thrown up into mountains beginning in the Miocene, 

 and the climate there became more diversified and 

 rigorous. Plant and animal species tended to segre- 

 gate into either the western or the eastern section of 

 the forest, depending on where habitat conditions and 

 community coactions were more favorable to them, 

 and isolation encouraged divergent speciation. The 

 western section continued to have sporadic contact 

 with the Eurasian biociation, especially during the 

 Pleistocene, but the eastern section was too far away. 

 Hence came the differentiation of the boreal forest 

 biociation in the eastern lowlands of the continent 

 and the western forest biociation in the western 

 mountains and on the Pacific coast. 



In this connection it is of interest that 52 per cent 

 of the breeding bird species in the boreal forest bio- 

 ciation are of Old World origin and 30 per cent of 

 North American origin, compared with 65 and 17 

 per cent, respectively, for the western forest biocia- 

 tion in Colorado (Snyder 1950). The difference is 

 even more striking when comparison is made between 

 the breeding populations. In the boreal forest bio- 

 ciation, only 20 per cent of the breeding pairs belong 

 to species of Old World origin while 79 per cent 

 belong to species of North American origin. In the 

 western forest biociation of Colorado the percentages 

 are 98 per cent of Old World origin and only 2 per 

 cent of North American origin. 



Pleistocene glaciation enhanced the differentiation 

 of boreal and western forest biociations since it al- 

 lowed independent subspeciation and even speciation 

 in the four refugia (Fig. 21-2). The boreal forest be- 

 came compressed with each glaciation into the Appa- 

 lachian refugium, but the western forest was segre- 

 gated three ways into the Rocky Mountain, Pacific, 

 and Alaskan refugia. At these times the Alaskan 

 refugium was probably connected by the Bering land 

 bridge to Asia. 



The present-day distribution of the four subspe- 

 cies of moose suggests that they were isolated during 

 at least Wisconsin glaciation in the Appalachian, 

 Rocky Mountain, and Alaskan refugia, and in the 

 unglaciated area of Wisconsin, Minnesota, and 

 Illinois. This area probably served also as the 

 refugium for the western subspecies of the wood- 

 land caribou, while the eastern subspecies was 

 isolated in the Appalachian refugium (Vos and Peter- 

 son 1951). Different subspecies of arctic shrews had 

 refugia to the south and east of the glacier and in 

 Alaska. The American marten and the red squirrel 

 apparently survived in the Appalachian refugium ; 

 the western marten and Douglas' squirrel, in the 

 Pacific refugium. Of the red-backed mice, Cletliri- 

 ononiys gapperi has apparently dispersed from the 

 Appalachian refugium, C. dazvsoni from the Alaskan 

 refugium, and C. zvrangcli from islands off the coast 

 of British Columbia (Rand 1954). 



Various subspecies or closely related species of 

 birds apparently differentiated as populations were 

 isolated in one or more of the four refugia. This 

 seems to have occurred with the spruce grouse, sap- 

 sucker, gray jay, boreal chickadee, myrtle and Audu- 

 bon's warblers, slate-colored and Oregon juncos, and 

 white-crowned sparrow (Rand 1948, Drury 1953). 



In each interglacial period, the coniferous forest 

 fauna previously isolated in the Appalachian, Rocky 

 Mountain, Pacific, and Alaskan refugia doubtless dis- 

 persed centrifugally from each center until they came 

 into contact with each other. Such segregation and 

 dispersal of the biota must have occurred four times 

 during the Pleistocene : the dispersal from refugia 

 after the Wisconsin glaciation is still going on. The 

 four subspecies of moose have come into contact 

 with each other (Peterson 1955), the least chipmunk 

 has entered Ontario and Quebec (Peterson 1953), 

 and the evening grosbeak has spread across Canada 

 from the western mountains only within the past 

 hundred years. 



Some of these changes in range may have been 

 hastened as a result of human interference. The 

 woodland caribou was formerly the principal large 

 ungulate present in the boreal forest (Vos and Peter- 

 son 1951), but as logging and fires opened up the 

 forest, the caribou has greatly declined in numbers 

 and the moose has become more abundant. The 

 white-tailed deer has also spread from the deciduous 

 forest well into the boreal forest in recent years, and 

 other species of mammals and birds appear in the 

 process of doing so. 



ANIMAL ADJUSTMENTS 



Animal adaptions for life in coniferous 

 forest are similar in many ways to those for life in 

 deciduous forest. Ecological niches in these two for- 

 ests are similar, although the species that occupy 

 them are different. Important differences are the stiff 

 needle-shape character of conifer leaves and their 

 arrangement around all sides of the twigs, which 

 hinder the movements and feeding of some birds, and 

 the poor decomposition of the shed leaves that accu- 

 mulate on the ground, not favorable to high popula- 

 tions of many species of small animals. In contrast 

 to the aspects of the deciduous forest, the vernal 

 and autumnal aspects are less well developed since 

 most of the trees retain their foliage throughout the 

 year. 



The woodland caribou is largely restricted to the 

 climax forest where it feeds on reindeer moss, a 

 ground lichen, and on tree lichens. Moose are found 

 throughout serai stages as well as in the climax. 

 During the summer they commonly feed on water 

 lilies, pondweeds, sedges, and grasses ; during the 



308 Geographic distribution of communities 



