Ecological Cycle in the Ocean 473 



bers, certain aspects of availability are of crucial concern in the eco- 

 system. Food organisms must be sufficiently concentrated in relation 

 to their size so that the predator can meet its nutritive requirement 

 within the time available. In other words, the abundance of the 

 food per unit area must be adequate in relation to the possible forag- 

 ing range of the animal concerned. If there were only one lion in 

 all of Africa and zebras lived 100 miles apart, plenty of food would 

 obviously exist for the one lion, but he would not be able to obtain 

 it fast enough to satisfy his needs. The same general point is illus- 

 trated by the facetious remark that in parts of Texas grass is so scarce 

 that the cattle must feed on the run. The necessity for adequate con- 

 centration as well as for adequate total amount of food is not so 

 immediately apparent for organisms in which feeding processes are 

 not directly observed. The total amount of planktonic food in an 

 oceanic area may be more than sufficient for the nutritive needs of 

 the clams or oysters living on the bottom, but in order for these 

 bivalves to survive the food must be concentrated to such an extent 

 that the volume of water that they can filter in a day will furnish 

 enough plankton to fill their daily needs. 



The necessity for an adequate rate of replacement of nutritive 

 materials is another aspect of availability in the food relations of a 

 community. The growth of terrestrial plants is dependent not only 

 upon the concentration of nutrient salts in the soil at the moment of 

 observation but also upon the replenishment of these nutrients after 

 the existing supply has been absorbed. The same dynamic de- 

 pendency exists in all natural situations where the nutrition of one 

 step in the food chain depends upon the rate of supply by the previous 

 step. In many situations the amount of nutriment supplied per unit 

 of time is larger than the amount of the material present at any one 

 moment. The same argument applies to the dependency of a car- 

 nivore on a prey species. Whether or not the food organism is 

 adequate in amount depends both on its abundance at the moment 

 and also upon the rate of replacement of the population. 



Ecological Cycle in the Ocean 



The ecological cycle in temperate regions of the ocean will be used 

 to illustrate the principles involved in the general operation of the 

 ecosystem as well as many of the relationships discussed in earlier 

 chapters. The open sea is especially suitable for this purpose since 

 a sampling can be made in the middle of a large uniform area with 

 fair assurance that no serious modification by interchange of materials 



