Origin of Biomes and Succession 293 



heat resistant, cold susceptible species A would have no range over- 

 lap with heat susceptible, cold resistant species B in a climate hav- 

 ing great seasonal extremes. If the climate moderated with resulting 

 cooler summers and warmer winters, the two species would have 

 extensive range overlap. These hypothetical climates could con- 

 ceivably move in either direction, resulting in separation or con- 

 gregation of species. 



Fossil evidence indicates that both of these processes have re- 

 sulted in past changes in biome composition and change. 



SEPARATION 



The arid southwestern flora analysed by Axelrod ( 1958 ) is an ex- 

 cellent example of the evolution of communities by separation. 

 Axelrod pointed out that today this flora represents at least five 

 moderately distinctive associations (Figs. 128 and 129): (1) a 

 California live oak and pine woodland, chiefly in California, (2) 

 an insular oak and pine woodland, on northwestern Baja Cali- 

 fornia and islands off the coast of southern California, ( 3 ) a Lagu- 

 nan oak woodland in lower Baja California, (4) a Sierra Madrean 

 woodland of evergreen oaks, piiion, and juniper, extending from 

 central Mexico to southern United States, and (5) a piiion pine- 

 juniper woodland in and around the Great Basin area. 



In the Cenozoic fossil records, components of these five associa- 

 tions were not segregated in this fashion but occurred in the same 

 localities. Every included species was not found in every fossil 

 find, but several distant stations such as Sonoma, California and 

 Florissant, Colorado, contained large assemblages of species which 

 included distinctive members of practically all the derived modern 

 communities. Axelrod believed that orogenic movements in the 

 mid-Cenozoic brought about greater regional differences in climate, 

 particularly extremes in aridity and winter temperatures and that 

 this in turn brought about a differential separation of the various 

 plant species according to their adaptive characteristics. 



The grassland biomes of central North America probably arose 

 in somewhat the same fashion. Prior to the Miocene the Great 

 Plains areas were either inundated or of lower elevation and warmer 

 climate than now. The more xeric of the terrestrial areas presum- 

 ably supported a semi-desert woodland merging into a temperate 

 deciduous forest to the north and east. During the Miocene this 

 central area was elevated to a height of 3,000 to 6,000 feet and 

 became colder. This probably produced a climate in which most 

 of the species in the older biomes could not survive, and, as a 



