336 



SCIENCE 



[Vol. LV, No. 1422 



then, may be considered as a subdivision of 

 a biota, with the same geographical limits. 

 Some communities will extend over two or 

 more adjacent biotic areas, and they may re- 

 ceive the same name in the various areas in 

 which they occur. But no community is likely 

 to be exactly the same in two biotic areas, for 

 between the various areas there are definite 

 general diiferences in the fauna and flora, and 

 usually also in climate and physiography. 



The terms ecologic habitat and eoologic com- 

 munity are here used to designate ecologic 

 divisions of any rank. Field workers dealing 

 with different systematic groups of animals 

 and plants will probably find it convenient 

 to use different grades of ecologic units, de- 

 pending partly on the size and mobility of the 

 organisms considered. The ecologic com- 

 munities recognized for ants will probably be 

 smaller in average area covered, and lesser 

 in ecologic rank, than mammal communities 

 will be. 



The rank of ecologic community which wLU 

 probably be most generally useful in field 

 work is the association, using this term in the 

 sense of any relatively stable community 

 whether climax or not. For finer distinctions 

 the association may be divided into commimi- 

 ties of lower rank, such as strata, societies, 

 and the like. However, if the habitats and 

 communities are carefully described, the field 

 worker need not worry about the rank of the 

 ecologic divisions. The important thing is to 

 record the field observations in such a way 

 that the environmental relations of the species 

 considered are clear. 



The discrimination of biotas and ecologic 

 communities is not easy. We must recognize 

 at once that there are few sharp divisions in 

 nature, and that the lines we draw must in 

 many cases be arbitrary ones. Communities 

 or biotas which are very distinct where typi- 

 cally developed, at their edges frequently 

 shade ofl: gradually into adjacent divisions. 

 But often taxonomie groups, such as sub- 

 species, pass gradually into other taxonomie 

 forms without sharp boundaries. The classi- 

 fication of biotas and ecologic communities is 

 no more difficult, and not essentially different 



in kind, from the classification of animal and 

 plant species and larger taxonomie groups. 



"Probably the best criterion for character- 

 izing faunal (biotic) areas is the dominance 

 of particular habitats. It is evident that in 

 passing from one area to another a situation 

 wiU be met where the dominant habitat of one 

 area wiU equal in extent the dominant habitat 

 of the other area. It is at this point that the 

 line separating the two must be drawn."^ 



In the western parts of the United States, 

 where the topography is often much broken 

 and where the climatic districts frequently are 

 sharply limited, it is often possible to mark 

 the boundary between adjacent biotic areas 

 with considerable precision. But in regions of 

 slight topographic and climatic diversity, such 

 as is the case over much of the eastern United 

 States, the limits of the biotic areas are often 

 not clearly defined. Indeed, in some cases, it 

 might be impossible to determine within several 

 hundred miles the position of the boundaries 

 between adjacent areas. The biotic area, how- 

 evei*, is still a useful concept, even though the 

 position of the boundaries of some areas can 

 not be stated exactly. To attempt an exact 

 definition of the boundaries of adjacent areas 

 between which there is a wide belt of over- 

 lapping is certain to result in confusion rather 

 than in precision. 



The distinguishing characters of animal 

 habitats are frequently based on the vegeta- 

 tion, though sometimes on the physical char- 

 acters such as the occurrence of rocks or 

 water. There is a close correlation between the 

 distribution of animal species and of types 

 of vegetation, and even in places where the 

 vegetation is not the dominant factor in the 

 en\'ironment it often can be depended upon 

 to give an index of the physical factors which 

 affect the distribution of animals as well as 

 plants. 



However, it is not yet certain that the small- 

 er animal communities correspond exactly in 

 distribution to the smaller plant communities. 

 The mollusks and insects and other inverte- 

 brates often seem to be restricted in distribu- 



1 Dice, 1916, Univ. Calif. PiM. Zoo!. 16: p. 299. 



