390 



COMMUNITY ECOLOGY: 



this reason, biotic provinces are described as "units 

 of evolution." 



Biotic provinces can be defined only in general 

 terms. They tend to be identified by their general 

 environment in terms of climate, soil, and topog- 

 raphy. However, each of these factors varies consid- 

 erably ufithin a province; the environment within a 

 province can be more variable than the average differ- 

 ence between provinces. For this reason, the best 

 diagnosis stresses their being units of evolution that 

 approximate the natural areas (distributions or 

 ranges) of flora and fauna. Biotic provinces are thus 

 said to be centers of evolution and dispersal; each 

 may have a unique race of one or more wide-ranging 

 species. 



Because of their close relationship, natural areas 

 and biotic provinces have many of the same charac- 

 teristics. Biotic province boundaries are defined by 

 tangible or intangible barriers; however, the barriers 

 never are absolute for all species within a single 

 province. Many species have the limits of their dis- 

 tribution beyond those of the province, and some 

 species have their margin of area within the confines 

 of a province. Such range restrictions are associated 

 with horizontal or altitudinal ecological differences. 



Ecological differences within a province are the 

 basis for forming lesser units. A bwlic district is a 

 horizontally distributed but ecologically unique 

 province subunit. A life belt is the altitudinal equiva- 

 lent of a biotic district. 



CRITICISMS 



This scheme represents areas of potential evolution, 

 something not attempted by the previous schemes. 

 For this reason, it is most used in studies of specia- 

 tion. However, it does not provide a foolproof method 

 for combining like organisms. Because different 

 species have unlike ecological amplitudes, they dis- 

 play independent reactions to their environments that 

 may lead to unique patterns of geographic variation. 

 On the other hand, the scheme does provide a useful 

 average distribution of potential units of evolution, or 

 natural areas. 



The biotic province also suffers from being a static 

 presentation of dynamic phenomena. The scheme 

 does not portray geographic changes in the areas of 

 species. This is an especially serious shortcoming in 

 a summary whose avowed purpose is to display the 

 dynamic nature of evolution. 



Although this is only secondarily an ecological 

 scheme, it has received many criticisms on an ecologi- 

 cal basis. The use of a strict geographic unit is criti- 

 cized, because some provinces include unlike habitats 

 and other provinces share much the same habitats 

 and life. Also, some provinces are dominated by a 

 single climatic climax, others by two or more climatic 

 climaxes, and a few cut across climatic climaxes. 



DYNAMIC FAUNAS 



K. L. Gordon in 1947 presented a close approxima- 

 tion to a truly dynamic scheme of distributional anal- 

 ysis. The dynamic nature is shown by the fact that 

 subunit mapping is not an important part of the 

 scheme and that single localities, normally stands, are 

 the usual unit of study. The study of such units al- 

 lows comparison of close to remote areas and later 

 investigations to appraise any changes in a stand. At 

 the present time, detailed development of the scheme 

 is limited to mammals. 



The major subdivisions of dynamic faunas are of 

 two types, faunal elements and faunas. A faurial ele- 

 ment is a broad geographic area of origin and is 

 roughly equivalent to a geoflora (Figure 19.28). The 

 individual elements of the North American mamma- 

 lian fauna are the South American, Tropical North 

 American, Austral North American, Boreal North 

 American, and Old World Elements. A fauna is a 

 dynamic assemblage of animals, the members of 

 which constantly shift their ranges in differing 

 amounts according to ecological amplitude (especially 

 in reference to mobility and adaptations) and to 

 availability of their habitats. As one might expect, 

 each fauna normally is associated with a certain type 

 or types of vegetation that in turn reflect the sum of 

 ecological factors (Figure 19.29). 



The individual fauna is ecological in nature. The 

 Boreal Faunas are the Tundran Fauna of Tundra 

 relationship and the Coniferan Fauna of Boreal Forest 

 relationship. Tlie Austral Faunas are the Deciduan 

 Fauna of Deciduous Forest affinity and the Sonoran 

 Fauna of non-forest alliance. The Sonoran Fauna 

 includes many different areas that are united because 

 they are arid Austral. The only Tropical F'auna re- 

 lates to the Tropical Forests and Savannas. 



Each fauna is further considered in two ways: ac- 

 cording to niche organization and/or as a center of 

 evolutionary differentiation. Specific consideration 

 depends on the particular use of the scheme; in many 



