10 



INTRODUCTION 



glaciers aflFects the location of the tundra. 

 Grasslands expand and contract on a vast 

 geographic scale; deserts wax and wane. 

 Bodies of water, including whole oceans, 

 overflow their basins; in another geological 

 age, the land masses stand high out of 

 water. These changes follow certain more 

 or less irregular periodicities that have a 

 geological time scale. Shorter temporal 

 progressions also occur. Given sufficient 

 freedom from man's interference, striking 

 vegetational changes may occur within the 

 life of a single human generation. Burnt- 

 over areas "heal," and, given longer time, 

 serai successions advance from pioneer 

 through intermediate stages to the climax 

 characteristic for the given climate. A com- 

 munity in this temporal series undergoes 

 development and maturation before the 

 succeeding one replaces it. The processes 

 of biotic development in combination with 

 those of physiographic succession are refer- 

 red to as "Community Ontogeny." "Com- 

 munity Phylogeny" involves the whole 

 range of continuing adaptational change of 

 the components of the community. Com- 

 munity evolution, in a broad sense, has 

 been made to include several meanings: 



1. The development of the climax 

 through successive biotic changes and 

 stages— a process comparable to the devel- 

 opment of the individual. 



2. The organic development of the cli- 

 max when there is a series of underlying 

 and correlated physiographic changes, suc- 

 cession in the strict sense. 



3. The convergence of community life- 

 forms, which is implied, so far as plants 

 are concerned, by speaking of the evolu- 

 tion of vegetation as contrasted with the 

 evolution of the species composition of the 

 community flora. The animal constituents 

 show the same kind of interrelations in 

 structures and in physiological adjustments, 

 and the whole biota can be similarly con- 

 sidered. 



4. The community evolves also as a re- 

 sult of converging immigration. Thus in the 

 Chicago area we have elements that have 

 come from both southeastern and south- 

 western centers of dispersal, immigrants 

 from the more northern grasslands and 

 from the northern forests, relicts from the 

 glacial age, and regional endemics. The 

 combination of this third sort of commu- 

 nity evolution with the convergencies al- 

 lows us to think of the evolution of the bio- 



sociological climax community as a whole 

 without giving particular consideration to 

 the evolution of the constituent species. 

 From this point of view the evolution of 

 forest or grassland, or other communities, 

 focusses on their evolution as biotic com- 

 plexes. Mesozoic and modern forests, for 

 example, have biotic equivalence, regard- 

 less of the great differences in the species 

 and higher groups of both plants and 

 animals. 



5. Such considerations lead to another 

 aspect of community evolution, namely, 

 "The phylogeny of the definitive grouping 

 of species within the community." The sub- 

 ject is too complex for thorough treatment, 

 and of necessity we have been essentially 

 limited to tracing the evolution of pairs of 

 ecologically related species, or at most to 

 small groups of species that have appar- 

 ently evolved under close mutual relation- 

 ships. This has the advantage of forcing us 

 to test fundamental interrelations that stand 

 near the simplest level of community organ- 

 ization, and it emphasizes our lack of 

 knowledge of more complicated ecological 

 phylogenies. 



Reconstruction of the cause of evolution 

 of the biosociological whole requires con- 

 sideration and integration of all these 

 aspects. We recognize and can outline the 

 problem without being able to advance far 

 toward its solution. 



In community relations it is important 

 to consider the fundamental relations of 

 protocooperation, disoperation, and, as a 

 somewhat different category, competition. 

 These are matters difficult to discuss with 

 clarity. In part the difficulty lies in the need 

 to consider both short-run operational 

 aspects and long-run evolutionary phases. 

 Aside from such complications, and from 

 the innate complexities, there is the lack of 

 sufficient exact and carefully documented 

 information with which one may test and 

 modify, and reject or strengthen, tentative 

 conclusions. 



The competition among plants for space 

 and light and for nutrients is obvious under 

 many conditions. Such competition is one 

 of the important relationships that find ex- 

 pression in the evolution of life forms with 

 resultant layering. There is also competition 

 for pollination when, or if, potential pol- 

 linators are scarce, and for effective mu- 

 tualism, if one of the mutualistic pair 

 lacks local abundance. Competition is 



