378 GENERAL DISCUSSION 



natural regulation comes in. Where protection operated, this was not the case. 



D. H. Chitty: This may be paralleled by the work on grouse, which 

 Jenkins and Watson have described. The main losses there are in late winter, 

 and cropping prior to this may perhaps not affect the spring population 

 density. 



G. C. Varley : Domestic hens show a very simple case. If you allow 

 them to accumulate eggs they stop laying and become broody. If you 

 continually abstract the eggs they continue to lay. 



J. G. Skellam : There is an interesting point about Klomp*s model. The 

 growth curve associated with the graphs of production and mortahty 

 employed is in two parts. Populations initially above a critical level grow in 

 a typical sigmoidal manner very similar to the logistic. Below the critical 

 level of unstable equilibrium the population declines to extinction. In most 

 appUcations I should imagine that this lower strip is very narrow indeed, as 

 many populations, natural and experimental, are known to survive and 

 recover if the habitat is favourable even when reduced artificially to levels 

 many times lower than those at which the animals normally prevail. 



D. H. Chitty : My work suggests that in dense populations the 'quality' 

 of the animals is different from that in expanding populations. In the vole 

 there is one year only in a four-year cycle in which a maximum rate of 

 increase prevails. Predictions from my data are the exact opposite of those 

 expected from Solomon's scheme. Since there is only one year in which 

 population increase occurs at a rate such as would be expected from studies 

 on captive animals, it should be possible to maintain a steady population if 

 they are exploited during that year, whereas exploitation during the other 

 years of decline would presumably accelerate the decline. 



M. Graham: There is a tendency in talking of this sort of topic to 

 assume that all situations are reversible. This is not the case. 



6. TERMINOLOGY 



G. C. Varley: Some considerable divergence has become apparent in 

 the way in which we use words like 'control' and 'self-regulation'. I think 

 we should attempt to agree on a uniform and consistent defmition for such 

 terms. 



D. H. Chitty: Nicholson used the term 'self-regulation' for populations 

 which induce changes in other species such as predators that increase the 

 level of mortality in the population. 



D. Jenkins: I think it implies a behavioural mechanism, whereby a 



