GENERAL DISCUSSION 377 



decreased to a level lower than P4, the population may be saved by stopping 

 the exploitation. 



The regulatory mechanism may precede exploitation, or exploitation may 

 precede the operation of a regulatory system. In the former case the mechan- 

 ism plotted above operates. In the second, exploitation is buffered by the 

 regulatory mechanism. 



D. H. Chitty: Do you suppose that if one exploits a population after 

 it has established stable numbers through a natural regulatory process the 

 results will be bad, while if exploitation precedes regulation it will be less 

 deleterious ? 



P P Ri P 



POPULATION DENSITy 



Pig. 5,— (H. Klomp) : A model of an exploited population — see text. 



H. Klomp: Regulation precedes exploitation in some cases, for example 

 where it operates on the larval stages but the adults are exploited. 



E. D. Le Cren: But life cycles are cychc processes: the regulation of the 

 following generation, in their larval stage, will follow the exploitation of 

 the adults of the first generation so that a balance will be attained. 



E. M. Nicholson: There is a simple example available in North 

 American duck populations. Two things have been done: firsdy, protection 

 in certain regions, and secondly a controlled exploitation. The result has 

 been firstly a steady rise in numbers, despite an increase in exploitation, and 

 secondly a concentration in the protected areas leading ultimately to over- 

 stocking there and natural regulation. This fits with Klomp's model. Where 

 there was heavy cropping the population never reached the point at which 



