376 GENERAL DISCUSSION 



gained from exploitation studies. Equally, they suggest preliminary tests by 

 means of which the exploitation ecologist might explore aspects of the 

 capacity of particular populations to withstand artificial reductions. 



G. C. Varley: May I enquire if anybody has thought of a population 

 which shows an increase following exploitation? 



M. Graham : I know of no case apart from the rabbit, and here the rise 

 during exploitation was probably due simply to more predators being 

 destroyed than the prey which they would have taken. 



In general, exploitation often leads to higher production, but not to a 

 higher total biomass. 



H. V. Thompson: I am surprised that no populations of species other 

 than the rabbit are said to increase under exploitation. I am also surprised 

 that the removal of predators is said to be responsible for the success of the 

 latter. In fact predators are markedly inefficient at catching rabbits. Foxes 

 proved useless in Australia, and Mustelidae in New Zealand. Resistance to 

 exploitation is in fact a characteristic of 'pests' : in the grey squirrel 40 per 

 cent may be taken and still the population will go on rising. 



M. E. Solomon: I have been told that in dense rabbit areas the number 

 surviving the winter is inversely proportional to the number entering the 

 winter, owing to food shortage. Are there any data showing that more 

 rabbits follow an autumn trapping campaign than if no exploitation goes on? 



C. W. Hume: Surely one would expect that a trapping campaign 

 killing 30-40 per cent of the population would aid the remainder to survive 

 the winter? 



H. V. Thompson: Unfortunately it is impossible to test such a hypo- 

 thesis by doing the converse experiment. 



H. Klomp: I would like to illustrate the relation of population numbers 

 to the level of exploitation by a simple model (Fig. 5). Mortahty is density- 

 dependent because of competition. Reproduction decreases at very low 

 population density due to the fact that an increasing part of the females does 

 not contact a male. 



Consider the situation in which the unexploited population is in equilib- 

 rium at density P^. Here the position is a stable one. If exploitation now 

 occurs the mortahty is raised, e.g. independently of density to the line Mg 

 or dependently of density to the line M3. This causes a shift of the stable 

 equihbrium to a lower population density {P^. 



With very great exploitation, however, population density may reach the 

 level P3. Then the population is in danger because a small random fluctuation 

 in density can cause a permanent decrease in numbers, due to the fact that 

 mortality is in excess of reproduction. However, if density has not yet 



