ECOLOGICAL SUCCESSION 25 



of prawns {Palcemonetes varians) left behind in some of the 

 shell holes. 



10. Ecological succession may easily be studied experi- 

 mentally by making a hay infusion in water and leaving it 

 exposed to the air for several weeks. Bacteria are the first 

 things to become abundant, since they live upon the decaying 

 vegetable matter. Then various protozoa appear, and it is 

 possible to see a whole animal community being gradually 

 built up, as each new species arrives and multiplies and fits 

 into its proper niche. In a hay infusion the bacteria are 

 followed by small ciHate protozoa of the Paramecium type, 

 which subsist upon bacteria and also by absorbing substances 

 in solution and in suspension in the water. Then there are 

 larger hypotrichous ciliates, which prey upon bacteria and 

 also upon the smaller ciliates. Eventually the whole culture 

 may degenerate owing to the exhaustion of food material for 

 the bacteria and therefore for the animals dependent on them. 

 On the other hand, green plants, in the form of small algae, 

 may arrive and colonise the culture. These will be able to 

 subsist for a long time, and may change the character of the 

 whole community by providing a different type of food. 

 Succession in hay infusions is particularly fascinating, since it 

 can be studied anywhere, and does not last over a very long 

 time. 



11. Another method of determining the course of animal 

 succession is to work from a knowledge of the succession 

 relations of the plant communities (gained either from direct 

 observation or from deduction and comparison with other 

 districts) and then work out the animal communities of each 

 plant zone. It is then possible to say in a general way what 

 animals will replace existing ones when succession does occur. 

 This was done by Shelford,20 who studied the tiger beetles 

 of the genus Cicindela (carnivorous ground beetles of variegated 

 colours) in a sere of plant communities on the shores of Lake 

 Michigan. The lake-level has been falling gradually of late 

 years, and there can be seen all stages in succession on the 

 bare areas left behind on the shores. On the lake margin was 

 Cicindela cuprascens^ whose larvae live in wettish sand. Young 



