Efficiencies 497 



used by the plants only once. A comparison may thus be made be- 

 tween the annual incident radiation and the energy content of the 

 plants produced. In the organisms at successively higher trophic 

 levels, both tlie materials and the energy are used again one or more 

 times. A summation of amounts of production at all trophic levels for 

 a long period, such as a year, therefore, similarly has little meaning. 



The procuring of significant measurements of production rate is 

 further complicated by the fact that growth, consumption, and decom- 

 position may all be going on simultaneously. This difficulty may be 

 dealt with by reducing the period considered to such a small size that 

 instantaneous rates for the various processes are obtained; in other 

 words, by obtaining the differentials of the curves describing these 

 processes (Clarke, 1946). The productive activity of an ecosystem is 

 therefore best measured as a set of rates applying to the gross pro- 

 duction, net production, and net increase for each category of or- 

 ganism or each trophic level. The term "productivity" has sometimes 

 been used specifically for the rate at which assimilation (gross pro- 

 ductivity) or growth (net productivity) is taking place since it is a 

 measure of the rate at which the wheels of the system are turning. In 

 view of the fact that this term is used in various broader senses it is 

 preferable to employ the more explicit term, "production rate," and 

 to specify the units of time and area considered. 



Efficiencies. In ecosystems in which the magnitudes of the rates of 

 production at the different trophic levels can be measured, calcula- 

 tions may be made of the quantitative relationships of the various 

 steps in the production. The ratios, or efficiencies, on page 498 

 are of interest. In addition ratios including several or all of the steps 

 within a trophic level, or embracing more than one trophic level, may 

 be calculated. For example, the efficiency of the growth of a plant 

 or animal component may be obtained as a percentage of the incident 

 radiation. 



Where the rates of the processes involved in the ecosystem have 

 been measured, the various ratios, or efficiencies, can be calculated. 

 Very few accurate measurements are available for organisms under 

 natural conditions, but the order of magnitude is suggested by the 

 following generalities. In ponds with dense algal blooms as much as 

 14 per cent of the light incident on the water surface may reach and 

 be absorbed by the plant cells; in the open ocean less that 1 per cent 

 of light energy is absorbed by the phytoplankton. Complete forest 

 canopies absorb up to 99 per cent of the sunlight, but desert vegeta- 

 tion obviously captures a far lower percentage. The efficiency of the 

 photosynthetic process, that is, of plant assimilation, for Chlorella 



