THE BIOTIC ENVIRONMENT OF ORGANISMS 



583 



of a successional sequence that will finally cover even the lake basins 

 with a beech-maple forest. Every type of area newly available for terres- 

 trial life has its own special group of pioneer organisms, and the successive 

 stages in each sequence, or sere, are more or less specialized and predict- 

 able. The pioneers and subsequent sequence of communities that change 

 a dry, wind-swept, sandy ridge into a moist, densely shaded forest are, 

 of course, very different from those that change a lake or pond into 



Fig. 33.18. An Arizona desert. Here the plants are all xerophytes, adapted for survival under 

 arid conditions. Leaves are generally small or absent, root systems are extensive, and the 

 plants may have water-storing stems or heavily cutinized surfaces or an abundance of hairs 

 or other adaptations against drought. Many are also protected against browsing mammals 

 by a formidable armor of thorns or spines. {Courtesy Carnegie Museum, Pittsburgh.) 



the same sort of beech-maple climax, and both of these differ from the 

 sequences that lead to the climax from a rocky cliff or from the bare, 

 muddy margins of a stream. 



We have used the ecologically classical "Chicago region" as a basis 

 for our brief discussion of succession, 1 but in a region as large and varied 



1 Because of the early and important work of Prof. Cowles and his students, of 

 Dr. Shelford and others, and partly because the exceptionally fine sequence of very 

 young to old dunes in northern Indiana and southwestern Michigan so clearly shows 

 all the stages of sand-dune succession. 



