[Illustration: Fig. 63. A typical gravel slide (talus) of the Rocky mountains, before invasion.] =305. Succession by animal agency.= Successions 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 furnish examples of similar successions in which the initial stages are subruderal, while overstocking and overgrazing frequently produce the same result with ruderal plants. =306. Succession by human agency.= The activities 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, irrigation, railroad building, etc. He destroys vegetation by fires, lumbering, cultivation, and drainage, and if he can not control climate, he at least modifies its natural effects by irrigation and the conservation of moisture. The operations of man extend from seacoasts and swampy lowlands through mesophytic forests and prairies to the driest uplands 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 upon 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.