DISCUSSION 299 



is not clear whether some of the changes are due to the herring formerly 

 having been distributed farther from the coast, and so outside fishing range, 

 or to a genuine change in stock. Latterly there has been a rapid fall in catch 

 which could be due to a decrease in population or to an effect of fishing. 

 There is certainly a great variation in the size of year-classes. 



T. B. Reynoldson: Among terrestrial animals I have been struck by 

 the stabihty of earthworm populations in undisturbed habitats. 



J. E. Satchell: Yes, but the variations in moisture content encountered 

 in the terrestrial habitat constitute an additional source of population 

 instabihty, which we should not overlook when we are comparing terrestrial 

 and aquatic organisms. 



T. B. Reynoldson: Over a ten-year period, the size of an earthworm 

 population at Bangor does seem to show some correlation with the rainfall. 



J. A. Gulland: There are various forms of stability. Marine populations 

 often remain stable although recruitment varies tenfold or even a hundred- 

 fold. Because these populations include many year-classes such variations are 

 damped down. Apart from recruitment, fishing is the main variable factor. 



C. Edelstam: Snakes often provide very stable populations, with few 

 major changes over whole decades. Their long life span is a factor — at 

 three years, a grass snake's expectation of life is another three or four years. 

 Such longevity is coupled by necessity with a low reproduction rate; 

 attained for example by giving birth to only a few young each year, or — 

 as in adders in central and north Sweden — by producing young only every 

 two years or perhaps with even longer intervals. 



A. C. Simpson: Similarly, in Crustacea, lobsters and crabs, which are 

 long-Uved, are stable, whereas shrimps, with short life-spans, are more 

 variable in population numbers. 



R. J. H. Beverton : I think we are confusing fluctuations with trends. 

 Recruitment may fluctuate widely about a constant mean or about a 

 changing trend, and the latter is by far the more significant type of population 

 instability. 



J. G. Skellam : The length of the period of maturity is more important 

 than the whole life-span, as a factor affecting population stability. 



M. E. Solomon : Isn't the essential point the degree of overlapping of 

 generations, which commonly means the number of years spanned by an 

 individual? In many insects each year's population is wholly at the mercy 

 of that year's conditions. In other, longer-lived, animals there is a buffering 

 effect. In connection with Le Cren's paper in which he mentioned the high 

 amount of reproductive products liberated by fish, do these serve as diet for 



