Production Rate 493 



smaller and smaller catch was obtained from each day of fishing. A 

 similar study of the drop in the catch per unit effort following de- 

 pletion of the stock has been made for the Antarctic whales by Ruud 



(1952). 



Production Rate 



A knowledge of the standing crop of an area or of the yield from 

 the area does not give a complete picture of productivity; in addition 

 the rates at which the different constructive and destructive processes 

 are going forward within the area must be considered. Since the 

 plant producers, herbivores, carnivores, and other components of the 

 ecosystem are all transforming energy and material simultaneously, 

 the evaluation of production rates is extremely complex. For an un- 

 derstanding of the interdependencies of the entire ecosystem we 

 should know the production rate for each of the trophic levels, that is, 

 the amount of material formed by each link in the food chain per unit 

 of time per unit area or volume, and we must distinguish between 

 gross and net production. To disentangle these relations the analyti- 

 cal approach will again be adopted. 



Let us consider first a very much simplified case such as an isolated 

 pond in which no gain or loss of materials occurs during the period 

 under consideration. The transformations going on within such an 

 area are indicated schematically in Fig. 13.11. At the left of the dia- 

 gram the energy represented by the standing crop at the beginning 

 of the period is indicated. To keep the problem simple the food 

 chain is considered to have only three links. The size of the rectangles 

 for the green plants, herbivores, and carnivores are progressively 

 smaller to indicate the typically reduced energy content of the stand- 

 ing crop as a pyramid of biomass. In the main part of the diagram 

 the energy absorbed at each level of the food chain and its transfor- 

 mations are shown. 



The largest rectangle at the bottom represents the total amount of 

 incident light reaching the area during the period under consideration. 

 Not all of the radiation is absorbed by the green plants. In the 

 aquatic environment the lion's share of the light energy is absorbed 

 by the water, and in the desert most of the sunlight falls on bare 

 ground. In regions where the vegetation forms a complete cover, 

 however, a large fraction of the incident radiation is absorbed. Thus 

 the various rectangles of the diagram are not drawn to scale but 

 simply illustrate the relations of the energy transformations among 

 the various components. Of the light energy actually absorbed by 



