A Common Concept of Population Dynamics Applicable to Both Thrips imaginis (Thysanoptera) and the Pacific Stock of the Japanese Sardine(Sardinops melanostictus)

Kazumi Sakuramoto

doi:10.4172/2150-3508.1000085

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.

Highlights

When discussing a fluctuation mechanism in biotic resources, one of the key issues is whether a density-dependent effect exists
If a density-dependent effect truly exists in the stock-recruitment relationship, which is the relationship between recruitment and spawning-stock biomass, or is a key factor in controlling population fluctuations, an optimal population size that achieves the maximum sustainable yield (MSY) can be defined, which is one of the most important concepts in fisheries management
68.7% of the trials erroneously indicated the existence of density-dependent effects in the relationship between population change and population size

Summary

Introduction

When discussing a fluctuation mechanism in biotic resources, one of the key issues is whether a density-dependent effect exists. Most scientists, at one time, have believed in the existence of a density-dependent effect and have discussed management schemes under the assumption that a density-dependent effect plays an important role in controlling population fluctuations. If a density-dependent effect truly exists in the stock-recruitment relationship, which is the relationship between recruitment and spawning-stock biomass, or is a key factor in controlling population fluctuations, an optimal population size that achieves the maximum sustainable yield (MSY) can be defined, which is one of the most important concepts in fisheries management. By using the concept of regime shift, Wada and Jacobson [4] analyzed Japanese sardines and concluded that density-dependent effects existed in the favorable and unfavorable regimes, and the carrying capacities during the favorable regime were 75 times greater than those during the unfavorable regime. Sakuramoto [10] reanalyzed the data used by Wada and Jacobson [4]

Objectives

Methods

Results

Conclusion