PLEISTOCENE ECOLOGY AND BIOGEOGRAPHY 407 



describe the pattern of herbivore extinctions, with the assumption 

 that extinction of certain wolf, cat, cowbird, vulture, and vampire 

 life forms followed inevitably. 



Figure 6 is a diagrammatic attempt at such a descriptive model, 

 showing high and low regions of extinction probability. The high 

 regions lie within the shaded portion of the solid ; the low regions lie 

 above in the unshaded portion. Following Dansereau (1957), habi- 

 tat can be divided conveniently into the four biochores: forest, 

 savanna, grassland, and desert. All terrestrial communities from the 

 equator to the Arctic fall within one of these units. Animal body size 

 is divided into four classes, represented respectively by (1) Norway 

 rat, (2) Norway rat to beaver, (3) beaver to pronghorn, (4) prong- 

 horn to mammoth. Total range of the species in square miles is 

 arbitrarily listed as very small (less than 2,000), small (2,000- 

 20,000), medium (20,000-200,000), and large (200,000 and more). 

 Three specific examples, which were used in constructing the model, 

 illustrate its predictive function : 



Case A. Upper-left-rear corner of the diagram. Small animals in a 

 small area of forest, Puerto Rico. 



Extinct. Nesophontes, an insectivore; Acratoc7ius, a small ground 

 sloth ; and six genera of hystricomorph rodents: Heptaxodon, Elasmo- 

 dontomys, Isolobodon, and Proechimys (surviving on mainland South 

 America), Heteropsomys, and Homopsomys. 



Surviving. Eleven genera of native bats; introduced Rattus and 

 Mus. 



Extinction intensity. Heavy. 



Case B. Upper-left-front corner. Small animals in extensive 

 forest, eastern North America. 



Extinct. None known. 



Range possibly reduced : Neofiber, round-tailed vawskrSit ; Erethizon, 

 porcupine; Didelphis, opossum. 



Surviving. Twenty-one genera of native rodents, various other 

 small terrestrial mammals and bats. 



Extinction intensity. Very light. 



Case C. Lower-center and right-front corner. Large animals in 

 extensive desert, grassland, and savanna habitats, the Basin and 

 Range province and western North America generally. 



Extinct. Mammuthus, mammoths of two or three species; Mam- 



