PLANT SUCCESSION 



213 



formation of humus; this, in turn, contributes to soil development, 

 modifies water relations, available nutrients, pH, and aeration, 

 and affects soil organisms. Thus every organism in a community 

 may have some reaction upon the habitat. By these reactions, the 

 habitat becomes changed and consequently is less favorable to the 

 organisms responsible for the changes, while, at the same time, it 



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LIKE ROCK SHORE 

 SUCCESSION 



BURN FORES 



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 SEEDLINGS 

 POPULUS 



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flREWEEDS 



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 ANAPHALI 



CLIMAX FOREST 



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BOO FOREST SWAM" FOREST 



( LARlX.PtCEA MARIANA ) CTHL'JA, LARIX, 



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SHRUBS 

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CRUSTOSE LICHENS, CREVKE HERBS 

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SEOGE MAT 



ROC* SURFACE CREVICE ROCK POOL 



5UB5UCCESSI0N SUBSUCCE5SI0N SUB5UCCE5SION 



Y 



ROCK SHORE SUCCESSION 



BEACH SUCCESSION , 



BOG SUCCESSION 



OELTA SWAMP SUCCESSION 



Y 



XERARCH SUCCESSIONS 



Y -- 



HVORARCH SUCCESSION 



Y 



PRIMARY SUCCESSIONS 



FlG. 102. A diagram of the trends of succession for the principal habitats 

 on Isle Royale, Lake Superior. This is one of the early complete condensa- 

 tions of a successional story for an entire region. On this pattern, similar dia- 

 grams have been worked out for many sections of the country. Note that the 

 system shows at a glance the kinds of habitats in which succession originates, 

 the interrelationship of trends, and the major dominants in each of the stages 

 of succession. Study of the diagram should help to clarify concepts of suc- 

 cession and climax. It must be remembered that not all trends progress with 

 equal speed.— After Cooper. 10 



has become more favorable for species that could exist there 

 previously only with difficulty. Under the changed conditions, 

 new species are able to compete successfully with the established 

 species and often even to replace them. 



The habitat may also be modified by forces quite apart from 

 the effects of organisms. A flood plain or swamp may become bet- 

 ter drained as a stream cuts more deeply into its channel. Silting in 

 of a lake or pond raises the level of mineral soil. Chemical changes 

 in the soil may result from leaching or accumulation of salts. Such 

 modifications of the habitat also produce vegetational changes. 



