ANALYSIS OF CHANGES OF INSECT-PLANT RATIO-IMPROVEMENT OF KEY-FACTOR ANALYSIS FOR EVALUATING BOTTOM-UP EFFECTS IN INSECT POPULATION DYNAMICS

摘要

A revised key-factor analysis was presented for analyzing the temporal changes in the ratio of insect absolute number to plant resource. Ten data sets for 5 insect species were then analyzed. In this key-factor analysis, the key factor is defined as the factor contributing highly to between year variation in R(r), the log rate of the inter-year change of the insect-plant ratio. The yearly change of plant resource was handled as a separate factor, expressed by r(pl), log ratio of plant resource in year n to plant resource in year n + 1. The following was revealed: 1) In 7 of the 10 data sets examined, r(pl) influenced variations of R(r); in particular, in 3 cases r(pl) was the main key factor. 2) Generation-to-generation fluctuations of absolute insect densities showed density dependence in 4 cases, while those of insect-plant ratios, in 8 cases. 3) The Royama model or a linear model explained well the relationship between log insect-plant ratio (X(r)) and R(r) and the relationship between X(r) and log yearly change rate of absolute insect density (R(abs)). However, in the 7 cases in which r(pl) was a critical factor for variations of R(r), with increase of X(r), R(r) showed a steeper decrease around the equilibrium point (the point for which R(r) is 0) than R(abs). This occurred because r(pt) tended to be negatively correlated with X(r). Consequently, in two cases X(r) fluctuated cyclicly or chaotically although without the changes in plant resource, fluctuations of X(r) would be damped oscillations approaching equilibrium.