CHAPTER III 



ECOLOGICAL SUCCESSION 



A number of changes ( i ) are always taking place in animal communities, 



(2) one of the most important of which is ecological succession, which 



(3) causes plant associations to move about slowly on the earth's surface, 

 and (4) is partly due to an unstable environment and partly to plant 

 development which typically consists (5) of a sere of associations starting 

 with a bare area and ending with a climax association. (6) Each 

 region has a typical set of seres on different types of country which 

 (7) may be studied in various ways, of which the best is direct observation 

 of the changes as in (8) the heather moor described by Ritchie or (9) the 

 changes following the flooding and redraining of the Yser region 

 described by Massart or (10) a hay infusion ; but (11) indirect evidence 

 may be obtained as in the case of Shelford's tiger beetles. (12) The 

 stages in succession are not sharply separated and (13) raise a number 

 of interesting problems about competition between species of animals, 

 which (14) may be best studied in very simple communities. (15) In 

 the sea, succession in dominant sessile animals may closely resemble 

 that of land plants, while (16) on land, animals often control the direction 

 of succession in the plants. Therefore (17) plant ecologists cannot 

 afford to ignore animals, while a knowledge of plant succession is 

 essential for animal ecologists. 



I. We have spoken of animal communities so frequently in 

 the last chapter that the reader may be in danger of becoming 

 hypnotised by the mere word " community " into thinking 

 that the assemblage of animals in each habitat forms a com- 

 pletely separate unit, isolated from its surroundings and quite 

 permanent and indestructible. Nothing could be farther 

 from the true state of affairs. The personnel of every com- 

 munity of animals is constantly changing v^ith the ebb and 

 flow of the seasons, with changing weather, and a number of 

 other periodic rhythms in the outer environment. As a 

 result of this it is never possible to find all the members of 

 an animal community active or even on the spot at all at any 

 one moment. To this subject we shall return in the chapter 

 on the Time Factor in Animal Communities, since its 



iS 



