ENVIRONMENT-COMMUNITY INTERREIATIONS 



399 



determining food taken is tlie quantity of food re- 

 quired by an animal. Food needs naturally are ex- 

 tremely variable both among species and within a 

 species. However, the main factors determining the 

 amount of food needed are an animal's body surface 

 area, body temperature, and degree of activity — "ac- 

 tivity" implying not only amount of physical exer- 

 tion but functional, or physiological, activity such as 

 reproduction, which requires more energy than nor- 

 mal. Moreover, increase in any of these -factors of 

 need increases chemical reactions (specifically, met- 

 abolic processes) and requires more energy. Finally, 

 in addition to the animal functional factors deter- 

 mining food needs, there is the already-mentioned 

 factor of nutritional value of food. 



PRODUCTIVITY 



The amount of usable food produced by a given 

 ecosystem is subject to many variables (Figure 20.4). 

 Of course, the primary criterion of food availability 

 is the amount of food produced. However, this does 

 not determine how much food is utilized; some food 

 is wasted. Various things cause the food consumption 

 rate to fall short of the basic production rate. For ex- 

 ample, some food usually is destroyed because ani- 

 mals trample the area. Also, waste may exist if more 

 of a particular food is produced than there are ani- 

 mals to consume the "crop." 



Total food availability determines the number of 

 individuals of each species possible in a community. 

 This possible composition of species is called the 

 carrying capacity and is allied to its community. 

 Carrying capacity can be related to a climax com- 

 munity. In a climax the area is in equilibrium be- 

 cause none of the species exceed the limits that would 

 cause the carrying capacity of the entire community 

 to be exceeded. In addition, carrying capacity is as- 

 sociated with certain possible efiPects of excessive 

 grazing practices. Overgrazing occurs as soon as the 

 carrying capacity is exceeded. Because food utiliza- 



PRODUCTIVITY 



waste 



CARRYING CAPACITY 



Figure 20.4 Relationship among productivity, waste, and carrying 

 capacity in an ecosystem. 



tion is greater than productivity, the most desirable 

 forage plants cannot reproduce, hence less nutritious 

 (because they make less energy demands and store 

 less energy) plants replace the original forage. 



The above is a bare indication of the fact that 

 available food determines the kinds and numbers of 

 organisms present in any community, and that food 

 is one of the most significant factors in determining 

 the characteristics of a community. 



BiOGEOCHEMICAL CYCLES 



The energy cycle of an ecosystem is dependent upon 

 the recirculation of all the essential elements of pro- 

 toplasm, an all-inclusive biogeochemical cycle (Fig- 

 ure 20.5). However, this recirculation, or over-all 

 biogeochemical cycle, consists of many smaller bio- 

 geochemical cycles, each related to the circulation of 

 one or more protoplasmic elements. By definition, 

 each restricted cycle must include the chemical proc- 

 esses of living creatures, which form organic com- 

 pounds, and chemical processes of the inanimate 

 world, which form inorganic compounds. These two 

 kinds of compounds include the more than thirty 

 elements that are essential in the formation of proto- 

 plasm. The elements needed in the largest quantities 

 are hydrogen, oxygen, carbon, nitrogen, phosphorus, 

 and sulfur. These commoner elements and some of 

 the less common ones are circulated in the water, 

 carbon, nitrogen, phosphorus, and sulfur cycles. 



In a sense, the biogeochemical cycles collectively 

 are identical with their local geomorphic cycles. Al- 

 though the biogeochemical cycles emphasize only the 

 chemical reactions of protoplasmic elements, these 

 reactions are identical with the chemical processes 

 in the geomorphic cycles. This is true because a 

 geomorphic cycle includes life and its processes as 

 well as physical features and their processes. 



Although the individual biogeochemical cycles are 

 part of a single, over-all cycle of nutrient circulation, 

 a consideration of individual cycles stresses the vary- 

 ing efficiencies of the circulation of diflferent ele- 

 ments. For example, the water, carbon, and nitrogen 

 cycles are very efficient; in each, circulation tends to 

 proceed regularly. However, in such cycles as those 

 for phosphorus and sulfur there are steps in which 

 materials might accumulate much more rapidly than 

 they are released. When such accumulations exist 

 there is an imbalance in the natural processes of the 

 ecosystem. This imbalance necessarily brings about 



