496 Dynamics of the Ecosystem 



period, and there has been no yield or other material removed. 

 Nevertheless, productive activity has been going on— the wheels of the 

 factory have been turning— but the products of the factory have been 

 taken in again as raw materials. An analytical scrutiny of the eco- 

 system reveals what processes are going on and at what rates, and 

 how they are interdependent. A basis is also obtained for judging the 

 productive capacity of the area and for deciding on the best pro- 

 cedure for the continued or improved use of the area for human needs. 

 In an unfished lake, for example, the yield to man is zero. If fishing is 

 to be initiated, information on the production rates of the various 

 living components of the lake is desirable in order to understand the 

 factors that would control the size of the fishery that could be per- 

 manently supported by the lake. 



In areas in which rates of production are higher than rates of de- 

 struction a net increase in one or more of the components will occur. 

 The surplus material may be removed by man, thus constituting a 

 yield, or it may be removed in other ways described in the previous 

 section, or it may be permanently added to the system. In the last 

 case an increase in the organic components of the ecosystem will be 

 experienced, but such an increase can take place only to the extent to 

 which an equivalent supply of materials is added to the area. The 

 amount of organisms and of nutrients in a lake, for example, are fre- 

 quently seen to increase in this way over a period of time with the 

 result that the lake becomes more eutrophic; the process is known as 

 eutrophication. Similar increase of the living and non-living com- 

 ponents of the ecosystem occurs in other types of habitat. 



Turnover. In our discussion of production rate nothing has been 

 said thus far about the length of the period considered. The time 

 required for each type of organism to complete its growth, to die and 

 decompose, and to start the cycle over again is known as its period 

 of turnover. The length of the turnover period usually differs widely 

 for the organisms at different trophic levels in the production pyramid, 

 and may differ for the same level in different situations and at differ- 

 ent seasons of the year. Herbaceous plants in a temperate terrestrial 

 area usually have only one turnover per year, whereas the phyto- 

 plankton of an aquatic area may turn over within a few days or a few 

 weeks. In the latter instance the same material may be used again 

 several times during the year, and it would thus have little meaning 

 to add up the increments of growth for the whole year in an attempt 

 to reach a "total" annual value. If the growth of the plants is meas- 

 ured as energy, it is permissible to sum up the amount of energy 

 which has been transformed during the year since the energy can be 



