502 Dynamics of the Ecosystem 



harvest per unit area. As would be expected from the previous dis- 

 cussion, the yield is greater for organisms at lower trophic levels than 

 for those further along in the food chain. The highest yields are 

 obtained when man can harvest the plant producers directly. The 

 value given for corn is a maximum crop after a growing period of 100 

 days (Transeau, 1926). The solar energy reaching the cornfield dur- 

 ing the rest of the year is lost as far as this crop is concerned. The 

 figure given for sugar cane in Hawaii, where a growing season of 11 

 months exists, represents one of the largest annual yields obtainable 

 from any crop. 



TABLE 22 



Comparison of Annual Yield to Man from Various Environments 



Vahies (from various sources) are in pounds per acre (roughly, kilograms per 



hectare) for fresh weight of whole organism. 



Grass Rich pasture 30,000 



Corn Ohio 40,000 



Sugar cane Hawaii (max.) 200,000 



Cattle and sheep Range 150 



Rich pasture 700 



Fish Georges Bank, Massachusetts 20 



Lake Mendota, Wisconsin 19 



Fertihzed ponds, Alabama 300 



Milkfish, Philippines 700 



Cultivated ponds, India and China 3,000 



MoUusks Clams, Cape Cod (max.) 2,000 



Mussels, Europe 4,000 



Since cattle, sheep, and other farm animals are products of a two- 

 link food chain that are raised in a relatively advantageous environ- 

 ment, it is not surprising that the yields are considerably higher than 

 those ordinarily found for animals in uncultivated aquatic environ- 

 ments. The harvest taken from natural populations of fishes such 

 as those from Georges Bank and Lake Mendota is thus much below 

 the yields obtainable from the growth of farm animals, but probably 

 it does not compare unfavorably with man's take from wild animal 

 populations on land. 



In the fertilized fish ponds of southeastern United States an average 

 of 300 kg of fish per hectare per year may be produced. An excellent 

 example of the effect of the length of the food chain is furnished by 

 the sizes of the yields of fishes of differing ecological position in these 

 ponds. When goldfish, which are herbivores, are raised a yield of 

 500 kg per hectare is obtained, but the yield for bluegill sunfish, a 

 primary carnivore, is 250 kg per hectare, and that for bass, a secondary 



