362 GENERAL DISCUSSION 



population with a four-year life-span. This is reasonable because the warm 

 water species may spawn twice a year but the cold water species only once. 

 But even so, the damping caused by the presence of several age-groups in 

 the population may be only a second-order effect. 



M. Graham: Why? 



S. J. Holt: Because despite this damping effect real variations in 

 population are usually due to differences in numbers in a few age-groups 

 only. This happens even when there are many year-classes present, as in the 

 North Sea plaice where the bulk of the fluctuation depends on that of 

 individual year-classes. 



S. W. Hurry: Many population cycles, like those in arctic mammals, 

 surely occur near the limit of range of a species? 



D. H. Chitty: No, that is not really true. 



J. A. Gulland: I think we must distinguish critically between regular 

 and predictable cycles like the mammaUan ones and irregular fluctuations 

 such as we have in fish. 



D. H. Chitty: There are certainly two separate phenomena here. First, 

 chance fluctuations such as those due to bad weather, often seen among 

 insects whose short hfe-span makes them especially vulnerable. Secondly, 

 fairly regular variations like those in lemmings or snowshoe hares. It is the 

 first type that we are really concerned with. 



G. E. Woodrofee: Could the distinction be made in terms of the 

 controlling mechanism? Comparatively stable populations are tied down by 

 self-regulatory systems whereas irregularly fluctuating ones are controlled 

 by external factors like food and environment. 



R. P. Silliman: I am sure that in large populations those with the largest 

 number of year-classes are the most stable : this is well seen in the Pacific 

 hahbut. 



J. G. Skellam : It is unfair to relate the fluctuations of a species to that 

 species alone. A species is a component of a system which itself may be 

 oscillatory, as is the classic predator-prey relationship. Others may form 

 part of a stable system with no oscillation — again one must relate this to 

 the whole system and not just to a part of it. 



M. E. Solomon: But generally it is practical to study only part of the 

 system. 



J. G. Skellam: Then you can only expect to get part of the answer! 



