Analysis of changes of insect‐plant ratio‐improvement of key‐factor analysis for evaluating bottom‐up effects in insect population dynamics

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 pl 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.