Material Removed 491 



desired to obtain the maximal yield of sheep (Fig. 13.9). Inereased 

 growth of the plant producers will result from the addition of fer- 

 tilizers, but it is essential that this added supply of nutrients reaches 

 the grass rather than the undesirable weeds. The grass supports a 

 flock of sheep but it usually also supports a population of field rodents. 

 If the rodents are excessively abundant, a reduction in their number 

 would allow a larger portion of the grass to be consumed by the sheep. 



SUPPLY 



YIELD 



Fertilizer- 



► Grass ^ Sheep 



Weeds v Rodents — 



->" Man 

 Z^ Coyotes 



Fig. 13.9. Schematic representation of the relation of man's harvest from an area, 



such as a natural pasture, to the supply of materials and to the reduction of the 



product in various ways. 



Often the best method of controlling the rodents is to allow the pres- 

 ence of predators such as coyotes, but should the coyotes become too 

 numerous they would compete with man for the sheep and their num- 

 bers might have to be reduced. A suitable balance must be main- 

 tained among all the members of the ecosystem. The population of 

 the species to be harvested should also be adjusted to the level of abun- 

 dance at which its growth rate will be as great as possible without 

 disturbing the equilibrium of the community. Study of the ma- 

 terials entering and leaving the area should reveal whether the area 

 is being managed to the best advantage from the point of view of its 

 desired use. The maximum potential yield is the largest maintained 

 harvest that can be removed from an area under the best population 

 and environmental conditions. 



An aspect of the yield of great practical importance is the ijield per 

 unit effort. This may be measured in terms of man power, cost, or 



