Research Article
Kazumi Sakuramoto
Abstract
The aim of this study was to discuss a common concept of population dynamics applicable to both Thrips imaginis (Thysanoptera) and the Pacific stock of the Japanese sardine (Sardinops melanostictus). First, I elucidate the mechanism that produces the false density-dependent effect on population changes of Thrips imaginis, I conducted simple deterministic simulations to discuss the qualitative viewpoints. Second, I conducted Monte Carlo simulations by using the average population size and standard deviation of the thrip data used by Davidson and Andrewartha for the quantitative discussion. In simple deterministic simulations, the resultant plots of population change against population size showed a decreasing trend for which the slope was statistically significant even though the true relationship between the population change and population size had no density-dependent effect. The results of the Monte Carlo simulations indicated that nearly 70% of the trials showed false density-dependent effects. The provability of the false decision, which was to recognize the existence of density-dependent effects, increased as the standard deviation in population size in a month i-1 increased. When the number of samples increased, the probability of the false decision greatly increased. The conclusion from these simulations was that the density-dependent effect that emerged in the population change of T. imaginis was artificially produced and invalid. Further, the thrip population size in month i was determined in proportion to that in month i-1; and environmental conditions in month i. This mechanism was completely same of that shown in the Japanese sardine (Sardinops melanostictus). Therefore, the fluctuations in populations of T. imaginis and Japanese sardine could be explained with the same concept of population dynamics shown here.