370 GENERAL DISCUSSION 



animals need less genotypic variation. Surely there is no marine animal 

 showing the regular oscillations drawn as type for terrestrial environments? 



R. J. H. Beverton : Haddock in the North Sea show wide fluctuations 

 in the size of year classes over a period. The environment may influence 

 young and adults very differently and the two groups should be considered 

 separately. 



G. C. Varley: I dispute the allegation that more is known about 

 aquatic than about terrestrial animals. Aquatic animals are in many respects 

 hard to study and the few data available often give a wholly spurious 

 smoothness to the curves. In terrestrial organisms there is often a trend to 

 more violent instabihty in simpler environments, as in the spruce budworm 

 in northern regions, where adult populations may vary by as much as lo^ 

 year by year. Similar wide variations are known in the tropics, as in pests 

 on coconut palms in Fiji where synchronous generations may build up to 

 high levels after escape from parasitic control. 



4. BEHAVIOUR AND COMPARATIVE POPULATION 

 DYNAMICS 



M. Graham: I do not think that ecology has yet reached the stage 

 where a Symposium of this kind can lead to the expression of a few clear 

 general principles. We are unhkely to reach much greater clarity than has 

 been obtained in this afternoon's discussions. 



I should like to hear more about the studies on populations of birds and 

 mammals — what have workers in these groups got out of the Symposium ? 



E. M. Nicholson : I am somewhat worried that the behavioural aspect 

 of regulation of numbers has so rarely been brought up. In birds there are 

 many short-term distributional changes, exemphfied by the 'eruptions' of 

 tits. The distance of movement in these birds is normally small — rarely 

 exceeding ten miles. In some years, however, an 'explosive' outburst of 

 population can occur over hundreds of miles. Behaviour changes are of 

 great importance in these. 



D. H. Chitty: Tinbergen (1957) refers to dispersion as 'the state of 

 living dispersed, as opposed to crowded and as distinct from being distri- 

 buted at random'. Many animals have dispersion mechanisms which operate 

 through hostility, either as a threat to or as an avoidance of, members of 

 the same species, and both these factors serve to limit numbers. 



One can see how this might operate by considering bird species in which, 

 due to territorial behaviour, surplus members are driven out into unfavour- 

 able habitats. Where there is a resistance to moving out there will be selection 



