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 
Ay ies ai aie DO ae Che 
EES eR ITO 
ei Dep ctinase 
® cae iS ae 6 
A B Cc D E F 
DraGRam 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 propellors 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. 
