584 INTERRELATIONSHIPS OF ORGANISMS 



as the United States, with so many types of climates, scores of different 

 regional climaxes exist. Each has its own more or less typical succes- 

 sional stages, which lead from the various pioneer situations to the local 

 climax. Westward from the Chicago region, as changes in the amount 

 and seasonal distribution of rainfall are encountered, the climax com- 

 munity becomes an association of prairie grasses; and still farther west, 

 it is an association of Great Plains bunch grasses. Southward, in the 

 Eastern states, other hardwood trees replace the beeches and maples as 

 dominants of the climax deciduous forest, and in northern Florida the 

 climax association is a broad-leaved, evergreen "hammock," dominated 

 by magnolia, holly, red bay, and laurel oak. 



Succession in northern Florida provides a good illustration of the not 

 uncommon circumstance in which, because of special conditions, some 

 preclimax community attains a semipermanence or permanence and, in 

 effect, becomes the actual climax. Such a conditional permanence appears 

 to hold for the longleaf pine forests so long as they are periodically subject 

 to fires. The latter are typically ground fires that destroy the invading 

 underbrush, the forerunners of the hammock community, and so maintain 

 the open, wind-swept, sunny forest floor that the pine requires for the 

 growth of its seedlings. Throughout historic times, the frequent burning 

 (at least once in every few years) of the pinewoods to provide better 

 pasture has been a standard and well-nigh universal practice; and there 

 is reason to believe that such periodic burning, both by prehistoric 

 Indians and as a result of lightning, has long operated to maintain the 

 pine-dominated communities. Today it is known that the elimination of 

 fire allows the pine forests to be replaced by hardwood hammocks, and 

 it seems evident that existing hammocks either owe their development 

 to some natural barriers that protected them from fires or were developed 

 from such preceding communities as swamps and seepage thickets. 



Some special phases of succession are so rapid that one may actually 

 follow them in detail. An infusion of dried grasses in water quickly 

 develops a huge population of bacteria, and within a few days, one or 

 more species of protozoa (introduced with the dried grasses or air-borne 

 with dust particles) appear, feed upon the bacteria, and multiply at a 

 prodigious rate. Then, within a few more days, other species of protozoa 

 appear, and the earlier species wane. This process may continue, each 

 form producing biotic, chemical, or physical changes in its environment 

 that pave the way for some succeeding form, until a sequence of perhaps a 

 dozen species will occur within the space of a few weeks. A similar but 

 somewhat slower succession can be seen in the stages of decay of a fallen 

 tree. Within a few years — the number depending upon the kind and 

 size of tree and the place where it falls — it will have changed from sound 

 wood to a moldering heap of humus and in the process will have formed 



