BIOME AND BIOME-TYPE m WORLD DISTRIBUTION 



583 



Fresh waters, on the other hand, invade the 

 sea and are invaded by components of the 

 marine community at river mouths (p. 

 542). They exhibit a graded transition in 

 brackish water sounds and lagoons and 

 marshes from fresh water to salt water, in 

 which certain forms enter from the sea and 

 others from the land and fresh water; while 

 some brackish waters may represent long 

 series of variously connected communities 

 of a quite distinct type, perhaps best re- 

 garded as a major ecotone (Pearse, 1936; 

 Smith, 1931). 



The regular north-south series of tem- 

 perature zones on the continents produces a 

 recognizable world pattern, especially when 

 approached from the north, with icy wastes 

 at the poles, tundra, temperate forests, 

 grasslands and deserts, and the more 

 sharply interrupted series of tropical forests 

 and tropical grasslands. Ecological equiv- 

 alence (Tables 35, 36, 41) is directly cor- 

 related with the existence of geographically 

 and climatically equivalent biomes. It has 

 already been indicated that the significant 

 biogeographic zonation within the larger 

 land masses is from north to south and is 

 ecological, whereas the world-scale east- 

 west faunal and floral partitions tend to be 

 historical in nature. 



A grouping of individual fresh-water and 

 terrestrial communities, whether extremely 

 uniform or considerably diverse, into inclu- 

 sive biomes involves a hierarchial classifica- 

 tion. For example, it is legitimate to group 

 into a biome the communities of the smaller 

 islands of the open Pacific, many of which 

 are almost exactly equivalent, and whose 

 biota has the operational connection of con- 

 tinuing dispersion. That biome, however, 

 must include also the larger and higher is- 

 lands with their richer biota, in which en- 

 demism reflects historically continued iso- 

 lation, and, further, must be related to the 

 still larger and more diversified islands and 

 archipelagoes that are the major sources of 

 the original and continuing dispersion. 

 Where to draw the boundaries for such an 

 extremely fragmented biome offers an al- 

 most insoluble question. 



The biome concept, in fact, is much like 

 the species concept in that it is useful when 

 used as a descriptive tool without attempt- 

 ing a sharp definition. The major problem 

 of definition for the biome lies, in essence. 



not in the geographic fragmentation of the 

 major types of biotic formation, but in the 

 interdigitation, overlap, and intergradation 

 of otherwise well recognizable terrestrial 

 biomes, such as that between desert and 

 grassland in North America. 



Some apology is necessary for the fact 

 that the characterization of the biomes as 

 to animal components is so much focussed 

 on the larger vertebrates. Critical regional 

 lists of the major groups of terrestrial inver- 

 tebrates are for the most part still unavail- 

 able. In the tropics and in many areas of 

 Africa, Asia, South America, and Australia 

 a vast amount of basic descriptive work is 

 still requisite for invertebrates. Thus, the 

 dependence of faunal definitions on the 

 more advanced floral studies and on verte- 

 brate distribution is again emphasized. 



THE PRINCIPAL BIOME-TYPES 



THE TUNDRA 



The northern circumpolar tundra** is rel- 

 atively the most continuous, and in some 

 respects the most sharply definable, of all 

 the biomes. Its vast extent from Labrador 

 to Alaska in North America is greatly ex- 

 ceeded in Eurasia by the segment extend- 

 ing from Kamchatka to Lapland. Its dom- 

 inant physiographic appearance is that of a 

 gently rolling plain, in which the depres- 

 sions are occupied by lakes, ponds, and 

 bogs (Fig. 218). The characteristic tundra 

 vegetation is sphagnum and various lichens 

 such as the "reindeer moss," with a strik- 

 ing flora of herbaceous higher plants in 

 sheltered places on the drier hillsides. 



Certain aspects of the tundra food web 

 have been discussed (p. 515). The charac- 

 teristic larger mammalsf of the tundra are 

 the musk ox (circumpolar until postglacial 

 times), the reindeer and caribou group, the 

 arctic wolf, the arctic fox, and smaller 

 forms, including the arctic hare and the 



" Tundra is the Siberian word for the zone 

 north of the timberline, known in boreal 

 America as the "barren grounds." Tundra is 

 the accepted ecological term. 



f Mammals and birds, as usually the more 

 conspicuous elements of the animal life of the 

 terrestrial biomes, are most easily used to char- 

 acterize the animal segments of the biome. We 

 acknowledge that this use tends to conceal or 

 perhaps to emphasize our greater ignorance of 

 the invertebrates. 



