form, and structure of the vegetation, although the species 

 composition may vary, as in various grasslands; e.g., the Great 

 Plains biome belongs to the same grassland biome type as the 

 South African, Central Asian, Brazilian, and other grassland 

 biomes.^ 



It is possible that species of different life-forms, formerly isolated 

 by geographic barriers, may succeed in new, closely similar 

 habitats as shown by the ability of Eucalyptus spp., which form 

 woodland in Australia, to persist in grassland in California. ^° It 

 appears questionable, however, that Eucalyptus spp., planted in 

 California, are steadily converting grassland into forest. It appears 

 that usually the species best adapted in the long run to the com- 

 plex physical conditions and to the species of plants and animals 

 with which they are associated are those that have developed close 

 interrelationships during the long period of evolution within a 

 specific geographic area or ecosystem. Even though climates may 

 be similar in geographically separated areas, other factors affect- 

 ing vegetation and soils are likely to be dissimilar. When intro- 

 duced species of different life-form grow successfully in a com- 

 munity, such as crested wheatgrass in native grassland in the 

 northern Great Plains and possibly Eucalyptus in California, or 

 when native species such as mesquite and cacti invade grasslands 

 in the southwestern United States (Figure 1-12) or shortgrasses 

 invade mixed and tallgrass prairie in Nebraska (as happened 

 during the 1930 decade) it appears that there have been changes 

 in environmental conditions or in associated species such as those 

 caused by excessive grazing, drought, or even absence of fire. 

 Dissimilarities in habitats commonly result in differences in the 

 life-form and structure of the vegetation, as illustrated by biomes 

 in different biome-types, and apparently this is true also in 

 bryophytic communities.^^ 



The classification of concrete communities or stands into ab- 

 stract communities or community types is valid and useful, partic- 

 ularly within a limited geographic area and often in a large area, 

 as demonstrated by numerous workers. The gradual transition of 

 one group into another, forming a continuuni,^° which may be 

 repeated in different locations in a large area, does not invalidate 

 the classification of stands into abstract communities or types, but 



68 • Species axmd PopimlatioBms 



