GENERAL DISCUSSION 



363 



2. OSCILLATIONS AND TRENDS 

 M. E. Solomon : Ordinarily, 'stability' means fluctuation about a mean 

 population level which is regarded as constant, as in Fig. i<i, with the 

 assumption that when the population density is either above or below the 

 line there are forces tending to restore it to the mean level. 



Fig.' I. — (M.[E.[Solomon) : Fluctuations and trends in populations with time — see text. 



In some cases a gradual change in the mean level about which the popula- 

 tion fluctuates can be detected (Fig. ib). This can be termed 'stability' in a 

 sense, comparable with the physicist's concept of 'moving balance'. 

 Nicholson goes further than this, and envisages oscillation about an 'equilib- 

 rium density' which is continually changing with environmental conditions 

 (Nicholson, 1954/j). Of course, until we knov/ a lot more than just the counts 

 of abundance, the choice of a line and the relating of the fluctuation to it 

 must be largely arbitrary. 



In a 'horizontal line' population (Fig. la) the stability of the adult numbers 

 may be partly or even wholly the result of regulation of the earher stages. 

 Le Cren has said that fish populations tend to have a constant number 

 surviving independently of the number of eggs produced, and Gulland has 

 suggested that the number of young recruited may be independent of the 

 adult stock. Beverton states that a tenfold variation in plaice egg production 

 has no effect on year-class size. 



G. C. Varley: It is most important to show the scales on the axes of 

 population fluctuation curves. For example, a seasonal peak of enormous 

 dimensions will appear if the total number of eggs is counted as individuals, 

 while the adult numbers may yet persist at a steady low level (Fig. 2). 



Where adults overlap several seasons there is a comparative stability: on 

 the other hand, in insects with a life cycle of one year the peak of adult 



