation between the
egg densities of successive generations (Fig. 33-A). Indeed, as shown in
Fig. 27, the egg density can change generation by generation from an
extremely high (1951) to an intermediate level (191i2), and in the opposite direction from a low (1953) to a high level (1954), thus giving quite
a different picture from most of the forest insects studied by Morris
(1959, 1963a, 1963b).
The addition of the effect of egg mortality to the influence of egg
density increases the correlation coe%cient to 0-3!1, and this improvement shows that this component supplies a clear, albeit not very pronounced contribution to egg density (Fig. 33-B).
The correlation is much more improved when larval survival is
plotted against egg density (Fig. 33-C), giving a coefficient of 0.85. This
shows that the density of the next generation is m,%inly determined by
the fate of this stage.
Further improvement of the coefficient is due to pupal mortality,
whereas moth mortality, and especially sex ratio and reduced fecundity
have hardly any effect (Fig. 33-D-F).
It will be evident that the addition of the final component, the reduction of fecundity, results in a correlation coefficienl; equal to unity, because of the fact that the values of log(Et . s1 . . . sI2) all equal log
Et+l/ log F = log Ettl/log 216 (Fig. 33-G).
To sum up it is shown that the results of this quantitative analysis
are in agreement with the conclusions reached by visual consideration
of Fig. 31. Egg density is mainly determined by the numbers of insects
surviving the larval stage in the previous generation. However, larval
mortality is due to a complex system of factors oporating on the population in succession over a long period of time. WCI have, therefore, to
concentrate on a more detailed analysis of this complex.
3. Analysis of Larval Mortality
As shown in Table XXIV, larval mortality is composed of five submortalities (k2-kE), which have been discussed on p. 271. From the life
tables it appears that the densities needed to compute the k-values of
the sub-mortalities have not been estimated completely annually.
Therefore we joined k, and k, as juvenile mortality (including all of the
larval mortality up to the end of August), and k, and k, as mortality of
advanced larvae. Prior to 1955 k4-5 could not be tieparated from prepupal mortality (kE), because of the fact that nymphal density was
inadequately measured (1954) or not measured at all.