168 TERRESTRIAL CONDITIONS 



re-established. The decrease of mice just noted might, however, cause 

 the coyote to eat more ground squirrels and thus cause an increase of 

 insects because of the removal of the ground squirrel as a check upon 

 their numbers. The numerous checks upon the numbers of insects 

 would tend to prevent their increasing greatly, but would no doubt 

 affect the greater part of the community. The reader will be able to 



A B C D E F 



DIAGRAM 7. Representing the food relations of the animals of a land community. 

 The circles represent life histories which come into contact or overlap at the point 

 where one species feeds upon another. The vertical shafts represent the animals 

 which feed upon the vegetation (herbivora and phytophaga). The extent to which 

 the shaft penetrates the community indicates its importance as food of the forms 

 whose life histories are represented. The letters refer to the vertical lines (shafts) 

 above them. These lines (shafts) represent the various central groups of Diagram 6 

 and other comparable groups as follows: A, large herbivores such as the bison; 

 B, the mice, rats, squirrels, and rabbits; C, vegetation-eating birds; D, boring insects 

 secured by the woodpeckers; E, the large plant-eating insects; F, the small soft- 

 bodied insects such as aphids, scales, etc. The animals represented by the shafts are, 

 figuratively speaking, the propellers which keep the life histories shown above them, 

 turning. 



trace out many such possible fluctuations and equilibrations. The 

 number of possibilities is great even in an arbitrary community, though 

 much greater in an actual one. 



Diagram 7 is a graphic representation of the relations of life his- 

 tories in land communities to elementary food substances. The number 

 of plant-feeders which serve to lock the inorganic substances to the main 

 part of the community is far greater than in aquatic communities. 



