THE BIOTIC ENVIRONMENT OF ORGANISMS 



575 



Fourth Link. Still larger and stronger animals that feed upon the 

 third link. Their size makes it unprofitable for them to hunt and eat 

 the small midges, and they depend upon the "third link" organisms to 

 concentrate their food into larger packages. They exist in still smaller 

 numbers than the "third-link" organisms and have a still smaller biotic 

 potential. Examples: frogs and fish 

 that feed upon spiders, dragonflies, 

 and robber flies. 



Fifth Link. Still larger and 

 stronger forms with still smaller 

 population and biotic potential. 

 Examples: the herons and other 

 large wading birds that live largely 

 on frogs and shallow-water fishes. 



Sooner or later, a given food 

 chain reaches its end in animals too 

 large and too powerful to be fed 

 upon by other predators. They are 

 also too few and too slow-breeding 

 to support another link. The num- 

 ber of links will vary but in most 

 instances will not exceed five or six. 

 The tracing of actual food chains 

 is greatly complicated by the exist- 

 ence of alternative possibilities and 

 by the large number of possible 

 short circuits and crossings over 

 with other chains. Thus a fox may 

 finally reap the benefit of photo- 

 synthesis via grass and rabbits ; via 

 grass, grasshoppers, and quail; or 

 via plant sap, sap-sucking insects (plant lice and leaf hoppers), lady 

 beetles, spiders, tree frogs, and snakes. 



The accompanying diagram (Fig. 33.7) shows some staple food rela- 

 tions among various common groups of animals and plants in the Saint 

 Johns River in northern Florida. The energy from the sun is captured by 

 the photosynthesis of two great groups of plants: the phytoplankto?i, 

 minute but immensely abundant microscopic algae that drift submerged 

 through the open, sunlit waters of the river and its lakes; and the larger 

 rooted or drifting plants of the shallow margins, bays, and inlets. The 

 latter comprise both a luxuriant growth of submerged and emergent 

 rooted plants and huge drifting "rafts" of water hyacinths and water 

 lettuce. Both the phytoplankton and the larger vegetation support large 



Fig. 33.9. The red mangrove, Rhizophora 

 mangle, growing on the shore of Biscayne 

 Bay near Miami, Fla. This is the dominant 

 plant of a pioneer community character- 

 istic of shallow, muddy seashores in tropical 

 and subtropical America. The tangled mass 

 of roots breaks the force of the waves and 

 holds silt, building the shore outward. The 

 spikelike seedlings, dropping from the over- 

 hanging branches, root first into the mud, 

 plant themselves and extend the seaward 

 margin of the thicket. {Photo by Prof. 

 A. M. Laessle.) 



