ORGANIZATION OF LAND COMMUNITIES 59 



increase, only to diminish again during the late 

 summer droughts until at the beginning of winter 

 there are only about a million invertebrates to 

 the acre. 



Along the sloughs and water courses are commu- 

 nities, characterized by willows, which have a 

 somewhat different composition. The bird life 

 shows the effect of the presence of water; warblers 

 are abundant and the red-winged black-bird and 

 bronzed grackle occur along with more truly 

 aquatic birds. Here leopard frogs breed in 

 tremendous numbers and from these centers 

 they migrate onto the surrounding prairie in 

 sufficient numbers to become of considerable, 

 though not of first rank importance. One esti- 

 mate showed about 200 per acre over the area 

 being studied. They are entirely carnivorous 

 as adults and eat mainly the prairie insects and 

 snails. 



The curve of insect abundance along these 

 moister regions is like that of the higher prairie. 

 The spring maximum of 9,500,000 is higher; the 

 June minimum of a million per acre is the same as 

 before. This rises to a summer maximum of 

 2,500,000 and falls off to an autumn minimum 

 of a half million per acre. Leaf-feeding beetles, 

 snails and gall-forming insects are notable addi- 

 tions to the community. Those who have per- 



