1. COMPONENT RELATIONSHIPS 



Trophic Dynamics, with Special Reference to the Great Lakes 



Trophic dynamics is that kind of 

 ecology which concerns itself with 

 energy flow through the component 

 organisms of an ecosystem. The ul- 

 timate source of energy for any living 

 system is, of course, the sun. Green 

 plants, converting the sun's radiant 

 energy into chemical energy, are said 

 by ecologists to constitute the first 

 trophic level within an ecosystem. 

 All photosynthetic plants, regard- 

 less of systematic affinity, are thus 

 grouped together by ecologists be- 

 cause they all perform this same 

 basic function. 



Animals that subsist largely by 

 eating green plants constitute the 

 second trophic level, be they aphid or 

 elephant. Their energy source is 

 once-removed from the initial fixation 

 of radiant energy. Although animals 

 of this trophic level are referred to 

 by ecologists as "primary consum- 

 ers," the lay term "herbivore" is of- 

 ten useful. Carnivores that prey 

 largely upon herbivores of any sort 

 constitute the third trophic level. 



There are usually no more than 

 five trophic levels in an ecosystem 

 because the inevitable loss of energy 

 in the shift from one trophic level 

 to the next higher means that the 

 total energy contained in the bodies 

 of organisms on the fifth trophic 

 level is small relative to the first. 

 This relatively small amount of en- 

 ergy at the top level is disposed 

 into a small number of large and 

 usually widely dispersed bodies, since 

 there is a tendency for the predators 

 at the top levels to be larger than 

 their prey. (See Figure VIII— 1) 



While the fifth level is often 

 reached in marine ecosystems, in the 

 Great Lakes it is not. Large lake 

 trout feeding upon fish would be the 

 top predators in the open-water com- 



munity. They operate on the fourth 

 trophic level. Smaller lake trout often 

 subsist largely on small crustacean 

 herbivores; they would be assigned 

 to the third level. Roughly speaking, 

 about half of the living material in 

 a large lake at any one time resides 

 in the tiny cells of the numerous 

 photosynthetic algae — the first level. 



In lakes as large and deep as the 

 Great Lakes, the overwhelming pre- 

 ponderance of life is found in the 



open waters — away from the shore 

 and bottom. Yet it is still desirable 

 to refer to this assemblage of life 

 in the open waters as a "community," 

 not an "ecosystem," because the open 

 waters lack full representation of 

 still a different trophic category — 

 "reducers." Reducers is the term 

 ecologists apply to the variety of 

 bacteria and fungi that derive their 

 energy from the complex molecules 

 in the dead bodies and feces of other 

 organisms of the system. Energeti- 



Figure VIII- 1 — TROPHIC LEVELS 



The figure illustrates an ecological pyramid showing various trophic levels. The 

 higher the step in the pyramid, the fewer the number of individuals and the larger 

 their size. In some environments, large animals circumvent some of the levels 

 in the food chain. For example, man takes from all levels below himself, including 

 that of the producers. 



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