79 



(2) Invaders from cultivated fields and rtideral associations 



Acaiithotlirips irrbasci Diabrotica spp. 



Adclphocoris rapidits Phytonoinus punctatus 



Lygiis pratciisis Gyinnetron tcter 



Mcgilla uiaculata Lasius niger aniericanus 



CoccincUa novcmnotata Apis mellifera 



Drastcrius clcgans Passer domesticus 

 Leptiuofarsa lo-lincata 



Certain of the above may originally have been true members of 

 the bunch-grass association, but they are now, at least, more abundant 

 in cultivated fields, and spread from them. The horses and cattle of 

 the pastured areas are really important bunch-grass animals, and 

 might well be added to the above list. 



(3) Invaders from marshy and aquatic situations 



Bphemerida CJiironoinidae 



TricJwptera Bufo americanus 



Odonata Amblystoma tigrinum 



The nearness of the Illinois River makes the aquatic element im- 

 portant. 



The bunch-grass association covers most of the sand prairie in 

 the Illinois River valley. It contains a larger number of plant and 

 animal species than any of the other associations, and presents a 

 greater variety of interrelations. It may be said to be the associa- 

 tion most representative of sand prairie. 



THE PANICUM PSEUDOPUBKSCENS ASSOCIATION 



The dominating factor in the Panicnm pseudopuhescens associa- 

 tion is the wind, which has gained control over the plants, and is 

 now destroying the vegetation. This association is the transition 

 stage in the succession from bunch-grass to blowsand, and is marked 

 by a large proportional area of bare sand, which is constantly being 

 removed by wind. The association is very commonly found on dune 

 summits, where wind exposure is pronounced. 



Paniciim pseudopuhescens is the last bunch-grass species to be 

 killed by the removal of sand from its roots. It seems to thrive 

 better under conditions of sand removal than in more stable sand, in 

 competition with the other bunch-grasses. It thus becomes the domi- 



