Toothed birds appeared in the Jurassic and dif- 

 ferentiation of types went on rapidly as modern 

 orders were already represented in the Lower Cre- 

 taceous, modern families in the Eocene, modern 

 genera in the Miocene, and modern species in the 

 Pleistocene. The great group of songbirds evolved 

 and dispersed through the world rather late, perhaps 

 in the Miocene; since the Pleistocene epoch, evolu- 

 tion of birds has been largely limited to subspecia- 

 tion. 



An important aspect of the geosere is the dispersal 

 of new types of animals and plants, for the impor- 

 tance of a new form depends on the extent of its dis- 

 tribution and the size of its population. A successful 

 species saturates the available niches at its center of 

 origin and thence spreads outward in all directions. 

 Dispersal continues until an impassable barrier is 

 reached. Tracing the origin of taxonomic groups, 

 their phylogenetic relations with other taxonomic 

 groups, and their dispersal is the subject matter of 

 zoogeography. This knowledge is desirable for the 

 ecological interpretation of communities, for it helps 

 to explain their species composition and the geologi- 

 cal history of the community itself. 



The mechanics of evolution at the species level is 

 called speciation, and is as much a problem of ecol- 

 ogy as it is of zoogeography and genetics. Speciation 

 in animals is initiated only when one population be- 

 comes isolated from another similar population so 

 that interbreeding does not occur. This permits vari- 

 ation and natural selection to proceed independently 

 in the two groups and to become fixed in the germ- 

 plasm. Isolation is usually effected by geographic 

 barriers and generally involves occupancy of new 

 niches and development of new coactions with other 

 members of the community. The history of the past 

 ecological relations of species and of whole communi- 

 ties is the subject matter of paleoecology. The dy- 

 namic forces involved in these processes of speciation 

 are among the determinants of the geosere. 



The climax 



We have described four types of succession as 

 if each were entirely independent of the other three. 

 This is not the case. All types of succession are going 

 on simultaneously, although the relative importance 

 of any one varies from one habitat to another. Biotic 

 succession is most conspicuous, since it proceeds most 

 rapidly and appears to reach a final, permanent stage 

 in just a few decades or centuries. The climax stage 

 of the biosere is undoubtedly more nearly stabilized, 

 self-maintaining, and in steady state in its particular 

 habitat than are the serai stages, yet it also is sub- 

 ject to gradual change over long periods of time. The 

 climax, as well as the serai stages, changes with cli- 

 mate, physiographic forces, and evolutionary proc- 



esses. However, the clisere usually requires a few 

 thousands of years before changes in the community 

 structure or composition become evident. The prog- 

 ress of an eosere is even slower ; geosere, slowest of 

 all. The climax is defined as the last stage in the 

 biosere : no absolute stability or final permanency 

 should be construed, since it is simultaneously a stage 

 in the clisere, eosere, and geosere. 



The climax may be recognized by the fact that 

 in a uniform climatic area all seres tend to converge 

 into it, and by its steady state in respect to structure, 

 species composition, and productivity. In the climax 

 community, all species, including the dominant spe- 

 cies, are continually able successfully to reproduce 

 and there is no evidence that new and different 

 species are invading. In serai communities, on the 

 other hand, the developing new growth, particularly 

 evident in the dominants, contains many individuals 

 of invading species which will eventually take over 

 and replace species already present. 



RECOGNITION OF COMMUNITIES 



The community as an organic entity 



Although the major community or ecosystem 

 is the generally accepted unit of analysis in syneco- 

 logical studies, there is a difference of opinion as to 

 whether the community constitutes a discrete organic 

 entity. Two different points-of-view are incorporated 

 in the organismic and individualistic concepts which 

 are usually associated with the names of F. E. Clem- 

 ents (1916) and H. A. Gleason (1926), respectively, 

 and more recently Phillips (1934-1935), Tischler 

 (1931), and Emerson (1952) on one side and 

 Bodenheimer (1938), Whittaker (1951, 1952, 1956, 

 1957) and Curtis (Brown and Curtis 1952) on the 

 other. Ramensky (1926) stated the individualistic 

 concept independently in Russia as early as 1924. 



The organismic concept considers the community 

 to be a supraorganism, a complex organism, or a so- 

 cial organism. As such, it is the highest stage in the 

 organization of living matter ; namely cell, tissue, or- 

 gan, organ system, organism, species population, 

 community. There is emergent evolution, so to speak, 

 at each higher stage in this hierarchy ; the whole is 

 more than merely the sum of its parts. Tissues have 

 properties, characteristics, and functions over and 

 above those of the individual cells involved ; the 

 organ, the organ system, or whole organism func- 

 tions in a way not to be predicted from a knowledge 

 of the parts of which each consists. The species pop- 

 ulation has inherent characteristics of density, rate of 

 natality, rate of mortality, and age distribution, while 

 the total community has such unique functions as 

 dominance, cooperation, trophic balance, competition, 



26 Background 



