468 Dynamics of the Ecosystem 



instances, but in specialized situations it may be of quantitative im- 

 portance. The consumers include all the other types of organisms in 

 the community. Free-living herbivores feed directly upon the green 

 plants. The principal herbivores in terrestrial habitats are insects, 

 rodents, and ruminants, and in aquatic habitats they are small crus- 

 taceans and mollusks. Since most other prominent types of animal 

 life in the community depend upon these herbivores, the latter have 

 been referred to by Elton ( 1939 ) as "key industry animals." The 

 herbivores serve as food for primary carnivores; these in turn may fall 

 prey to secondary carnivores, and several more stages of nutritional 

 dependency may exist. The tissues of the various plants and animals 

 in the community may also be eaten by parasites, and after the organ- 

 isms are dead, by scavengers and saprophytes of many sorts. The 

 plants and animals that depend successively one on another form the 

 links of a food chain. 



The dead bodies of the producers and of the consumers mentioned 

 above are attacked by decomposers, consisting of bacteria and other 

 types of fungi. The decomposers render the organic matter soluble 

 and break it down chemically. The material is then attacked by 

 transjormers—oihex types of bacteria that change the inorganic com- 

 pounds into forms suitable to serve as nutrients for photosynthetic 

 plants once again. Among the living components only the photosyn- 

 thetic plant producers, the decomposers, and the transformers are 

 essential. Some communities could theoretically maintain themselves 

 indefinitely without the presence of any animals whatsoever, and, 

 although it may injure our ego to state it, we realize that man is not a 

 necessary part of the ecosystem. However, animal life is ordinarily 

 present in most natural communities, and sometimes animals hold a 

 controlling position or take a prominent part in the operation of the 

 ecosystem. 



Niches. Different species of animals and plants fulfill different 

 functions in the ecological complex. The role of each is spoken of as 

 its niche. The term so used stresses the function of each organism 

 in the community rather than its physical place in the habitat. The 

 term originated, however, from the characteristic location of different 

 types of organisms in the area under consideration, and niche is still 

 used by some ecologists in this sense. The "functional niche" is more 

 fundamental than the "place niche," but both concepts exist and 

 should eventually be given different names. 



Niche may be used in a broad sense to refer to the principal func- 

 tions involved in the operation of the ecosystem, or it may be employed 

 to describe the subdivisions of these and the various methods of 



