ISO ANIMAL ECOLOGY 



successful in breeding and starting a permanent popula- 

 tion of its kind. Frequently a species may reach some new 

 place and breed, and may establish itself for a short time, but 

 then is wiped out. This often happens because the animal is 

 not adapted to some periodic factor which acts at fairly long 

 intervals, e.g, a very bad winter, or an epidemic. Or the 

 species may die out simply because its numbers and rate of 

 increase are not suitably adjusted to the new environment in 

 which it finds itself. For instance, after big invasions of 

 sand-grouse or of crossbills, pairs of the birds have been known 

 to breed in some localities for a year or two after their first 

 appearance. ^^^ But they usually die out in the end, and no 

 more are seen until the next invasion. Some botanists employ 

 the word " ecesis " (from the Greek word which means 

 dwelling at home) instead of " establishment." It is rather a 

 useful word, owing to its clear meaning and shortness, but at 

 the same time rather an ugly one. 



The combined results of dispersal and of the establish- 

 ment of the individual and then the species in a new place we 

 call the *' spreading " of the species. The area covered by 

 it at any one time is its '' distribution." These terms may be 

 summed up as follows : 



Dynamic . . . . Dispersal ^ 



Establishment as individual J- Spread. 

 Establishment as species J 



Static . . . . Distribution. 



The loose and undefined use of words like dispersal, spread, 

 and distribution, by faunists and palaeontologists has led to 

 a good deal of confused thinking on the subject of past move- 

 ments in the animal world, and it is well to be quite clear on the 

 subject, and to realise that the spreading of a species involves 

 at least three fairly clear-cut phases, and that successful spread- 

 ing involves overcoming a series of obstacles at each of these 

 phases, so that it may be absurd to attribute the final result 

 to any simple factor or factors. 



5. One of the important ways in which slow dispersal takes 

 place is through the migration or spreading of the environ- 

 ment, i.e. by ecological succession. The patch of vegetation 



