ECOLOGY AND ZOOGEOGRAPHY 



633 



vironmental variation is almost endless in 

 such cases, and since the limiting factors are 

 often extremely subtle, a special section of 

 this chapter is devoted to such relationships. 



POPULATIONS AND 

 COMMUNITIES APPROACH 



Ecology can be subdivided in another 

 manner with regard to the ways in which it 

 is approached. If one studies the interre- 

 lationships of a single individual or species 

 with its environment, he is concerned with 

 autecology. If, on the other hand, whole 

 populations are studied as units, the ap- 

 proach is called synecology. Today, the 

 synecologic approach has largely replaced 

 its autecologic predecessor, and, since this 

 method deals with various levels of popula- 

 tion organization, these should be well 

 understood. 



A group of individuals composed of mem- 

 bers of a single species or several closely as- 

 sociated species, and occupying a definite 

 environmental area, is called a population. 

 All of the populations occupying a given 

 geographic area constitute a biotic com- 

 munity. A biotic community together with 

 its nonliving environment forms an eco- 

 system. It is on the level of the ecosystem 

 that the principles of ecology can best be 

 understood. 



Biological principles with 

 application to ecosystems 



The part of the earth in which life exists 

 is called the biosphere. Compared with the 

 earth as a whole, this is a shallow surface 

 region. It includes both the oceans and the 

 atmosphere. The biosphere itself can be con- 

 sidered an ecosystem, consisting of smaller 

 ecosystems, which in turn are made up of 

 still smaller ones. In practice, ecologists 

 study ecosystems of various sizes, depending 

 upon the problems of interest at the time. 



Also, the larger ecosystems provide exam- 

 ples of the broader ecologic principles while 

 the smaller ones exhibit more clearly the 

 specific and limited relationships. 



Some of the biological principles which are 

 a part of the ecosystems and which are also 

 important are habitat and niche, chemical 

 cycles, biological interrelationships, limiting 

 factors, and energy transfer in ecologic 

 systems. They will be discussed in this order. 



Habitat and niche 



Two of the most important concepts of 

 ecology are those of habitat and niche. A 

 habitat is the particular environment in 

 which a population lives. Thus the catfish 

 lives in slow-moving streams and lakes; these 

 are said to be its habitats. A niche is the role 

 of a population within its community and 

 ecosystem. It includes such factors as what 

 it eats, what its predators are, and how it 

 changes the physical environment. 



Chemical (inorganic-organic) cycles 



The chemical elements of which all or- 

 ganisms are composed come from the en- 

 vironment, and eventually return to the 

 environment after the organism dies. There 

 are continual cycles of the chemical ele- 

 ments. Three examples will suffice to illus- 

 trate this principle. 



Carboti cycle. Both plants and animals 

 respire and give off carbon dioxide gas. This 

 gas is of vital importance in the web of life. 

 Tlie carbon cycle is illustrated and discussed 

 on page 510. 



Nitrogen cycle. The proteins constitute 

 one of the most important of the animal 

 foods. All proteins contain the essential ele- 

 ment nitrogen. The nitrogen cycle is illus- 

 trated and described on pages 510 and 511. 



Mineral cycle. The importance of mineral 

 salts to animals is discussed on page 518. 

 Minerals dissolved in river or soil water 

 form the basic supply for continental plants 

 and animals. Upon their death (Fig. 444), 

 the mineral components are again received 

 by soil or water. 



