368 GENERAL DISCUSSION 



in the diagram? Surely not. In all species many regulatory mechanisms 

 operate at many stages, some before and some after recruitment. 



J. G. Skellam: Distinguishing between trends and oscillations is a major 

 problem. 



E. Broadhead : Does this mean that populations under many independ- 

 ent influences, such as predators and the hke, can show several variations of 

 this kind? 



J. B. Cragg: Is this the meaning of Cole's work? 



L. B. Slobodkin: Cole took a system of random numbers and smoothed 

 them to get a pattern comparable with population fluctuations. Field data 

 for fluctuations at least seem of the same type. Yet the random numbers, 

 once smoothed, are no longer random but in a sense a proper oscillating 

 mechanism. The cycles in his paper give a superficial impression that because 

 random numbers that have been smoothed give an apparent oscillation 

 comparable with the population, therefore natural oscillating populations 

 are oscillating randomly. 



M. E. Solomon: It should be possible in suitable cases to measure 

 regulation and random fluctuation against each other by raising or reducing 

 the population density and observing the rate of return towards the normal 

 level. 



J. G. Skellam : The tendency to come back is equivalent to the stability 

 of the population. 



N. Waloff: How many generations are needed before a population 

 can be considered in a stable state? The environment itself is often not 

 stable — this is so for insects. What is a stable population related to ? 



M. E. Solomon: This cannot be laid down in general. It must be made 

 clear with reference to the particular case. 



3. THE INFLUENCE OF ENVIRONMENTAL CHANGE 



E. B. WoRTfflNGTON: I notice that whenever a fishery biologist comes 

 to the blackboard he draws even lines to represent population numbers, 

 while terrestrial biologists draw irregular ones. Surely there are in fact 

 fundamental differences between land and water environments, and one 

 cannot divorce a population from its environment. In water, there is much 

 more stability of habitat; things are buffered so much that slow changes 

 occur as compared with gross oscillations on land. 



E. D. Le Cren: I am not convinced by Worthington's distinction 

 between terrestrial and aquatic habitats. Isn't an undisturbed tropical forest — 



