and the boundaries between them practically coincide with the boundaries 

 of the equatorial biogeographic province of C. W. Beklemishev (Heinrich, 

 1968). Each boundary of the equatorial province is the midline between 

 two boundaries, one of which separates the 50°/ core of the equatorial 

 species, the other the corresponding core of the central species (C. W. 

 Beklemishev, 1969). Consequently, the equatorial, central and distant 

 neritic communities, in addition to the specific species which are 

 theirs alone, are also distinguished by the set of species which reach 

 maximum abundance there. 



The existence of groups of species with different types of large- 

 scale changes in abundance is directly or indirectly (through biologic 

 interrelationships) related to the peculiarities of the biotopes of the 

 central, equatorial and distant neritic communities. The combination of 

 these peculiarities is determined by the hydrologic and hydrochemical 

 environments and productivity. It is possible that their combined 

 influence is significant (Fager, McGowan, 1963). 



The increase in the abundance of certain species around the periphery 

 of the tropical area apparently results from a decrease in competition, 

 since many tropical species disappear here, and the number of some others 

 decreases (Tokioka, 1959; Voronina, 1962: Heinrich, 1964, 1968; C. W. 

 Beklemishev, 1967). 



Relatively small-scale changes in the abundance of species occur 

 within individual communities. In the equatorial communities of the 

 Pacific and Indian Oceans, the position of the band of high abundance 

 of various species is related to the position of divergences and con- 

 vergences. Transport by currents is also important, as a result of which 

 the bands and spots of high abundance of species, developing one after 

 another in time, also diverge in space (Vinogradov, Voronina, 1962. 1963). 

 Within the northern central community in the western Pacific, the dis- 

 tribution of the maximum abundance of wide-spread tropical species of 

 copepoda of the second and third types of distribution has been found 

 to be connected with the areas of greater productivity located around the 

 halistasis. In the center of the halistasis, where the biomass of zoo- 

 plankton was minimal (less than 3 g/m2 in the 0-500 m layer), none of 

 these species produced spots of high abundance (Heinrich. 1968). 



Number of species and dominance . The tropical oceanic communities 

 are richest in numbers of species. The number of species varies within 

 the tropical communities. In the tropical regions of the Pacific Ocean, 

 the number of species of euphausiids, chaetognaths and foraminifers 

 changes, when we consider the impoverished peripheral regions, by a 

 factor of 2-3, or if we ignore the periphery, by a factor of 1.5-2 

 (Heinrich, 1974a). In various taxonomic groups of animals, the changes 

 occur differently. The outlines of regions with the greatest number of 

 species recall the outlines of the ranges of planktonic animals, since 

 in each such group, species predominate with certain types of distribution, 

 However, these regions agree rather well with the boundaries of the 

 equatorial, central and distant neritic communities. Thus, for the 

 chaetognaths, the greatest number of species in the distant neritic 



112 



