574 



INTERRELATIONSHIPS OF ORGANISMS 



but visit and feed in both of them. In the same way, the primary soil, 

 leaf-mold, floor, herbage, shrub, and tree-inhabiting communities of the 

 hardwood forests combine to form a larger hardwood forest community, 

 to which animals like the gray squirrel and raccoon (at least partly) 

 belong, although these animals are not confined to any of the primary 

 communities of the forest. 



Food chains. Many of the internal relationships of biotic communities 

 are due to specific sequences of various organisms in the energy cycle. 

 Although all animal food is dependent upon the anabolism of plants, not 



all kinds of animals can eat plant 

 food. In fact, most of the plant- 

 eating animals are adapted to feed 

 on only a few species or types of 

 plants, and most flesh eaters are 

 limited to a very small list of 

 suitable prey. Most organisms are 

 specialists in their food habits and 

 are confined to particular kinds and 

 sizes of food. This results in a wide 

 variety of food chains. 



All food chains have their basis 

 in some dependable supply of a 

 definite type of food, either (pri- 

 marily) in some continuously re- 

 newed supply of plant protoplasm 

 or (secondarily) in some con- 

 stantly accumulating supply of 

 organic detritus such as occurs in 

 the soil or on the bottoms of lakes, 

 ponds, and pools. Given such a 

 food supply, the resulting food chain is somewhat as follows: 



First Link. The constantly renewed supply of plant protoplasm or 

 the steady accumulation of detritus. 



Second Link. One of a few different species of "base-industry" 

 animals that feed upon the plant protoplasm or detritus and convert 

 it into animal protoplasm. All base-industry animals exist in large popula- 

 tions and have a high biotic potential. Example: the "midges" (Chiron- 

 omus) that feed on lake-bottom detritus and occur in tremendous 

 numbers. 



Third Link. Larger and stronger animals that feed upon the base- 

 industry population. They are much less numerous and have a smaller 

 biotic potential than the base-industry animals. Examples: the spiders, 

 dragonflies, and robber flies that feed on midges. 



Fig. 33.8. The walking fern, Camptosaurus 

 rhizophilus, growing in a rock crevice, 

 Chester County, Pa. This pioneer plant is 

 able to endure the severe environmental 

 conditions provided by rocky cliffs and 

 outcrops which are more or less moist and 

 partly shaded. (Photo by Prof. A. M. 

 Laessle.) 



