Niche

The niche is a theoretical concept first fully developed by G. E. Hutchinson and brought forward by numerous other distinguished ecologists. It attempts to define all physical, chemical and biological factors that influence a single organism throughout its life. Energy consumption is also part of the complex equation defining the “essential” niche. This global concept cannot be tested by experimental approach, yet it is a useful notion when considering the interactions of all life forms within a given ecosystem. The statement that no two organisms can occupy the same niche suggests that competition plays a significant role in the interactions between assemblages and communities of organisms. Defining the essential niche of any macro invertebrate is an impossible task, considering its constantly changing environment. This fact, alone adds to the romance and mystery of river ecology.

Links
  http://www.geog.ouc.bc.ca/physgeog/contents/9g.html