synthesis. L'lulcr ordinary daytime lifjlit intensities, 

 the amount of sugar formed by pliotosynthesis greatly 

 exceeds the amount oxidized in respiration. Plioto- 

 synthesis ceases during darkness, but loss of sugar 

 at night, because of respiration, continues. Sugars 

 may be converted to starch or fat or upon combina- 

 tion with nitrogen, sulphur, and ])hosphorus be 

 changed into proteins. The amount of sugar resulting 

 from photosynthesis represents the primary produc- 

 tion of the ecosystem. Accurate measurement of the 

 rate of primary production is one of the most impor- 

 tant problems of trophic ecology, for the activities of 

 all plant and animal organisms in the community de- 

 pend on the energy thus supplied. It is necessary to 

 distinguish between gross production, the total 

 amount of energy captured, and net production, the 

 amount that remains after that used for respiration. 

 Primary production is commonly expressed in terms 

 of glucose or carbon, or indirectly in the amount of 

 oxygen released or carbon dioxide absorbed, all of 

 which can be converted into calories of energy. 



Vse 



Net energy at the producer level becomes avail- 

 able for use of animals when it becomes transferred 

 to the higher trophic levels through predation, here 

 considered also to include consumption of plants 

 (Riley 1940, Lindeman 1942, Clarke 1946, Clarke, 

 Edmondson, and Ricker 1946, Birch and Clark 1953, 

 Macfadyen 1957). In order to measure the transfer 

 and use of energy at each trophic level (A), it is de- 



siralilc to know the size of the standincj crop or bio- 

 inass (l<) at each level, l-'nergy is accjuired by ani- 

 mals only through the consum])tion of food, which 

 may be indicated as the ijross energy intake (I ). 



A good portion of the energy ingested is used for 

 existence, that is for basal metabolism, temperature 

 regulation, procurement and digestion of food, and 

 other normal activities. There is almost continuous 

 loss of heat energy from the body and in liomoio- 

 therms this must be compensated for by increased 

 iieat production. luiergy is used for the production of 

 eggs and s])erm, reproductive behavior, and other ac- 

 tivities in the normal life of the individual animal. 

 Kven the process of converting raw food into proto- 

 plasm is work and requires energy. In transfer of 

 energy from one form to another, there is always loss 

 of free energy. No transfer is 100 per cent efficient. 

 This is the second law of thermodynamics. The total 

 energy that is utilized to perform work and to produce 

 heat IS called respiratory energy (R). 



Transference 



The gross energy intake, /, less the respiratory 

 losses of energy, R. gives the net production of the 

 consumer levels, the same as at the producer level. 

 The rate at which net production accumulates is net 

 productivity or simply /)ro(/»ffw(Vy (X). Net produc- 

 tion may be lost to the trophic level in the decompo- 

 sition of excreta and dead animals; it may become 

 evident in the growth and increase of population ; or 

 it may be transferred to a higher trophic level. 



^" ,i' 



FIG. 14-1 Isolines of average 

 solar radiation (g-cal/cm'/ 

 day) received in July on a 

 horizontal surface In the 

 United States during days of 

 average cloudiness. In 

 December, isolines run nearly 

 straight and parallel across 

 the country, and are of 

 values less than 100 in the 

 North and over 250 in the 

 South (Fritz 1957). 



Exchanges, productivity, and yield 201 



