.SECONDARY SUCCESSIONS 117 



mation is destroyed, and the new vegetation consists largely 

 of ruderal plants. A peculiar effect of climate occurs in 

 regions with poor drainage, where the result of intense 

 evaporation is to produce alkaline basins and salt lakes, in 

 which the succession becomes more and more open, and is 

 finally represented by a few stabilized halophytes, or dis- 

 appears completely. 



13. Succession by animal agency. Succes- 

 sions of this class are altogether of secondary importance, 

 the instances in which animals produce denudation being 

 relatively few. Such are the heaps of dirt thrown up by 

 prairie dogs, and other burrowing animals, upon which 

 ruderal plants are first established, to be finally crowded out 

 by the species of the original formation. Buffalo wallows 

 f Ornish examples of similar successions in which the initial 

 stages are subruderal, while overstocking and overgrazing 

 frequently produce the same result with ruderal plants. 



14. Succession by human agency. The activ- 

 ities of man in changing the surface of the earth are so 

 diverse that it is impossible to fit the resulting successions 

 in a natural system. While man does not exactly make new 

 soils, he exposes soils in various operations, mining, irri- 

 gation, railroad building, etc. He destroys vegetation by 

 firefi, lumbering, cultivation and drainage, and, if he cannot 

 control climate, he at least modifies its natural effects by 

 irrigation and the conservation of moisture. The oper- 

 ations of man extend from sea coasts and swampy lowlands 

 through mesophytic forests and prairies to the driest up- 

 lands and inlands. Since the adjacent formations determine 

 in large degree the course and constitution of a succession, 

 it will be seen that the effects of any particular activity up- 

 on vegetation will differ greatly in different regions. For 

 convenience, all classes of successions arising from the 

 presence and activity of man will be considered in this 

 place, though, as indicated above, some might well be 

 regarded as producing primary successions, while others 

 produce anomalous ones. 



