400 



ECOSYSTEM ECOLOGY: 



plant 

 functions 



environment - 



plant 

 compounds 



plant 

 synthesis 



t 



chemica I 

 compounds 



t 



bacterial 

 chemosynthesis 



animal ingestion 

 digestion and 

 transformation 



plant and 

 animal death 



chemica I 

 compounds 



anima I 

 compounds 



anima I 

 functions 



saprophytic 

 bacterial reduction 



Figure 20.5 Processes found in biogeochemiccl cycles. 



a change in the composition of the ecosystem that 

 usually causes habitat destruction and leads to some 

 form of secondary succession. 



Ecosystem imbalance as a result of the accumula- 

 tion of biogeochemical materials in particular parts 

 of the cycle results from disturbance. For example, 

 human exploitation removes materials from an eco- 

 system much faster than materials are replaced. This 

 means that certain protoplasmic elements are not in 

 sufficient supply to meet the nutritional demands of 

 the energy cycle, and soon there will be a collapse of 

 the ecosystem. Again, the consequences of this col- 

 lapse is secondary succession, or regression to a com- 

 munity capable of entering a balanced energy cycle 

 in the available environment. 



GENERALIZED CYCLE 



Any biogeochemical cycle involves the nourish- 

 ment of the various organisms in an ecosystem. Basic 

 to this nourishment is the production of food by green 

 plants, green flagellates, blue-green algae, and certain 

 bacteria. These organisms, mainly by photosynthesis 

 but also by other complex synthesis, are producers of 

 energy used by the organisms themselves, by animals, 

 and by the parasites of both. Bacteria are of many 

 nutrition types. Some autotrophic bacteria, those 

 forming their own food from inorganic materials, are 

 photosynthetic and fulfill the same functional niche 

 as do other photosynthetic organisms; these bacteria 

 are producers for the microscopic world. Other auto- 

 trophic bacteria are chemosynthetic, using oxygen in 



a process called oxidation to obtain nutrients and at 

 the same time forming compounds either simpler or 

 more complex than those they use. The other bac- 

 teria are heterotrophic saprophytes. By a process 

 called reduction these saprophytes obtain nutrients 

 and form simpler substances from these ingested ma- 

 terials. Therefore, bacteria have three important 

 nutritional roles in an ecosystem. First, they act as 

 producers, forming by photosynthesis and other 

 syntheses the nutrients that are needed by micro- 

 scopic primary consumers. Second, saprophytic 

 forms act as primary consumers and decomposers 

 (reducers) of dead and waste organic materials, free- 

 ing the materials for resynthesis. TTiird, chemo- 

 synthetic forms act as transformers of decomposed 

 products by oxidizing these products into substances 

 that are either further transformed by other chemo- 

 synthetic bacteria or are used directly by plants. 



The final phase of any biogeochemical cycle is the 

 contribution of materials from the physical environ- 

 ment. This environmental contribution comes mostly 

 from the atmosphere; however, such geological proc- 

 esses as sedimentation and vulcanism are also impor- 

 tant contributors. 



SELECTED READINGS* 



Dansereau, Pierre, 1957. Bwgeography, an Eiological 



Approach. 

 Odum, E. P., 1963. Ecology. Modern Biology Series. Holt, 



Rinehart & Winston, New York. 



*.See also the General Ecology References on p. 338. 



