Consumers 



All 



However, food dependencies are difficult to determine, and several 

 divergent views have been held as to the critical relations. Since 

 observations at sea indicated that diatoms were often abundant when 

 copepods were scarce and vice versa, some investigators argued that 

 no direct food dependency occurred. The quandary of this apparent 

 contradiction represents the danger of generalizing from isolated ob- 

 servations or from statistical tabulations of the occurrence of animals 

 and plants without due regard to the dynamic aspect of their growth 

 and life cycles. If diatoms are scarce at a given point where 

 copepods are abundant, does it mean that the copepods have just 

 finished consuming the population, or does it mean that copepods are 

 primarily dependent upon some other source of food? 



U. 



Light 



Nutrient 

 supply 



Clarke, 1943 



Fig. 13.5. Simplified representation of main ecological cycle in the sea. 



As data on this problem were gathered, two theories emerged. 

 One was the theory of animal exclusion proposed by Hardy ( 1936 ) 

 that high concentrations of phytoplankton are harmful to zooplankton 

 and, therefore, that water masses containing large numbers of diatoms 

 are actively avoided by copepods. Exclusion was originally believed 

 to have taken place chiefly in the vertical direction and thus to curtail 

 the upward phase of the diurnal migration of the zooplankton. Hori- 



