GENERAL DISCUSSION 373 



G. M. Dunnet: There can be no doubt that many species have surplus 

 animals moving around and unable to settle due to population pressure. I 

 have observed this in opossums. 



G. V. Nikol'skii: (a) There are two aspects to intraspecific relationships: 

 on the one hand competition, and on the other co-operation, as for example 

 the schooling of fish which results in improved defence against predators. 

 Shoals may disperse at night when this is no longer effective. 



{b) We should remember that population properties like other properties 

 of the species are adaptive. 



(c) Host/parasite relationships in fish are of two types; those in which the 

 host is killed — in which case only a proportion can be parasitized — and 

 those in which it is not — where parasitism can be complete. Improved 

 adaptation results in a change from the first to the second. 



5. THE RESPONSE OF POPULATIONS TO EXPLOITATION AND 

 THE OPERATION OF DENSITY-RELATED FACTORS 



M. E. Solomon: Most ecologists who have been concerned with the 

 question would agree with the view that the abundance of animals is regu- 

 lated, at least in a rough way, by density-related factors. These factors (or 

 processes, as I prefer to think of them) are not considered as operating 

 independently of the general environment, but rather as gearing the size of 

 a population to the capacity of its envirormient (or to this capacity as modified 

 by the presence of predators, competitors, etc.). 



On the other hand, a few of the theorists of insect ecology have claimed 

 that certain insect populations simply fluctuate in respose to changes in the 

 environment, in a manner not related to population density (Thompson, 

 1956; Andrewartha, 1957; Birch, 1957). That is to say, if regulation means 

 control through density-related processes, they claim that these populations 

 are not regulated. Some of us have objected that the chances of a population 

 remaining for a long period within the observed limits of abimdance seem 

 too remote, unless there is at least occasional regulation by density-related 

 processes. 



Supposing, however, that such an unregulated population did exist, it 

 seems clear that any exploitation would impose a reduction from which 

 such a population would in a sense never recover: whatever its subsequent 

 fluctuations, it would always be larger if it had not been exploited. More- 

 over, the effects of successive imposed reductions would be cumulative. 



Going to the other extreme, imagine a population in which each adult 

 female produced 100 female offspring yet the numbers of adults were kept at 

 a constant level by density-related processes. Clearly, a high rate of exploita- 



