38o 



ELEMENTS OF BIOLOGY 



larvae and worms, and themselves afford food for turtles and large 

 fish. Parasitic animals, for example tape and round w^orms, and 

 bacteria attack fish and other animals. The dead bodies of these 

 larger animals in turn afford food material for saprophytic bacteria. 

 So the cycle of food goes on. Any animal community may be an- 

 alyzed in a similar way (Fig. 220). 



I N/TROGEN 



I protozoa\ 



i BACrER/A'Y ^ — I DUNG H j GULLS \ 



» | NITRATES I 



DEAD PLANTS 



PLANTS 



/NSECTS-^SP/DERS\ 



GEESE AND OTHER 

 BIRDS 



DUCKS, AUKS 



AND OTHER 



BIRDS 



I riSH I 



I SEALS I 



POLAR 

 BEARS 



WORMS j 



I ALGAE \ I protozoa] -» | DECAYING MATTER 



ROTIFERS 

 SMALL ARTHROPODS 



ALGAE \ 

 1 ' 



PROTOZoT^ 



INSECTS I 



^^^T^ H INSECT LARVAE \ 



ANNELIDS 

 NEMATODES 

 I MOSS H ROTIFERS 



Fig. 220. — The food chain in an isolated community, the arctic Bear Island. Follow 

 the arrows. (Adapted from Summerhayes and Elton: Journal of Ecology.) 



The relative numbers of individuals in each of the various species 

 of plant and animal life that make up communities regulate the 

 food supply of the various types, that is, relative numbers establish 

 a food balance. And the food supply in turn regulates to a consider- 

 able extent the number and size of individuals in the various species. 

 Thus if the number of any one species is increased, the food bal- 

 ance is disturbed and may be restored or a new balance arrived at in 

 several ways. The excess may die as a result of failure to find suffi- 

 cient food, or migrations to establish new communities may occur 



