250 DRY AND MESOPHYTIC FOREST COMMUNITIES 



spring. Evaporation in the various associations varies directly with the order 

 of their occurrence in the succession. The differences in the rate of evapora- 

 tion in the various plant associations studied are sufficient to indicate that 

 the atmospheric conditions are most efficient factors in causing succession 

 (Fuller, 131). 



A comparison of Fuller's (131) data with the tables or lists of ani- 

 mals shows that the distribution and succession of animals is clearly 

 correlated with the evaporating power of the air. Further comparison 

 with the description of different forest stages shows that the evaporating 

 power of the air may be taken, in this case, as an index of the materials 

 for abode, etc. 



2. CHARACTERS OF THE COMMUNITIES 



It is possible to characterize the formations of the forest in physio- 

 logical terms, though these cannot be of a very definite kind until the 

 mores have been studied in detail, and accurate measurements made. 

 Taking them stratum by stratum, we may note the following obvious 

 characters: 



a) Pioneer communities. — The communities of the cottonwood, pine, 

 and black-oak stages may be designated as pioneer because of the 

 presence of bare mineral soil. 



Subterranean and ground strata: (a) The cottonwood community 

 is characterized by animals which breed and spend the dark and cloudy 

 days chiefly below the surface of the sand. They are very largely 

 diurnal and predatory, and are exceedingly swift and wary. The bur- 

 rowing spider (Geolycosa pikei) is one of the few nocturnal animals. 



(b) The pine community is characterized by similar mores, but is 

 to be distinguished from the preceding by the presence of many animals 

 which prefer sand that is less shifting and which is slightly darkened by 

 humus (170). Animals requiring "cover," such as the lizard, the blue 

 racer, a few ground squirrels, etc., give character because of their absence 

 from earlier and later communities. 



(c) The black-oak community represents the climax of diversity 

 of the subterranean and ground strata. The bare-sand mores continue 

 in the open spaces, which we have designated as transition areas. Leaf- 

 cutters are now present, while among the burrowers the root-borers 

 (prionids and lucanids) work on the roots of the decaying trees. The 

 behavior differences between this and the preceding community are 

 differences of detail which, for the making of deductions, would require 

 much careful study. 



