30 THE ECOLOGY OF ANIMALS 



In the North Sea, diatoms (which are algae) in the 

 plankton are eaten by the copepod Pseudocalanus, 

 which is eaten by sand-eels, and the latter by the 

 herring, while the herring is captured by man and 

 sea-birds and dogfish, the latter itself being also caught 

 by man. 



This phenomenon of food- chains was first pointed 

 out by Shelford (1913), who constructed a theoretical 

 food-cycle diagram for some of the animals of some 

 IlHnois habitats. It was not until some ten 3^ears 

 later that more exact food- cycles began to be worked 

 out for animal communities. In no instance has it 

 been possible to work one out completely, but the 

 method is a useful one, and enables the organization 

 of an animal community to be more clearly seen than 

 would otherwise be possible. Food- chains are seldom 

 simple and self-contained. Usually several preys and 

 several predators interact at each stage. Two genera- 

 lizations can, however, be made about them. First, 

 there are seldom more than half a dozen stages in 

 the sequence from herbivore to the final predator 

 that has no predatory enemies. Usually there are 

 not as many as this. Thus the snowshoe rabbit in 

 Canada is a vegetarian, and is preyed upon by the 

 lynx and the fox and various birds of prey, none of 

 which have further enemies. When we start with 

 a smaller animal such as a copepod or a springtail 

 or an aphid, there are usually more stages in the 

 complete food-chains which come to depend upon 

 them. The Hmited number of stages is due primarily 

 to size considerations. There is considerable differ- 

 ence between the sizes of most predators and their 

 preys, and after a food- chain has passed through 

 several stages, size hmits begin to be reached. The 

 second point, which further explains the first, is that 

 the numbers of individuals gets smaller as each stage 

 in a food- chain is reached. The reason for this is 

 that every animal has to produce not only enough 

 young to provide against non-biotic checks, but also 



