490 Dynamics of the Ecosytem 



farming or hunting in which products are not taken to the market, the 

 human agent may be regarded as an integral part of the local eco- 

 system; but in most farming, fishing, hunting, and forestry operations 

 a large part of the organisms harvested is carried away, and hence 

 the material of which the plants or animals are composed is perma- 

 nently removed from the area. 



Permanent emigration of animal or plant life similarly represents a 

 loss of material from the ecosystem. Organisms may leave the area 

 by their own locomotion or they may be carried away by wind or 

 water currents. When the activity of natural predators results in the 

 one-way transfer of organic material from one ecosystem to another— 

 as when a fish hawk removes fish from a pond— the loss of material 

 from the ecosystem under consideration is also to be included in this 

 category. Another type of loss is represented by dead organic matter 

 that is buried in the soil or in the mud of an aquatic biotope in such 

 a way that it is functionally removed from the system. Also in- 

 cluded is material that has gone into an inert chemical form and thus 

 is essentially unrecoverable during the period under consideration. 



If a community is to maintain itself, material in some form must be 

 supplied to it in an amount at least equal to the total amount of 

 material removed from the area in the various ways discussed. The 

 supply in the form of nutrients and immigrants usually must be con- 

 siderably larger than the removal because of the inevitable loss of 

 energy and of material from the system at each level in the food chain. 

 Ordinarily, man's harvest, which is only one portion of the total 

 amount of material removed from the area, is very much smaller than 

 the total supply to the area from all sources. The ratio of yield to 

 supply gives one measure of the efficiency of the use of the area. 

 Such a measure can show whether a needlessly low utilization is 

 taking place or overexploitation is under way. 



The yield to man may be improved in one or more of several ways. 

 Although practical limitations must be considered in each specific 

 situation, a summary of the theoretical possibilities is desirable as a 

 basis for realistic procedure. A maintained yield can never average 

 larger than the supply. However, the natural supply of materials 

 entering the area may be augmented artificially by adding fertilizers 

 or by stocking; and the amount of materials removed or lost in ways 

 other than man's harvest may be curtailed. In addition, the degree 

 of crowding of the desii'ed species may be adjusted to the optimal 

 level, and the competitors of this species may be removed. 



The foregoing points will be illustrated by a schematic considera- 

 tion of the interdependencies of a natural pasture from which it is 



