THE BI()LO(;iC,\L CONQUEST ()\- nil] \\(,|u,|) 



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The question of how long seeds will survive, uiidci- whal condilions 

 they will germinate, and how fast they will grow is of g,vat inipor- 

 tance in the repopulation of areas after soil erosion oi- fire. Beale 

 reports an experiment where 

 ten out of twenty-two species 

 of seeds sprouted after hav- 

 ing been buried in open bot- 

 tles in moist sand at a depth 

 of three feet for over forty 

 years. After a coniferous for- 

 est has been devastated by 

 fire, an entirely new series of 

 plants spring up in the area ; 

 first herbs, such as fireweed 

 or wild mustard ; then trees 

 or bushes, the seeds of which 

 may be brought by birds, as 

 raspberry, blackberry, or wild 

 cherry ; later a stage of trees 

 having wind-blown or bird- 

 carried seeds, such as aspen, 

 cottonwoods, or birches. Still 

 later the forest may become 

 repeopled by its original in- 

 habitants, which becomes the 

 climax. 



Conditions of wind, mois- 

 ture, sunlight, and weather, the sum total of which constitutes climate, 

 play a most important part in succession. If drought destroys life in 

 a given region, an entirely new group of plants may come to occujiy 

 that area, bringing with them a new group of animals. Migrations 

 of animals may be brought about by changing seasons. 



The biotic conditions governing successions are many. Man, 

 through clearing forests, throwing wastes into ri\ers, or introducing 

 new plants or animals which may compete with existing species, often 

 completely upsets the balance of life and causes succe.'^sioiis. Indus- 

 trial pollution may completely depopulate streams of fish life, bac- 

 terial growth replacing the original plants and animals. Sometimes 

 new organisms add so many competing mouths to feed in a gix'cn terri- 

 tory that it becomes necessary for some to break away if any are to li\('. 



Wriijlu I'itrct 



A lypiciil undt'CKruwtti succession after a 

 I'orcsl (ire. 



