370 



COMMUNITY ECOLOGY: 



fluence and basic nature of the biome, when the new 

 dominants do alter biome influence, they may repre- 

 sent the evolution of a new climax community. The 

 new climax often causes the previous climax to be- 

 come a serai stage just prior to the new climax and 

 also causes the original biome to be altered. 



Perhaps maximum change in biomes is brought 

 about by climatic change, which normally stems 

 from geomorphic change. Shifts in climate tend 

 either to separate species of a single biome into many 

 biomes or to congregate species from adjacent bi- 

 omes (isolation-recombination cycle) into new biomes. 

 Both are eventualities, because the unlike ecological 

 amplitude of different species can allow them to occur 

 together in certain environments but not in others. 

 Hence, a single biome as a result of climatic change 

 can become fragmented into two or more distinct 

 but closely related biomes, especially when the earth 

 proceeds from geomorphic periods lacking climatic 

 zonation to periods having climatic zonation. On the 

 other hand, the opposite geomorphic cycle can lead 

 to fusion of biomes, extinction and evolution of many 

 species, and creation of a very complex biome or 

 simple biome-class. However, the individual biome 

 components of a biome-class probably differ from 

 any preexisting biome, the unique physical and biotic 

 factors leading to unique combinations of species into 

 communities and, perhaps, to unique successional re- 

 lationships among species. 



Biome-classes, then, probably are due to no unique 

 process. From the time of the primary biosere to the 

 biome-class — and even to the biosphere, if one wishes 

 to carry discussion that far — isolation-recombination 

 cycles are sufficient to build one level of complexity 

 into the next higher level; thus biomes become biome- 

 classes, and biome-classes become related within a 

 biosphere. 



EXTINCTION AND ORIGIN 



Biogeographical succession is probably intimately 

 associated with the extinction of certain groups of 

 organisms and the origin of new groups. Since Cam- 

 brian times, two remarkable periods of extinction 

 and origin have occurred. The first of these was at 

 the end of the Permian and the start of the Triassic, 

 a time of the extinction of the archaic amphibians, the 

 formation of many new reptile groups, and the origin 

 of the mammals. The second period was at the end 

 of the Cretaceous and start of the Paleocene, a time 



featuring the extinction of most reptiles and ancient 

 mammals and the origin of modern reptiles and 

 mammals. These two occasions mark the separation 

 of Paleozoic, Mesozoic, and Cenozoic eras. 



Extinction at both times probably was neither 

 catastrophic nor devastating, as is often believed; 

 each period of extinction and origin probably lasted 

 ten million years or more. However, the tempo and 

 rate of extinction were greater than usual. In animals 

 it occurred generally at the ends of eras; in plants, de- 

 cline or extinction of forms often was during eras. 

 Also, quite logically, the origin of certain plant taxa 

 preceded that of animals associated with them. For 

 example, during the last of the Mesozoic Era, the 

 Upper Cretaceous Period, flowering plants entered 

 as dominant species; however, modern animals seem 

 not to have reached their present role until the start 

 of the Cenozoic Era. 



Unfortunately, it is very difficult to provide a single 

 reason for these between-era happenings. Among 

 various possibilities are great geological activity, in- 

 cluding vulcanism, diastrophism, and changing cli- 

 mates, ocean currents, or ocean levels. However, 

 there is no geological evidence that catastrophies were 

 extensive enough to have caused between-era changes 

 in animals, but not in plants. For this reason, many 

 paleontologists have sought biological reasons for 

 extinction. Until fairly recently, the assumed bio- 

 logical causes were racial senescence and overspecial- 

 ization. With moderately changing environments 

 such features could cause extinction. However, once 

 again, it does not appear that these biological phe- 

 nomena singly, or even together, could explain what 

 the fossil record portrays. Although certain groups 

 that became extinct could be considered senescent or 

 overspecialized, others appear to have been general- 

 ized, quite vigorous forms. Therefore, although 

 catastrophies, senescence, and overspecialization 

 might have contnbuled to widespread extinction, they 

 do not provide an unequivocal explanation for the 

 phenomena involved. 



Recently, a new proposal was made. This hy- 

 pothesis does not exclude previously suggested geo- 

 logical and biological causes for extinction; rather, 

 these previous causes might have acted sufficiently to 

 trigger the final, more dramatic phases of extinction 

 of animals, explain the different response in plants, 

 and also explain the conditions where chance muta- 

 tion and selection of genes would produce remarkably 

 new taxa in a very short period of time. 



