64 THE ECOLOGY OF ANIMALS 



trial forms. We can see how there are also ecological 

 reasons for very low densities being biologically 

 inefficient for a species. For instance, if there were 

 only one male and one female aphid in a ten-acre 

 field the chance of their meeting and mating would 

 be extremely small. The special interest of Alice's 

 work lies in his suggestion that such physiologically 

 conditioned aggregations (which he assumes to be 

 ecologically advantageous also) give rise to conditions 

 favourable to the evolution of more complex social 

 organizations of the type discussed in Chapter III. 

 The theory put forward is that such loose aggre- 

 gations would tend to exploit through natural selec- 

 tion the potentiahties of evolving into more power- 

 fully knit groups, and finally into such dominant 

 ' civilizations ' as those of the ants. 



This concept of optimum densities can also be 

 considered from another point of view. All animals 

 tend to increase rapidly in numbers if their normal 

 checks are temporarily removed. This sort of rapid 

 increase is commonly observed during ' plagues ' of 

 animals such as field-mice, cockchafers, locusts, and 

 aphids. We have just seen that there are physio- 

 logical reasons why the density tends to remain 

 above a certain very low level. There are equally 

 strong reasons preventing the density rising above a 

 certain level. The ultimate limits to increase lie 

 either in factors of space (as with some sessile marine 

 intertidal animals such as the mussel Mytilus and 

 the barnacle Balanus) or in the exhaustion of food 

 supplies, as occasionally may be seen during plagues 

 of caterpillars or in protected herds of deer in 

 national parks. Thus, it has been found that one 

 reindeer in Alaska requires about forty acres of 

 pasture to maintain it continuously throughout the 

 year. The phenomenon of over-parasitization some- 

 times occurs in insects, and leads to the destruction 

 both of the host and the parasite through lack of 

 food sufficient to complete development. Other 



