89 



and Panicuiii virgatiiiii, which are among the first of the bunch- 

 grasses to become established. 



The growth of annual plants in parts of the blowsand complex is 

 sufficient to support a considerable animal population, usually similar 

 to that of the blowsand association of lee slopes. Where the plants 

 are very few and scattered, however, as is more often the case, the 

 animals are in large part .roving forms, more being predaceous than 

 phytophagous. These animals (practically all of them insects) are 

 quite numerous, too, and it is clear that their food supply must come 

 from outside the blowsand area. It is then observed that in large areas 

 of bare sand the animals are very much more numerous towards the 

 margin, and when it is remembered that the blowsand species are 

 the same as the interstitial species of bunch-grass, the conclusion is 

 reached that there must be a continual shifting of individuals from 

 one area to the other. This has been observed in the case of tiger- 

 beetles and grasshoppers. As the blowsand animals get their food 

 largely in the bunch-grass area, it might appear that they are mem- 

 bers of two associations, being at one time blowsand animals, and 

 at another time interstitial members of the bunch-grass association. 

 The question is raised w'hether they may properly be spoken of as 

 blowsand animals wdien they depend absolutely on the near-by pres- 

 ence of the bunch-grass association. But if in reality they are mem- 

 bers of the bunch-grass association, why do they not stay there? 

 Why should they enter the bare areas at all, when the food supply 

 is lacking? A possible explanation is that in their random activities 

 the bunch-grass animals move about in every direction, and, accord- 

 ingly, those individuals near the margin of the bunch-grass are con- 

 tinually wandering out into the blowsand complex. The junction of 

 the two areas is seldom abrupt, and would probably never be recog- 

 nized by animals so small as these interstitial insects. There is, 

 therefore, no well-defined or appreciable environmental difference 

 which might influence the animal to turn back, but sooner or later 

 it returns to the bunch-grass. In this way the margin of the bare 

 sand-area becomes populated with transient interstitial animals of 

 the bunch-grass. 



Not all the animals of the blowsand complex are necessarily de- 

 pendent upon the bunch-grass association, even indirectly, though it 

 so happens that most of them are. It is to be remembered that 

 there is almost always some plant growth, which allows development 

 of a self-contained animal assemblage. Few sand expanses of con- 

 siderable size are absolutely devoid of vegetation ; but w'here such 

 areas do exist, animal life is almost entirely absent except near the 



