RELATIONS AMONG SPECIES 



391 



Figure 19.28 Mammalian Founal Elements in references to their 

 centers of origin: 1, Old World, 2, Boreal North American, 3, Austral 

 North American, 4, Tropical North American, 5, South American. 



instances both means of subdivision might be used. 

 Niche organization can imply either a microhabitat 

 (environmental niche) or food relationship (functional 

 niche), and both kinds of niches are utilized. 



FAUNAL ELEMENTS 



The South American Element originated in South 

 America during the Paleocene or Eocene to Pliocene 

 ocean isolation of South America from North America. 

 At the present time most of the North American emi- 

 grants of this element are in Central America and 

 Mexico. The element includes anteaters, monkeys, a 

 large and distinctive group of rodents (including 

 cavies, capybaras, spiny rats, and porcupines), arma- 

 dillos, and marsupials. Among this group, the arma- 

 dillo is in the southern Gulf States, the opossum is 

 widespread, and the porcupine is mostly in coniferous 

 forests. 



The Tropical North American Element originated and 



developed with the Neotropical-Tertiary Geoflora. 

 Owing to changing climatic conditions and the open- 

 ing of the Panamanian land bridge in the Pliocene, 

 this element invaded South America and now is an 

 important part of the fauna of that continent. It con- 

 tains many tree squirrels, members of the New World 

 rat and mouse family, members of the dog family, the 

 ocelot, the jaguar, the coatimundi and other racoon 

 relatives, peccaries, the American tapir, and New 

 World deer. 



The Austral North American Element has two sub- 

 units, one developed with the Deciduous Forest and 

 the other with the Grassland and Southwestern 

 Deserts flora. The Deciduous Forest unit has some 

 shrews, the eastern mole, eastern chipmunk, eastern 

 gray squirrel, and fox squirrel. The Grassland and 

 Desert unit has ground squirrels, prairie dogs, pocket 

 mice, kangaroo rats and mice, grasshopper mice, the 

 badger, and the pronghorn antelope. This element 



Figure 19.29 Mammalian Founos; 1, Tundron, 2, Coniferon, 3, 

 Sonoron, 4, Deciduan, 5, Tropical North American. The wide-ranging 

 Afounal group is not shown and the Grassland area that does not con- 

 stitute a fauna is shown but not numbered. 



