CYCLE OF CAUSE AND EFFECT 29 



Although the question of the biotic habitat presents no serious 

 difficulties, the delimitation of habitats in lake and sea is compli- 

 cated by a lack of visibility. In general, the two major principles 

 of limits of dominants and definite changes in conditions appear to be 

 as applicable in water as on land. The primary divisions of the 

 ocean, for example, should be in close correspondence with the marine 

 communities, and the dominants of the latter should furnish the chief 

 indications of climax limits, with the aid of factor measurements as 

 necessary adjuncts. In the littoral reaches down to 200 meters, or 

 thereabouts, the great density of the medium produces major physical 

 differences in smaller spatial limits, in proportion to the greater density 

 of water as compared with that of air. As a result, the major habi- 

 tats are correspondingly smaller than on land. The deeper bottom 

 ones are known only in a very fragmentary way, but the impression 

 afforded by the great marine expeditions, as that of gradual change 

 over large areas, may not be correct. 



The pelagic communities of the sea have no counterpart on land, 

 and the greater mobility of the ocean, as expressed in currents, up- 

 welling, tides, storms, icebergs and pack ice, appears to render bound- 

 aries broad or even vague, a consequence possibly augmented by the 

 mobility and motility of pelagic dominants and influents. The vast 

 depths of the ocean further complicate the problem in a fundamental 

 fashion and probably make it necessary to limit pelagic communities 

 both vertically and horizontally (Murray and Hjort, 1912:617). Such 

 features of the ocean floor as the Wyville Thomson or Iceland-Faroe 

 Ridge (1,500 meters above the Atlantic floor at the south) exert a dis- 

 tinct influence on the deeper pelagic communities, but it is hardly com- 

 parable with the striking effects of this ridge upon the benthic com- 

 munities of the same area or of some ridges of similar height on the 

 land communities near the coast of California. 



AVhether it is possible or desirable to recognize serai habitats in 

 the pelagic realm of the ocean remains an open question at present. 

 How^ever, it appears probable that the movement of great masses of 

 water by currents, or through upwclling, produces areas relatively free 

 of organisms, into which invaders may press, and it is not impossible 

 that similar effects may be produced by drifting ice. On the other 

 hand, while they often result in great variations in abundance, the 

 seasonal and annual cycles of the microplankton are not to be re- 

 garded as successional processes, but are characteristic phenomena of 

 aspection and annuation (p. 315). 



In the tidal area, the question of habitats is much simpler and 

 with close parallel on land. The importance of surface for attach- 



