stored by the producers. Decomposers play a vital role in the recycling 

 of matter within an ecosystem by reducing the energy-rich compounds 

 of feces and the dead bodies and remains of plants and animals to 

 simple molecules that producers can reutilize. 



Chemical elements tend to circulate in the biosphere in characteristic 

 paths from environment to organisms and back to the environment. 

 These paths are variously called "inorganic-organic cycles" or 

 "biogeochemical cycles." Of the nearly 100 elements known to occur in 

 nature, we have found some 30 to 40 are required by living organisms. 

 Some of these elements (e.g., carbon, hydrogen, oxygen, and nitrogen) 

 are needed in large amounts; others in lesser or even balance than 

 others, and materials are returned to the environment as rapidly as 

 they are removed. Some cycles are unevenly balanced and portions of 

 the nutrients may become lost or bound in sediments or in organisms 

 and thus are not available to the ecosystem. 



Man's activities have accelerated the movement of many materials, 

 e.g., phosphate, into lakes. Fortunately some cycles, such as carbon, 

 have so many compensating mechanisms that man has successfully 

 used them but done little to alter them. Application of radionuclide tags 

 has facilitated quantitative studies of ecological food webs and 

 pathways of both nutrients and toxicants in the environment. 



The numerous species found at each level in a food chain are competing 3 . Species Co m position 



for energy and nutrients. Which of the competing species will be the r Fro«;v«;t<am 



numerical dominant in an ecosystem is, to varying degrees, ' 



determined by the food-selection characteristics of the animals of the 



next higher trophic level. That is, the second-order consumers can often 



determine the dominance at the herbivore level and the herbivores 



determine dominance at the plant level. Granted that the numerical 



dominance among competing species is determined, in part, by the 



selective feeding behavior of the next higher trophic level, then 



consumers of the highest levels must both directly and indirectly 



influence the composition of the entire ecosystem. 



This fact may provide mechanisms by which man can regulate the 

 great living mass of the producer and herbivore levels by manipulating 

 the smaller populations of larger individuals at the top levels. There is 

 evidence, for example, that the predatory fish of a lake (such as trout or 

 bass) may influence the algal populations through their direct effect on 

 the plankton-eating fish and their indirect effect on the animal 

 plankton that feed by sweeping algae from the water. Changes in fish 

 stocks may have aggravated eutrophication problems in some lakes, 

 but proper manipulation of these fish stocks may also be a means to 

 alleviate these same problems. 



In some natural terrestrial ecosystems herbivores play an important 

 role in determining the abundance, spatial distribution, and temporal 

 patterning of the competing species of producers. This is equally true 

 for large grazers on a prairie or for insects in an evergreen tropical 

 forest. Insect herbivores are themselves subject to an often highly 

 selective attack by predatory insects. The stability of predatory insect 

 population with complex behavior depends on the spatio-temporal 



31 



