Relation to Population Growth 483 



to the number of organisms that can be harvested. In addition to 

 the yield taken by man other material may be removed from the area 

 by active or passive emigration or by the withdrawal of organic matter 

 from circulation through the formation of deposits. The second con- 

 cept of productivity is the material removed from the area per unit 

 time, and includes the yield to man, organisms removed from the 

 ecosystem by emigration, and material withdrawn as organic deposits. 

 When the modern ecologist enters the area, he views the situation 

 in a still different light. He should, of course, be aware of the num- 

 bers and types of organisms present, and of the extent of their removal 

 from the area by man and in other ways, but his attention is directed 

 chiefly to the dynamic aspect of the growth and interdependency of 

 the inhabitants as members of a functioning system. The ecologist 

 is concerned with natality and mortality in the various components 

 of the population and with rates of metabolism and growth. He in- 

 quires not only into the different types of machinery in the factory 

 but also into the speeds at which the wheels are turning. The third 

 concept of productivity is the production rate, or the rate at which 

 growth processes are going forward within the area. Each of the 

 fundamental concepts of productivity will be considered separately 

 in further detail. 



Standing Crop 



Measurements of the standing crop reveal the concentration of 

 individuals in the various populations of the ecosystem. This in- 

 formation is essential for judging whether the degree of crowding in 

 each species is exerting a harmful or a beneficial effect. Further, in 

 the study of the dependence of one species on another the enumera- 

 tion gives a measure of the intensity of predation and of the avail- 

 ability of the forage species. The size of the standing crop is also of 

 vital concern in relation to the exploitation of natural populations by 

 man. The abundance of the plant or animal influences the efficiency 

 with which the enterprise is carried forward. However, since a 

 knowledge of the standing crop gives no information on its replace- 

 ability, other aspects of productivity must also be taken into account 

 for intelligent use of natural resources of this kind and for a main- 

 tained harvest. 



Relation to Population Growth. The relation of the size of the 

 standing crop to the rate of growth of the population will be con- 

 sidered for the three types of situations illustrated in Fig. 13.8. In 

 the first a rapid increase in population size is indicated, leading to a 



