Production Rate 495 



is actually assimilated by the herbivores, for a certain amount of food 

 that is eaten remains undigested and frequently more food is destroyed 

 than is actually eaten. Sometimes this unassimilated portion is the 

 greater of the two. A colony of beavers, for example, may cut down 

 large numbers of aspens, but the bark that they consume is obviously 

 only a very small fraction of the total ])iomass of the trees. Similarly, 

 as mentioned earlier, the grazing activities of copepods often result 

 in the destruction of a quantity of diatoms far greater than that actu- 

 ally assimilated. 



We have assumed that in our simplified ecosystem the new plant 

 growth was entirely removed by being consumed or by decomposing. 

 Under natural conditions the net plant production per unit of time 

 would rarely be exactly balanced by the loss due to herbivore feeding 

 and to death of the plants from other causes. Any portion of the net 

 production remaining at the end of the period constitutes a net in- 

 crease to the standing crop. Under other circumstances the amount 

 of foraging and of other destruction may exceed the amount of new 

 growth during the period, with the result that a net decrease will take 

 place in the standing crop. 



We can apply the same line of reasoning to an analysis of the energy 

 transformations at the herbivore level. The plant material assimilated 

 is equal to the gross herbivore production. This amount, less that 

 lost in respiration, is a measure of the actual growth of the herbivores, 

 or the net herbivore production. At the end of the time unit one 

 portion of the new herbivore growth has died and decomposed, and all 

 of the other portion may have been destroyed by carnivores, as indi- 

 cated in the diagram— or part of the growth increment may remain as 

 a net increase in the population. 



At the third trophic level the primary carnivores often eat only part 

 of the prey that they kill, and, of course, only a fraction of the material 

 is digested and assimilated. The gross carnivore production is equal 

 to the total assimilation, and the net carnivore production is equal to 

 the actual growth increment of these animals. Since the primary 

 carnivores are at the top of the production pyramid in this assumed 

 three-link food chain, no part of the carnivore production is consumed 

 by predators, but all decomposes and goes back into the system after 

 death. In most natural situations secondary and tertiary carnivores 

 and other high-level consumers would also be present. The same 

 types of relation apply to the gross and net production of these further 

 links in the food chain. 



In this simplified example the standing crop at all trophic levels at 

 the end of the period is the same as it was at the beginning of the 



