Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 87 



in human blood, a squash bug on the vine, or a citizen in the town, it is con- 

 cerned with a biological environment, human or otherwise. The animals of an 

 environment are roughly divided into producers of food and competitors in 

 the consumption of food. Some of the consumers are predators that rob and 

 kill. 



Search for Food. Numerous and widely distributed animals are apt to live 

 on common foods. Rodents — squirrels, field mice, and rabbits — all abound in 

 great numbers; so do the shrubs, grasses, and clover which they eat. Grass- 

 hoppers and crickets live surrounded by grass and grain. At the height of their 

 season the only grass-eaters that compete with them in open fields are cattle 

 and sheep. During the great migrations of grasshoppers nothing stands in 

 their way (Chap. 30). Birds, small mammals — shrews, ground moles, and 

 chipmunks — commonly prey upon them. But their reproductive capacity is 

 so high that these predators do them the good turn of keeping the population 

 to a size which the space and food can support. Animals multiply greatly in 

 regions where they have few or no competitors for the particular food on 

 which they live. This is strikingly true of penguins in the Antarctic. The same 

 principle applies to nocturnal animals such as owls and skunks that hunt by 

 night when there is less competition. 



Biological environments obviously depend on the chemical and physical 

 ones. Plant populations rely particularly upon water and temperature and 

 animals follow the plants. Animals abound at river mouths to which the river 

 brings rich organic deposits. Rivers and their valleys have always determined 

 the location of animals just as they have always determined locations for man- 

 kind. 



Size of Food. Man is the only animal that can catch all sizes of animals, 

 from frogs to cattle, oysters to whales, and use them for food. He can eat 

 small, large, and medium-sized animals indiscriminately: an important control 

 to have over the environment. The scavengers — vultures (turkey buzzards), 

 lobsters, pigs, and chickens — approach mankind in the variety and sizes of 

 food which they appropriate. With the exception of parasites and scavengers, 

 other meat-eaters must deal with food that is adequate but not too large to be 

 manipulated. Fierceness and skill may take the place of size in capturing prey, 

 so may social behavior. Packs of wolves will attack a moose but a solitary 

 wolf seldom does so. Millions of South American army ants will set upon and 

 kill small mammals but no one of them could do it alone. 



Food Relations. The food relations of a community are exceedingly com- 

 plex, changeful, and affected by factors in the immediate environment as well 

 as others far outside it. The complexity of the human food market is an exam- 

 ple with its many and remote causes of undersupply and oversupply and 

 resulting prices. The food relations between animals are expressed as food 

 chains, food webs, and pyramids of numbers (Fig. 5.20, 5.21 ). A food web is 



