ANIMAL ECOLOGY OF JOHNSON COUNTY = 15 
ing the causes of the treelessness of the prairie, which are here 
quoted because of the light which they throw upon an under- 
standing of both the forest and the prairie of this region: 
‘*1. Exposure to evaporation as determined by temperature, 
wind, and topography, is the primary cause of treelessness of the 
prairies. 
‘*2. The prairie flora persists on the exposed areas because it 
is xerophytic. 
‘*3. Rainfall and drainage, while of importance, because de- 
termining the available supply of water in both soil and air, are 
not a general determining cause, both being frequently equal on 
contiguous forested and prairie areas. 
‘‘4. Soils and geological formations are of value only in so 
far as they affect conservation of water; the porosity of the 
former determining its power of holding moisture, and the latter 
often determining topography. 
‘*5. Prairie fires were an effect rather than a cause, and 
where acting as a cause were local. 
‘6. Seed-dispersal probably accounts largely for the group- 
ing of plant societies on the prairies, but does not account for the 
presence of the prairie as a whole. 
‘‘7. Other assumed causes, such as the bison, sea, etc., are of 
remote interest and not to be taken into account in any attempt 
at an explanation of the prairie as a whole.”’ 
The foregoing quotation serves to explain the occurrence of 
the prairie on all land not forested, in the original conditions. 
As the reduction of this great area of prairie to a few narrow 
strips is discussed under the heading of prairie habitats, it will 
be merely mentioned here. The composition of the prairie, 
however, merits some consideration at this point. In his classi- 
fication of the prairies Shimek* refers to three types as having 
been existent in Johnson County: the rolling prairie, the alluvial 
prairie, and prairie openings in the forested areas. Of these, 
however, the present consideration need not deal separately, as 
the most typical xerophytic plants which compose them are gen- 
eral. The prairie plants represent a high type of plant. special- 
ization, the larger percentage of species being members of the 
family compositae. In number of individuals the grasses and 
*Shimek, The Prairies, p. 174 
