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continents, down thousand mile river valleys or along mountain 

 ranges. But, most invaders of a bare area come from nearby. Migra- 

 tion is limited by mobility, type of carrier agent available, distance, 

 and topography. Not only are spores and seeds distributed, but in 

 other cases the fruit, offshoots, or the entire plant may be transported. 



Having invaded the area, the plant must succeed in growing and 

 reproducing if it is to exert any influence. If it does, the next step is 

 increasing its population, and exerting its influences, i.e., modifying 

 the environment. Greater population also gives rise to competition 

 and the establishing, by the most efficient species and individuals, of 

 dominance over the community. As the environment is modified 

 (improved, usually) other invaders come, succeed, establish dominance. 

 This continues through a series of phases, determined by the soil, 

 climate, and available invaders, until a climax is reached. At this 

 point the dominant species do not improve or change the environment 

 further, no other species can take over control of the habitat, and 

 relative stabilization is achieved. This situation remains until and if 

 one or more forces cause a return to complete or relative bareness. 



Secondary Succession. Secondary succession is what follows the 

 original succession when its climax is destroyed. A common sequence 

 of events in the dissected Appalachian Plateau and elsewhere will 

 illustrate what occurs when forest is converted to cropland. After 

 the forest is cleared the first few farm crops are phenomenal. HOW- 



FIG-. 26. The hill is being- rapidly reduced in fertility as topsoil is washed off. 

 The up and down corn rows accelerate the vicious process. The bottom is, in a 

 sense, being 1 enriched at the expense of the hill; but, the excess water runoff 

 from the hill drowns the bottomland crops in spots where it collects. The hill 

 will soon be retired to low grade pasture. If overgrazed, abandonment will 

 follow, and then secondary succession will begin. 



