-I mice of energy is solar railiation. This energy is 

 captured by green plants in photosynthesis. Some of 

 tiiis energy is transferred to higher trophic levels 

 through predation, but the amount that is transferred 

 decreases at each higher trophic level in spite of the 

 greater efficiency of predation, until none remains. 



Gross productivity is the total energy intake per 

 unit area and unit time at any trophic level. It is 

 called primary productivity at the producer level and 

 secondary productivity at the consumer levels. Gross 

 energy intake minus respiratory losses is the net pro- 

 duction. 



N'et production may be lost to the trophic level 

 i!i excreta and dead animals, produce growth and in- 

 crease of populations, or be transferred to a higher 

 trophic level. Energy lost in excreta, non-predatory 

 deaths, and wastage of predatory kills is utilized by 

 the transformers to activate the nutrient cycles 

 (Chapter 11), and some of it recirculates again 

 through the ecosystem. A variety of methods are be- 

 ing developed to measure quantitatively the various 

 uses and flow of energy through the ecosystem. 



In serai stages, annual production exceeds total 



utilization so that accunuijation of energy and nu- 

 trients results. This increases the fertility of the sub- 

 stratum. In climax communities total utilization may 

 balance total jiroduction so that the ecosystem is at 

 an e(|uilibrium. 



Productivity of populations is sustained at a faster 

 rate over a longer time if the surplus production 

 above a certain level is removed by predators, or man. 

 Theoretically, the population level giving greatest 

 absolute productivity should come at the point of in- 

 flection in the growth curve of the population, but 

 disturbance of age ratios or other conditions may 

 jilace the level of o]jtinnini sustained yield at some 

 other point. 



In this chapter we iiave been primarily concerned 

 with basic princijiles and methods rather than with 

 results. Data are not available to permit broad gen- 

 eralizations about the total energy relations within 

 ecosystems. Much more work needs to be done. The 

 need for a better understanding of productivity and 

 yield has both theoretical and economic incentives and 

 provides one of the main challenges in the future re- 

 search of ecologists. 



Exchanges, productivity, and yield 209 



