Conclusion 503 



carnivore, is only 100 kg per hectare per year. The herbivorous 

 milkfish in the PhiHppines, raised under very favorable tropical con- 

 ditions, provides an annual yield of 500 to 1000 kg per hectare without 

 the addition of fertilizer (Frey, 1947), and even larger harvests are 

 reported from ponds in India and China. The very high yields of 

 shellfish in the littoral zone are partially due to the fact that these 

 animals act as food concentrators. They feed on plankton and detritus 

 brought to them by currents from regions far beyond the area in which 

 they are harvested. The efficiency of their production is thus not 

 exactly comparable to the other items in Table 22, but the harvesting 

 of these shellfish is clearly a very efficient use of marine resources 

 by man. 



CONCLUSION 



The comparative productivity of land and water is a fitting topic 

 with which to close our discussion of the elements of ecology. In this 

 chapter we have shown how the individual physical and biological 

 influences of the environment interact in the functioning of the eco- 

 logical complex. We have considered the availability of solar energy 

 for green plants in various habitats and the circumstances under which 

 organic compounds can be synthesized by these primary producers. 

 We have traced the transfer of materials and energy through succes- 

 sive links in the food chain and have discussed the qualitative and 

 quantitative restrictions placed on the various plant and animal com- 

 ponents of the ecosystem by the specific conditions of each habitat 

 and each type of dependency. Thus the concept, with which we 

 began, of the living community and its environment as a reciprocating 

 system in dynamic balance has been analyzed and illustrated. We 

 have seen how the intricate relations between the organism and the 

 environment underlie the striking diflFerences in the abundance and 

 kind of plants and animals that populate contrasting habitats, in 

 the harvest of hving material that may be removed, and in the rates 

 of production that are displayed. An understanding of these ecologi- 

 cal relations enables man to live in adjustment with his natural en- 

 vironment, to preserve and develop it, and to derive the greatest 

 benefit from it. 



