ECOLOGICAL SUCCESSION 19 



discussion comes more suitably under the structure of animal 

 communities than under their distribution. 



2. There is another type of change going on in nearly all 

 communities, the gradual change known as ecological succession, 

 and with this we will now deal. Such changes are sometimes 

 huge and last a long time, like the advance and retreat of ice 

 ages with their accompanying pendulum swing from a tem- 

 perate climate with beech and oak forests and chaffinches, to 

 an arctic one with tundra and snow buntings, or even the 

 complete blotting out of all life by a thick sheet of ice. They 

 may, on the other hand, be on a small scale. Mr. J. D. Brown 

 watched for some years the inhabitants of a hollow in a beech 

 tree, and the ecological succession of the fauna. At first 

 an owl used it for nesting purposes, but as the tissues of the 

 tree grew round the entrance to the hollow it became too small 

 for owls to get into, and the place was then occupied by nesting 

 starlings. Later the hole grew smaller still until after some 

 years no bird could get in, and instead a colony of wasps 

 inhabited it. The last episode in this story was the complete 

 closing up of the entrance-hole. This example may sound 

 trivial, but it is an instance of the kind of changes which are 

 going on continuously in the environment of animals. 



3 . If it were possible for an ecologist to go up in a balloon 

 and stay there for several hundred years quietly observing the 

 countryside below him, he would no doubt notice a number of 

 curious things before he died, but above all he would notice 

 that the zones of vegetation appeared to be moving about 

 slowly and deliberately in different directions. The plants 

 round the edges of ponds would be seen marching inwards 

 towards the centre until no trace was left of what had once 

 been pieces of standing water in a field. Woods might be 

 seen advancing over grassland or heaths, always preceded by 

 a vanguard of shrubs and smaller trees, or in other places 

 they might be retreating ; and he might see even from that 

 height a faint brown scar marking the warren inhabited by the 

 rabbits which were bringing this about. Again and again 

 fires would devastate parts of the country, low-lying areas 

 would be flooded, or pieces of water dried up, and in every 



