494 Dynamics of the Ecosystem 



the plants only a small fraction is used in photosynthesis and appears 

 as carbohydrate formed since the process is far from 100 per cent ef- 

 ficient especially under natural conditions. 



The energy of the carbohydrate formed represents the gross plant 

 production and therefore the gross production of the producer com- 

 ponent of the ecosystem. Not all the energy present in the photo- 

 synthetic product takes part in the realized growth of the plants since 



Initial standing crop 



Energy transformations during period 



1 



R I 



-(Po)- 



Herbivores 



(Pi)- 



Green plants 



Growth rPz) * 



-Assimilation (P^ )- 



Plant growth (P2) 



Carbohydrate formed (Pi)- 



Energy used in photosynthesis 



■< Absorbed light * 



■* Incident light - 



Fig. 13.11. Energy transformations during unit of time in an idealized ecosystem 



witli no net increase or decrease (not drawn to scale). Pi = gross production; 



P2 rr net production; R = respired; D = decomposed. Cross hatching indicates 



total portion of food organisms killed by consumers; a fraction, U, of this is un- 



assimilated. (Clarke, 1946.) 



the growth process also is not 100 per cent efficient. The accompany- 

 ing catabolism of the plant is measured by the amount of respiration. 

 Actual plant growth is represented by an amount of energy equal to 

 the total carbohydrate formed minus the respiration and is designated 

 as the net plant production. Since the formation of additional links 

 in the food chain depend upon the elaboration of organic compounds 

 by the green plants, the photosynthesis of plants in the ecosystem is 

 sometimes referred to as "primary production" or "total production," 

 but the more specffic terms used here are preferable. 



Part of the plant material formed in the ecosystem is consumed by 

 herbivores, and part dies from other causes and undergoes direct de- 

 composition. Only a portion of the plants designated as "consumed" 



