RELATIONS AMONG SPECIES 



MS 



the energy cycle of the invaded ecosystem. ) However, 

 this gives only a general and oversimplified idea of 

 what must take place. In actual practice, success 

 applies to those species that can respond synergisti- 

 cally to the total environment, physical and biotic, 

 during each step necessary for success. 



BIOGEOGRAPHICAL SUCCESSION 



The distribution of individuals, species, and com- 

 munities must be dealt with in terms of the dynamic 

 relationships between the environment and organisms 

 through time. These dynamics appear to proceed in 

 an orderly sequence of events that can be termed a 

 kind of succession. Although any segment of the 

 over-all biogeographical succession varies according 

 to the nature of a particular environment and its 

 organisms, it is possible to assume certain general 

 tendencies in the events of most restricted sequences. 

 These restricted events, or biogeographical serai 

 stages, here are arbitrarily termed abiotic area (any 

 space without life), primary biosere (an unstable or- 

 ganism congregation lacking clear community organi- 

 zation), consolidating biosere (presence of a definite 

 community organization but at the serai level), 

 bwtic climax (a climax community as considered in 

 biotic succession), biome (a group of similar climaxes), 

 biome-class (a group of similar biomes), and biosphere 

 (the part of the earth containing life) (Figure 19.10). 

 The biosphere will only be touched on in the follow- 

 ing discussion. 



In the discussion that follows a single successional 

 sequence, or biosere, can be assumed. However, the 

 sequence really involves a sequence of bioseres. For 

 this reason the term "biosere" was and will be used 

 somewhat loosely to recognize what might be called 

 phases (each, part to more than a biosere) of the 

 over-all biogeographical succession. 



ABIOTIC AREA 



For practical purposes, the areas now lacking life 

 are hardly worth mentioning. Although there are 

 places such as glaciers where kinds of species and 

 numbers of individual creatures are very few, one can 

 be excused if he says that the lower part of the at- 

 mosphere, the entire surface of the land, and the en- 

 tire depths of the waters constitute the biosphere. 

 Hence, the concept of an abiotic area or areas treats 

 a rather nebulous condition. Areas lacking life 

 existed in the past, in primeval seas prior to life and 



on the land prior to its invasion by marine organisms. 

 Furthermore, after life invaded these places, for some 

 time only certain locales had living creatures. There- 

 fore, it appears that one of the trends of biogeography 

 was gradual dispersal and population of the earth's 

 land and water. However, this does not necessarily 

 mean that now there are more species and numbers of 

 organisms than ever before. In the past there were 

 tremendous fluctuations in the density and diversity 

 of life on earth, fluctuations afTecting both local 

 and world-wide aspects of biogeography. 



The likelihood that most of the land and water have 

 had life for many million (perhaps a few hundred mil- 

 lion) years, necessitates some mental gymnastics in 

 regard to recent abiotic areas. In other words, one 

 often gains understanding by assuming abiosis in a 

 situation where it really does not exist. For example, 

 a vast area might be elevated and change from ocean 

 bottom to land. When ocean, the area would contain 

 many marine creatures; all during the process of be- 

 coming land, it would contain a sequence of other 

 organisms. However, one might perform the mental 

 gymnastics of ignoring the sequence of life between 

 strictly marine and land creatures. If this is done, 

 biogeography is more easily understood. Therefore, 

 one might assume that any area that loses its old life 

 and gains a new type of life was an abiotic area just 

 prior to invasion by the new life. 



Abiotic areas are the product of geomorphic change 

 and of the main consequence of geomorphic change, 

 climatic change. Such changes also modify the 

 nature of past barriers and highways, thereby setting 

 the scene for the invasion of new life. However, of 

 primary importance is the nature and distribution of 

 the created highways. Recall that highways can be 

 broad, fairly long-lasting, connections between two 

 favorable environments (corridors); narrower and 

 less permanent than corridors and allowing only 

 certain species to pass (filter bridges); or chance 

 paths along which one to a very few species can 

 pass (sweepstake routes). Each kind of highway has 

 a part to play in populating an abiotic area. 



When a particular site becomes open to life, im- 

 mediately adjacent areas are connected by corridors; 

 more remote areas are connected only by filter 

 bridges; and most remote, perhaps only by swee{> 

 stake routes. Therefore, an abiotic area normally will 

 acquire the majority of its life from adjacent commu- 

 nities via corridors, some of its life from more remote 

 areas via filter bridges, and a very few (if any) species 

 from the most remote areas via sweepstakes routes. 



