Productivity of the Ecosystem 481 



are bottom feeders and depend upon worms, mollusks, crabs, and 

 other benthic animals for food. As mentioned above, certain of the 

 mollusks are primarily herbivores, but other types of benthos are 

 secondary or tertiary carnivores, scavengers, or mud feeders. Many 

 worms and echinoderms pass large quantities of mud through their 

 intestines, extracting nutriment from its components. Other types of 

 fish feed primarily upon smaller members of their own class; the 

 growth of a shark, for example, may represent a food chain of 5 or 

 more links (Fig. 13.5). 



(c) Decomposers and Transformers. The ecological cycle in the 

 sea is completed by the activities of a large group of decomposers and 

 transformers. Material excreted by living organisms and the bodies 

 of dead plants and animals from every link in the food chain undergo 

 decomposition as the result of the activity of certain groups of marine 

 bacteria. Dead tissue is rendered soluble and decomposed to in- 

 organic form. The material is then attacked by the transformers, as 

 represented by other types of bacteria, and converted to forms suit- 

 able for the nutrition of green plants once more. Living members of 

 the oceanic community are thus necessary to carry out each step in 

 the chemical regeneration of the organic material. However, before 

 the resulting nutrient salts can be assimilated again by the phyto- 

 plankton, the water containing them must be restored by currents to 

 the euphotic zone where light is sufficient for photosynthesis. On 

 certain submarine banks, such as Georges Bank, and in regions of 

 permanent upwelling, such as along the western coast of the United 

 States, physical regeneration may take place almost continuously. 

 In other regions the deep stirring of winter is required before renewed 

 development of phytoplankton can initiate constructive growth in the 

 oceanic ecosystem once more. 



PRODUCTIVITY OF THE ECOSYSTEM 



In the previous sections the functional interdependencies be- 

 tween the various living and non-living components of the ecosystem 

 have been discussed. We shall now consider relations of productivity 

 both for the individual links in the food chain and also for the eco- 

 system as a whole. When reliable methods of measurement have 

 been developed, the productivity of one type of organism may be 

 compared with another, and the productivities of different communi- 

 ties or regions can be evaluated. From the practical point of view it 

 is desirable to ascertain whether the production of a given area is as 

 great as it could be. In order to determine whether an area is being 



