Abstract: Niche theory is important both in theoretical and applied ecology,but a generally-accepted definition has not been established yet. During the past century, three ecologists have made great contribution to development of the niche concept. Grinnell's (1917,1924, 1928) “spatial niche”, Elton's (1927) “functional niche” and Hutchinson's(1957)“n-dimentional hypervolume niche” have been the major categories of the definition of niche. The present paper reviewed the development of niche concept and analysed the problems that have confused and bothered ecologists for almost 100 years.Finally, a new definition of the niche concept was proposed. Niche can be described as the functional role that a particular species plays in all or a special stage of a given species' invasion, colonization, propagation, development, decline, and extinction process'. Niche theory can be applied to the conservation of endangered,threatened and declining species in the 21st century.

Key words: offshore ecosystem health, assessment index, assessment method.