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ECOSYSTEM ECOLOGY: 



CONSUMPTION LEVELS 



ESSENTIAL COMPONENTS 



Figure 20.1 Components of an ecosystem energy cycle. 



rocks. Therefore, steps three and four are essential 

 to photosynthesis and more complex synthesis in step 

 two. 



NONESSENTIAL STEPS 



Various other steps add to the complexity of the en- 

 ergy cycle; however, none of them are necessary for 

 an ecosystem to operate. These steps are nonessential 

 in the sense that life could go on without them. 



The nonessential portion of the energy cycle is a 

 complex rather than a series of steps. It is a complex 

 because no direct, or step-by-step, relationship exists 

 among the basic components of herbivores, carni- 

 vores, omnivores, parasites, scavengers, and sapro- 

 phytes. It is true that herbivores are a logical next 

 step in energy transfer from photosynthetic plants, 

 and carnivores tend to rely mainly on herbivores for 

 their food. However, omnivores use both plants and 

 herbivores for food, thus sometimes acting as carni- 

 vores and at other times, as herbivores. Parasites, 

 scavengers, and saprophytes gain nutrients from 



plants, herbivores, and carnivores. A more detailed 

 presentation of the interrelationships among essential 

 and nonessential "steps" is portrayed in Figure 20.1. 

 Omnivores are not included as a separate component 

 in the figure; they are indicated by the collective func- 

 tions of herbivores and carnivores. 



FUNCTIONAL NICHES 



The discussion of steps in the energy cycle men- 

 tions the functional niches present in an ecosystem, 

 and Figure 20.1 portrays these same niches. This il- 

 lustration emphasizes two phases of function: pro- 

 duction and consumption. Although there is a single 

 level of production, that by green plants, green flagel- 

 lates, blue-green algae, and certain bacteria, there are 

 many possible levels of consumption. For example, 

 because herbivores generally are the first, or direct, 

 users of living plants, herbivores are primary con- 

 sumers: because carnivores obtain produced food sec- 

 ond hand when they eat herbivores, they are second- 

 ary consumers. In addition, a larger carnivore eating 



