Origin of Biomes and Succession 275 



Long-term events. The examples of succession on the sand dunes 

 and the sphagnum bogs concern phenomena which arise only 

 through the action of long-term events. In certain arid areas new 

 bare sand dunes may indeed arise continuously and apparently 

 ad infinitum through local wind action, but sand areas in the decid- 

 uous forest of eastern North America with its relatively high rain- 

 fall have a different history. These sand areas and the bogs bring 

 out some interesting aspects concerning the life of communities. 



The large upland sand areas in the temperate deciduous forest 

 of central North America resulted chiefly from the deposition of 

 sand by the tremendous rivers of melt-water issuing from the spent 

 Wisconsin ice sheets, and strong winds during the period of 

 glacial decay. Many of these sand areas are unstable to the point 

 that unusual wind, rain, or drought lead to fresh exposures of sand 

 surfaces, and this is followed by the establishment of a new series 

 of local successions leading from bare sand to hardwood forests. 

 Many of the plants and animals of the earliest stages apparently 

 require sand for their existence. As this process of succession con- 

 tinues, organic material is constantly being incorporated into the 

 upper layers of sand, which in some localities have reached the 

 state of sandy loams. Many herbs which do not require sand thrive 

 on these loams and out-compete the obligate sand dwellers. It is 

 only a matter of time before the upland sands of this area will 

 lose their distinctive physical character whereupon the upland 

 sand community will no longer be represented in the region. The 

 habitat itself will not be present again until another glacier and 

 its rivers, or some equally drastic change, reworks the present and 

 future loams and deposits large amounts of pure sand. Local streams 

 will continue to create new sand bars along their courses, but these 

 sand bars support only river edge plants, not the more xeric types 

 found on the upland sand areas. 



Comparable upland sand areas were undoubtedly left as the 

 aftermath of glaciers preceding the Wisconsin ice sheets. What 

 happened to the sand biota of these areas when the sandy habitats 

 presumably turned into loam soils? We can get some clues from 

 the known distribution of some of the species confined to the Illinois 

 sand areas. Each sand species of the herb Lithospermum has a wide 

 distribution and occurs to the west in sandy prairie communities 

 (Fernald, 1950). The grass Aristida tuberculosa occurs slightly to 

 the west and north but chiefly in the sandy areas along the Atlantic 

 coast (Hitchcock and Chase, 1950). The majority of the distinctive 

 insects which in Illinois are obligate to sand areas are either chiefly 



