THE EVOLUTION OF ANIMAL COMMUNITIES 79 



controlled in the winter by the lake waves with 

 their waterborne sand, while in the summer the 

 wind-carried sand-blast is a controlling feature. 



Even in the face of this adverse environment, 

 pioneer plants invade the shifting sand. These 

 include the sand-binding grasses. If these grasses 

 gain a foothold, they tend to break the force 

 of the wind and to cause a deposition of sand 

 around their roots. Similarly the birds may 

 bring seeds of the sand cherries which frequently 

 start to grow just back of the winter or storm 

 beach and again form the basis for a growing dune. 

 In these more stable dunes, the burrowing spiders 

 (Geolycosa) and the digger wasps continue their 

 burrowing, with the resulting mixing of humus 

 with the sand. 



Around moist places in the fore-dune system, 

 seeds of the cottonwood trees may germinate. 

 These trees as they grow form a most potent means 

 of collecting sand. Like the sand-binding grasses 

 and the sand cherries, they too are able to grow 

 without being smothered by the sand that ac- 

 cumulates about them. Frequently only the 

 upper branches of a tall tree extend above the 

 level of the sand; the rest is all buried. In this 

 way great dunes are built up which normally ex- 

 tend parallel with the lake shore. With the 

 increasing stability of these dunes, the animal 



