PALEO-ECOLOGY 



Grasses did not evolve until the Upper 

 Cretaceous period of the Mesozoic, and did not be- 

 come important in North America until the elevation 

 of the Western mountains in mid-Tertiary produced 

 a semi-arid climate in the middle of the continent. 

 Several of the mid grasses are circumpolar in dis- 

 tribution and may have constituted serai stages in 

 the Arcto-tertiary flora. They probably segregated 

 out to form the true, mixed, and bunch grass prairies 

 when the forests belonging to this flora retreated 

 southward and eastward. The close relation of grass 

 species on the prairies of North America and north- 

 ern Eurasia is doubtless due to their similar deriva- 

 tion from the Arcto-tertiary flora. The tall grasses 

 of the Andropogoneae may be of tropical origin. The 

 short grasses were probably derived from the Madro- 

 tertiary flora to form the short grass and desert 

 plains. Although one cannot be sure because of the 

 paucity of grasses in the fossil record, it appears 

 that the grassland is of mixed and relatively recent 

 origin (Axelrod 1950, 1952). 



During portions of the Pliocene and Pleistocene 

 (Fig. 21-2), rainfall was heavy; grassland probably 

 extended through the Great Basin and into the Mo- 

 have desert. There is little information available to 

 indicate whether the California grassland was ever in 

 broad contact with the rest of the biome, but there 

 was probably a narrow and irregular contact through 

 mountain valleys at either or both the southern and 

 northern extremities. 



During the post-glacial xerothermic period, grass- 

 land doubtlessly retreated in the southwestern states 

 and in Mexico as the desert biome became extended, 

 but it is difficult to draw boundary lines for the ex- 

 tremes of these advances and retreats. In the eastern 

 part of the continent during the xerothermic period, 

 prairie advanced as a peninsula far into the deciduous 

 forest (Fig. 21-7). Relict patches, including the 

 Illinois prairie, still remain after later cool and more 

 humid climate permitted the forest to recover much 

 of the area it had previously lost. Doubtless the prai- 

 rie also advanced northward into the area now domi- 

 nated by the boreal forest of central Canada during 

 the xerothermic period, where relict patches of grass- 

 land may still be found. Reinvasion of prairie areas 

 was slow at first because of frequent fires caused by 

 lightning and Indians and to poor drainage, but since 

 settlement of the area by white man and artificial 

 lowering of the water table it has become rapid 

 (Gleason 1922). 



In spite of its vast extent, only a single grass- 

 land biociation can be recognized at present in North 

 America, although it varies in composition between 

 different regions. 



NORTH AMERICAN GRASSLAND 

 BIOCIATION 



The North American grassland biociation 

 extends in reduced form as a biocies or serai stage 

 into the deciduous and coniferous forest biomes. 

 Common species are the following, sometimes re- 

 placed locally by related species : 



Mammals 



Masked shrew 

 White-tailed jack rabbit 



(North) 

 Black-tailed jack rabbit 



(South) 

 Eastern cottontail 



(East) 

 Desert cottontail (West) 

 Nuttall's cottontail 



(South) 

 Black-tailed prairie dog 

 Ground squirrels 

 Northern pocket gopher 

 Plains pocket gopher 

 Pocket mice 

 Banner-tailed kangaroo 



rat (South) 



Ferruginous hawk 

 Great prairie chicken 

 Lesser prairie chicken 

 Sharp-tailed grouse 

 Long-billed curlew 

 Upland plover 

 Burrowing owl 

 Short-eared owl 

 Horned lark 



Sprague's pipit (North) 

 Bobolink 



Harvest mice 

 Deer mouse 

 Northern grasshopper 



mouse 

 White-throated wood rat 



( South ) 

 Meadow vole 

 Prairie vole (East) 

 Meadow jumping mouse 

 Coyote 

 Swift fox 



Long-tailed weasel 

 Black-footed ferret 

 Badger 



Prairie spotted skunk 

 Pronghorn 

 Bison 



Western meadowlark 



Dickcissel (East) 



Lark bunting 



Savannah sparrow 



Grasshopper sparrow 



Vesper sparrow 



Lark sparrow 



McCown's longspur 

 (North) 



Chestnut-collared long- 

 spur (North) 



In addition to the more strictly grassland spe- 

 cies above, many species from the deciduous forest- 

 edge biociation extend their ranges varying distances 

 into the open country. Many desert species also ex- 

 tend their ranges into the grassland biome, especially 

 where sagebrush, mesquite, and other shrubs come 

 into overgrazed areas. Because of these various in- 

 fluences, the long North-South range through vari- 

 ous temperatures, and the isolated nature of some 

 portions of the grassland, several faciations may be 

 recognized (Carpenter 1940). They have not yet 

 been clearly defined but may correspond to the biotic 

 provinces described by Dice (1943) and Blair 

 ( 1954) ; vie: Illinoisan in the East : Saskatchewan in 

 the northern Great Plains ; Kansan in the central 



326 Geographic distribution of communities 



