INTRODUCTION 



to repeat the process with another principle. 

 The two approaches continually tend to 

 become mixed when the documentation of 

 principles is given in any detail. Recogni- 

 tion of the existence of a physical environ- 

 ment as contrasted with a biotic environ- 

 ment illustrates the principles approach; 

 even when the physical environment is 

 broken down into component parts, the 

 treatment continues to present principles, 

 when, within the subdivisions such as tem- 

 perature, light, and moisture, the discussion 

 centers about principles such as the tem- 

 perature "laws," Bergmann's rule, and 

 Corioli's force. 



A fresh definition of the community con- 

 cept is offered in the present work: In 

 large, the major community may be defined 

 as a natural assemblage of organisms which, 

 together with its habitat, has reached a 

 survival level such that it is relatively 

 independent of adjacent assemblages of 

 equal rank; to this extent, given radiant 

 energy, it is self-sustaining. 



This definition places special restrictions 

 on a term that has often been a useful 

 catch-all, correctly applicable to any 

 ecological assemblage ranging from the in- 

 habitants of a small clod of earth to the 

 animals and plants living in the northern 

 evergreen forests of the world. Under the 

 older usage, "communitv" might refer to a 

 simple ecological unit illustrated bv a thin 

 mat of floating algae as well as to the com- 

 plicated, multistoried tropical rain-forest 

 (J. R. Carpenter, 1938). A practical solu- 

 tion seems to be to recognize the usage of 

 the term "community" both in the restricted 

 sense indicated by our definition, and in the 

 extended loose sense. It will occasionally be 

 necessary, under the conditions, to add or 

 to imply "s.s." or "s.lat.," "in a strict 

 sense" or "in a broad sense." We have 

 wished to avoid further implementation of 

 the facetious definition of ecology as being 

 that phase of biology primarily abandoned 

 to terminology. 



There are two fundamental approaches 

 to ecological communities that are best pre- 

 sented by considering the two extremes. As 

 biocoenoses, they may be organized 

 primarily by the interrelations of the plants 

 "nd animals as associates; in contrast, the 

 basic organization may rest on the com- 

 mon habitat in which the constittient or- 

 ganisms serve primarily as indicators and 

 secondarily as associated individuals. Both 



types of communities exist in fairly pure 

 form, and there are closely graded intercon- 

 nections. The biota of the desert presents 

 many aspects of a community controlled by 

 its physical habitat, and the oyster bed is 

 a classical example of a biotically con- 

 trolled biocoenosis. Both types present 

 many different orders of complexity and 

 size; one of the larger of these, the biome, 

 requires further mention. 



The biome, represented by the northern 

 coniferous forest in North America, includes 

 three major plant associations: viz., the 

 spruce-pine forest of Alaska and northwest- 

 em Canada; the spruce-balsam fir forest of 

 northern Canada from the Mackenzie River 

 through Labrador and southward; and the 

 pine-hemlock forest of southeastern Canada, 

 the region around Lake Superior, and 

 northern Michigan. The climax dominants 

 of the last two associations are radically 

 different, but they resemble each other 

 closely in having a large number of 

 identical animal constituents that charac- 

 teristically range through both. 



Shelford and Olsen (1935, p. 395) list 

 the common animals of the coniferous for- 

 est biome, pointing out that they range 

 through the three maior plant associations 

 without conspicuous change. Their analysis 

 shows the importance of the animals in 

 definincr biotic units and the weaknesses 

 inherent in biome concepts based solely on 

 data concerning plants. The vegetation is 

 not the sole key to the biome. Furthermore, 

 the pine-hemlock community has a clear 

 unity with the transcontinental spruce-bal- 

 sam fir forest and even with the Alaskan 

 spruce-pine association. This unity is based 

 on subclimax stages and on animal con- 

 stituents some of which may be relatively 

 unimportant ecologically. 



The universality of the biome concept 

 meets a severe test in the ^eoejraphic frag- 

 mentation of the major biotic formations. 

 New Guinea and northern Australia, for 

 example, tend to be separated by plant 

 geographers into two areas (Scrivenor et 

 al., 1943). Contrariwise, most students of 

 animal distribution unite the two into a 

 common major zooijeogranhic region. The 

 concept of the biome, like manv other 

 ecological generalizations, must be accepted 

 with proper reservations and adjusted to 

 the historical prolilems involved. 



Ecological formations are not static. 

 Given time, the advance and retreat of 



