ENVIRONMENTAL RELATIONSHIPS 



obtain stored solar energy very indirectly. In any food chain, there may be 

 two or three, but seldom more, successive carnivorous links, until at last the 

 chain terminates in a large carnivore which does not serve as prey for any 

 other member of the community. These largest carnivores, while living, 

 support only their indigenous populations of ecto- and endoparasites, although 

 they may be preyed upon by transient carnivores, such as birds or bears, 

 from other communities. If the pickerel die in the pond, their bodies are 

 decayed by microorganisms, and their substance re-enters the food cycle as 

 inorganic compounds required for the growth of aquatic plants. 



Let us now consider size of food as it relates to the food chain. Note that 

 in our example the successive links are: algae — protozoans — small insects — 

 large insects — small fishes — large fishes. That is, there is a stepwise increase 

 in the size of food from the beginning to the end of the chain. The large 

 fishes do not feed on protozoans, or normally even on small insects; it is 

 virtually impossible for them to capture enough of these tiny animals to sat- 

 isfy their food requirements. On the other hand, neither do the small insects 

 prey on the large fishes, which are beyond their ability to capture or ingest. 

 Thus, every carnivore is restricted by its own limitations of size and strength 

 to kinds of food small enough for it to manage; but there are also lower limits 

 to the size of food which it can profitably handle. In short, the size of an 

 animal largely determines its place in the complex predator-prey sequence 

 called a food chain. 



Related to the food chains, and also to the size of food, are certain consider- 

 ations of numbers of animals. Every food chain represents what has been 

 termed a "pyramid of numbers"; at every upward step in the chain, there is 

 an increase in the size of organisms but a corresponding decrease in their 

 numbers. In our pond, very large numbers of herbivores are required for the 

 support of a smaller number of insects; these, in turn, suffice to maintain a 

 few hundreds of small fishes, which furnish only enough food for a few dozen 

 large fishes. In such an environment as that of the pond, limitations of space, 

 sunlight, and available chemicals impose finite restrictions on the amount of 

 plant growth that can occur. These restrictions are transmitted stepwise up- 

 ward through the food chains, with the result that the maximum growth of 

 plants will support, at the third or fourth remove, only a small number of the 

 largest carnivores. 



But why should this be so? The really significant factors underlying these 

 numerical relationships appear to be considerations of mass and energy. As 

 we have seen, the basis of food relations is the requirement of energy; but at 

 every step in the food chain, about 80 to 90 per cent of the available energy 

 is dissipated. For example, the flesh of the small fishes, upon which the large 

 fishes prey, contains only about 10 to 20 per cent of the energy present in the 

 bodies of the many insects supporting the small fishes. This accounts for the 

 relatively small numbers of large carnivores in the community; in view of the 

 inefficiency of energy transfer along the food chain, there is not enough energy 

 left at the end of the chain to maintain more than a few terminal members. 



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