RELATIONS AMONG SPECIES 



371 



Think of these times prior to the ends ot eras as 

 periods ot more than normal extremes of geomorphic 

 change. Under extreme conditions dominant plant 

 species, owing to their specialization, might become 

 extinct. If dominants are lost in some serai stages as 

 well as in climaxes, biomes and even biome-classes 

 would be subject to chaotic relationships in their 

 ecosystems. To support this view, it is known that 

 present-day ecosystems are composed of many closely 

 interdependent organisms. Moreover, if certain key 

 organisms are removed from these ecosystems, it is 

 known that the ecosystem might collapse. In a sense, 

 this is no more complex than the disturbance of a 

 single climax causing the area to revert to some serai 

 stage. However, if physical and biological factors 

 in the past eliminated key members of many serai 

 and climax communities, there would be widespread 

 extinction of organisms in many biomes. 



The consequence of the collapse of ecosystems on 

 this scale would be a rather chaotic recombination 

 stage of survivors. The survivors would be hard 

 pressed for survival, and thus would be subjected to 

 conditions optimum for rapid evolution. 



Carrying speculation further, the Lower Cretace- 

 ous opened with an expansion of the gymnosperms. 

 However, by late Lower Cretaceous gymnosperms 

 were definitely declining. The reason for this is pure 

 speculation, but it might be that the Lower Cretace- 

 ous was the start of a change from a lack of climatic 

 zonation to climatic zonation, or conditions similar 

 to those prevailing today. Evidence indicates that 

 there were progressive diversity, seasonal changes, 

 and zonation of climate. Assuming this was true, a 

 strong isolation-recombination cycle could start and 

 end with the Upper Cretaceous dominance of flower- 

 ing plants. Moreover, during most of this cycle, 

 animals probably could exist in many areas where 

 older conditions prevailed. This would be the case 

 especially with animals associated with serai stages 

 rather than climaxes. Finally, however, the end of 

 the Upper Cretaceous, with its complete dominance 

 of flowering plants, could prove too much for archaic 

 animals to survive; hence, widespread extinction 

 could take place and conditions be created for new 

 animal evolution in the Paleocene. 



This interpretation of extinction as the removal 

 of key organisms resulting in the collapse of various 

 ecosystems fits in with known physical and biological 

 paleogeographical records. It does not hypothesize 

 truly catastrophic geological changes — conditions that 



do not appear to be supported by present data about 

 those eras. 



HISTORICAL BIOGEOGRAPHY 



The area occupied by a species or community is 

 the function of interplay between organism and en- 

 vironment through time. The important species re- 

 sponses to environment through time are accom- 

 plished by adaptations that allow rapid dispersal and 

 penetration of environments. These adaptations con- 

 tribute to an over-all ecological amplitude that de- 

 termines the success or failure of a species in expand- 

 ing or maintaining its range and, more important, its 

 very existence. An additional possibility of success, 

 even greater success than that achieved previously, 

 can come about through chance hereditary changes 

 and selection of these changes, resulting in evolution 

 of a new ecological amplitude that is either broader 

 or better adapted to environmental conditions. 



Previous discussion described how variations in 

 ecological amplitude of single species and commu- 

 nities resulted in their establishment in particular 

 habitats. Inferences about the origin and develop- 

 ment of new areas could be made from those now 

 existing. However, such minutae tend to be lost in 

 an over-all view of the past history of organisms, 

 especially in a synopsis of part of the past history of 

 life on our planet. Such is the case in the discussion 

 that follows. For this reason, the reader may want to 

 review earlier chapters before continuing. 



NORTH AMERICAN FLORAS AND LAND 

 VERTEBRATES 



One of the better examples of the past history of 

 life is the Late Cretaceous to Recent record of the 

 floras and land vertebrates of North America. 



This historical discussion of plants involves geojhras. 

 A geoflora is an extremely large unit of vegetation 

 that usually can be diagnosed on the basis of the 

 vegetation type or growth form that is present. Each 

 geoflora is composed of many elements, each element 

 often being roughly equivalent to a fairly large unit of 

 vegetation. In addition, a geoflora is a historical 

 unit, which means that both the individual species 

 and the unit as a whole evolve through time. There- 

 fore, a geoflora never is exactly the same thing 



