32 THE ECOLOGY OF ANIMALS 



(as distinct from plants which have in the main a 

 common food supply), food-chains which are built 

 up into complex food- cycles for a whole community, 

 the limited number of stages in each chain, the idea 

 of niches (which applies also to other habits than 

 feeding), the pyramid of numbers in which each 

 species in a progressive food- chain becomes lower in 

 its density of numbers, the existence of habits which 

 partly control population among the species at the 

 end of such chains, the corresponding chains of para- 

 sites, and throughout these the dominant importance 

 of the size of animals. As a result of these principles 

 we find that the organization or structure of an animal 

 community is not widely different in almost any 

 habitat which supports a rich fauna at all. Even 

 in intertidal marine communities where sessile animals 

 play a dominant part, food- chains form the most 

 striking feature in the life of the community. Food- 

 cycle maps have so far been published for all or 

 parts of the following communities : temperate forest 

 communities in Illinois ( Shelf ord, 1913), high Arctic 

 fjaeldmark, and associated habitats (including fresh- 

 water) in Spitsbergen, North-East Land and Bear 

 Island (Summerhayes and Elton, 1923, 1928), aspen 

 parkland and prairie in Manitoba (Bird, 1930), young 

 pine trees on Oxshott Common in England (Richards, 

 1926), the vertebrates of the Kara Kum desert (Kash- 

 karov and Kurbatov, 1930), and for the plankton of 

 the North Sea in relation to herring food (Hardy, 

 1924). 



These intricate series of inter-relationships of whose 

 nature we are only now beginning to see a glimpse, 

 have inevitably given rise during evolution to in- 

 numerable special adaptations connected with attack 

 and defence. These adaptations have been studied 

 in great detail, and form a subject of their own — 

 especially in connexion with the widely debated but 

 firmly established theory of mimicry (see Carpenter 

 and Ford, 1933, in the present series). Mimicry forms 



