CHAPTER X 



DISPERSAL 



The study of animal dispersal involves (i) a large number of subjects 

 besides biology, and we can only deal here with the general ecological 

 aspects. (2) Most animal dispersal is directed towards the ordinary 

 feeding, breeding, and other requirements of animals, not directly 

 towards spreading the species, as shown (3) by the example of the 

 capercaillie in Scotland. (4) The terms used in discussing the subject 

 need to be carefully defined : the " spread " of any species involves 

 " dispersal," " establishment of the individual," and " establishment of 

 the species." These three phases sum up to control the " distribution " 

 of a species at any one time. (5) Ecological succession plays a large 

 part in slow dispersal of animals. (6) Movements of animals are often 

 periodic and regular, but these do not necessarily extend the range 

 of the species. (7) Besides active migration, many animals become 

 dispersed by " accidental " means (which are often very definite and 

 regular) ; they employ wind, water, logs, ships, seaweed, etc., and 

 (8) other animals. Direct evidence on this subject is hard to obtain, 

 and (9) is usually encountered by accident in the course of other ecological 

 work. It is most important to publish such incomplete observations, as 

 they may be unique. (10) Dispersal may take place as described above, 

 through the ordinary activities of the animals, but where it is a definite 

 large-scale phenomenon resulting in spread of the species, it may be 

 either for the purpose of getting away from an overcrowded population 

 or for getting towards some new place, or for both. (11) Much abortive 

 colonisation is always taking place, partly owing to ordinary ecological 

 factors being unsuitable and (12) partly to the difficulty of finding a mate 

 upon arrival at a new place. (13, 14) Three methods of finding the 

 right habitat are employed by animals : broadcasting huge numbers so 

 that a few by chance reach the right habitat ; directive migration, by 

 means of special instincts or tropisms ; and a combination of general 

 broadcasting and local direction-finding. 



I. When we are studying any particular animal or com- 

 munity of animals, we are brought up, sooner or later, against 

 questions connected with dispersal : with the movements of 

 animals in search of food, of shelter, or of their mates. This 

 movement, on a large or small scale, is characteristic of animal 

 communities, as compared with plant communities, and it 

 forms a very important part of the lives of wild animals. Dis- 

 persal is such a large subject, and has so many sides to it, that 



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