Life-zones 



The life-zone system was developed between 

 1890 and 1910 by C. Hart Merriam (Kendeigh 

 1954). Merriam postulated that animal life in North 

 America had dispersed during past geological time 

 from two great centers ; one in the far North, the 

 boreal, and the other in the Southwest, the sonoran. 

 Boreal animals dispersed southward along the higher 

 elevations in the mountains, thence over the conti- 

 nent generally as they became acclimatized to the 

 progressively higher temperatures of lower altitudes 

 and more southerly latitudes. Sonoran forms dis- 

 persed northward through the lowlands as they be- 

 came acclimatized to cold. Believing, contrary to 

 Allen, that temperature was more important than 

 moisture and types of vegetation in controlling the 

 distribution of animals, Merriam extended the fauna! 

 areas indicated as Alleghanian, Carolinian, and 

 Louisianian on Allen's map westward as belts to the 

 Pacific and re-designated them the Transition, Upper 

 Austral, and Lower Austral life-zones. The boun- 

 daries of the life-zones coincided closely with iso- 

 therms. Each life-zone supposedly contained boreal 

 and sonoran organisms in a characteristic and defi- 

 nitely proportioned mixture. Each of these three life- 

 zones he then divided, at about the 100° meridian, 

 into eastern humid and western arid faunal areas to 

 indicate the secondary role played by moisture and 

 vegetation. 



Flaws in this system have become apparent in 

 recent years. For one thing, the life-zone, as a trans- 

 continental belt, is not a unit that can be recognized 

 everywhere by a characteristic and uniform faunal 

 composition. The Lower Austral zone in Georgia or 

 Florida, for instance, is composed of animal and plant 

 species almost totally diflferent from the composition 

 obtaining in the same zone in Arizona. This has led 

 to the realization that while temperature is of un- 

 doubted importance in controlling animal distribu- 

 tion, differences in type of vegetation, in moisture and 

 terrain, as well as in geological history of community 

 groups are factors usually equally as important as tem- 

 perature, sometimes more so, in determining what 

 species will be present. Furthermore, while the past 

 history of some genera and species can be traced di- 

 rectly to boreal and sonoran distributional centers, 

 it is clear that many species and especially subspecies 

 evolved in various smaller centers (biotic provinces) 

 elsewhere over the continent. Finally, evaluation of 

 local, especially mountainous, areas in terms of biotic 

 communities has tended to confuse the life-zone sys- 

 tem with the biome system. 



Biotic provinces 



A concept of biotic provinces first began to 

 attract attention about 1911, and the provinces were 

 defined and mapped by Dice in 1943 and again in 

 1952a. A biotic province is a continuous geographic 

 area that possesses a fauna distinguishable, at the spe- 

 cies and subspecies levels, from the fauna of adjacent 

 areas, at least to a certain degree. The boundaries of 

 biotic provinces are more likely to coincide with 

 physiographic barriers than with types of vegetation. 

 Unlike faunal areas, life-zones, and biomes, biotic 

 provinces never occur in discontinuous geographic 

 fragments since they are intended to show regional 

 areas of differentiation. 



A biotic province which includes a mountain may 

 have several types of vegetation or life-belts, each 

 serving as a center of differentiation for its charac- 

 teristic fauna when compared with similar life-belts 

 in other provinces. The biotic province system is be- 

 ing used at the present time by some mammalogists, 

 ornithologists, and herpetologists in the study of par- 

 ticular taxonomic groups, but there has been no gen- 

 eral synthesis of these studies and of plant groups to 

 render the provinces truly "biotic" in nature. 



Faunal groups or elements 



In all the systems described, faunal distribution 

 has been analyzed in terms of geographic areas, and 

 the chief criterion for determining the limits of an 

 area has been that of distinctiveness between the 

 fauna of different areas. Recently, largely from the 

 impetus of herpetofaunal studies by Dunn (1931) ; 

 avifaunal studies by Stegmann (1938), Mayr (1946), 

 and Miller (1951): of Simpson's studies of mam- 

 malian fauna (1947) ; and the plant studies of Wulff 

 (1943) and Cain (1947), a different analytical ap- 

 proach to faunal and floral problems has developed. 

 By this more recent approach, those species with sim- 

 ilar centers of origin, dispersal routes, geological 

 histories, and habitat preferences are the elements 

 of a fauna or flora which are analyzed. The objec- 

 tives are to learn the place where and time at which 

 these groups of species originated, how the groups 

 came to occupy a particular part of the world, be- 

 came adapted to live in new environments, and how 

 they evolved into the present-day living species. A 

 type of vegetation, life-zone, or biotic province may i 

 contain species from several different faunal elements 

 that have come to live together. Thus in the bird 

 composition of California one can recognize boreal. 



274 Geographic distribution of communities 



