324 Relations within the Species 



creased by recopulation, and recopulation increases with the density 

 of the population. At densities higher than four, however, the fact 

 that the beetles eat their own eggs progressively curtails the success 

 of reproduction. This cannibalism eventually sets the upper limit of 

 the population (Park, 1941). In experiments with ciliates the re- 

 productive rate was found to increase with the size of the initial popu- 

 lation. This effect may be due to the release into the medium of a 

 beneficial substance produced by the protozoans, to the low absorp- 

 tion per individual of some inimical material originally present, or 

 to more adequate control of the bacterial population (Mast and Pace, 

 1946; Alice, 1951). 



Another effect of numbers on success in reproduction involves re- 

 actions, some of which may be considered to be in the realm of 

 group psychology. In many animals breeding activity is initiated 

 only when an aggregation of more than a certain minimum size has 

 been formed. The necessity for the existence of a sizable group in 

 order that reproductive behavior will be called forth is clearly seen 

 among social insects and among certain fishes, birds, and mammals, 

 as well as among other types of animals. 



This effect of numbers is particularly striking in birds that breed 

 in colonies ( Darling, 1938 ) . Anyone who has visited an island where 

 terns are nesting or a cliff where gannets have established a rookery 

 (Fig. 1.2) will have obtained an indelible impression of the atmos- 

 phere of excitement caused by the sight of thousands of wheeling and 

 darting birds and by the sound of their screaming calls. The visual 

 and auditory stimuli produced by this uproar are necessary, or at 

 least valuable, in preparing the birds psychologically for mating as 

 well as for setting off and integrating the elaborate behavior patterns 

 involved in nest building, incubation, and feeding the young. If 

 there are too few birds in the colony, the reproductive cycle will not 

 be successfully completed. Furthermore, the large concentration of 

 birds often induces synchrony in reproduction, and this provides a 

 certain amount of protection since predators cannot destroy a serious 

 proportion of the eggs and young if they all appear within a short 

 period. Even without synchronous reproduction large colony size 

 has the advantage of a relatively smaller periphery where young may 

 wander away or be attacked by enemies. This need in certain species 

 for group stimulation accounts for the failure of isolated pairs to breed 

 and for inefficient breeding as seen, for example, in small colonies of 

 gannets (Fisher and Vevers, 1944). An extreme illustration of these 

 principles is the report that a minimum of 10,000 birds, nesting at a 

 mean density of 3 nests per sq m, is necessary for the establishment 



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