GENERAL DISCUSSION 375 



exploitation studies revealed about the degree of density-related regulation in 

 particular populations, as measured by their speed of recovery after artificial reduction ? 



This question refers to drastic reductions of the standing crop. But pre- 

 sumably in the exploitation of a natural resource the aim is often to exploit 

 the potential turnover without reducing the standing crop (or without reduc- 

 ing it below a level from which it soon recovers), i.e. to employ a regime of 

 frequent rather small 'takes'. We may enquire (Question 2) : What is known 

 of the degree of density-related regulation in particular populations, as measured by 

 the rates of exploitation they can sustain without reduction of the standing crop? 

 Fig. 4c illustrates the sort of test envisaged. 



A glance at Fig. 4a suggests that the procedure of conserving the level of 

 the standing crop may not necessarily give the maximum yield. For if the 

 numbers are kept down by cropping to a level where the curve of increase 

 is steeper (being not so subject to density-related hindrances as at the upper 

 levels), then, in this model at least, the yield should be greater (Fig. 4J). 

 This leads to the enquiry (Question 3): How is yield affected when exploitation 

 is increased to a rate which keeps the standing crop well below its natural level? 



In formulating these elementary questions, I have not tried to anticipate 

 the difficulties that no doubt arise when one tries to apply them to real 

 populations. These difficulties vary with the circumstances; I hope that they 

 are not too often insuperable. 



For analogies, on an exaggerated scale, of what the above considerations 

 lead us to expect in natural populations, it may be appropriate to refer 

 briefly to some of the laboratory experiments on insect populations as 

 examples of the relationships I have been discussing. In one of the experi- 

 ments Nicholson (1954^) performed with populations of the blow-fly 

 Lucilia cuprina, the main limiting factor was the supply of food provided 

 for the larvae. When he systematically removed 99 per cent of all emergent 

 adults, the consequent alleviation of adult crowding and of larval competition 

 allowed greater numbers to develop through to the adult stage — about six 

 times as many as before. This is an extreme example of the principle of 

 Fig. 4J — severe exploitation evoking an increased rate of production of 

 exploitable individuals. 



Watt (1955) used populations of flour beetles as a model for the study of 

 the optimum yield problem. As with Nicholson's flies, the productivity of 

 Tribolium cultures increased wdth the rate of exploitation, sometimes up to 

 an exploitation rate of about 90 per cent; the result varied according to the 

 age-distribution of the residual population. 



The foregoing questions represent aspects of one approach to population 

 dynamics — an attempt to assess the degree of regulation of some natural 

 populations whose ecology is not exhaustively known, using the information 



