MIGRATION. 



67 



at least several miles in such quantities as to produce dominance. Dominance 

 in the development in secondary areas, especially, is directly dependent upon 

 the number of seeds which enter, and hence upon the migration device. If 

 seeds or one-seeded fruits migrate singly, the resulting individuals stand 

 separated, and dominance results only from the movement of large numbers. 

 In a relatively large number of cases, several-seeded or even many-seeded fruits 

 migrate, and upon germination produce the nucleus of a community. Often, 

 also, fruits become tangled with each other, as in Stipa, Erodium, Xanthium, 

 Desmodium, etc., and are transported to new areas, when they produce families. 

 This is particularly true of tiunble-weeds (Salsola, Cycloloma, Amarardhus, 

 etc.), and of tumble-grasses (Panicum cajdllare, Eragrostis pectinacea, etc.). 



Eole of migration agents. — ^It is significant that the agents which carry 

 migrules, viz, wind, water, gravity, glaciers, man, and animals, are also the 

 initial causes of bare areas. Thus, the force which produces an area for suc- 

 cession also brings the new population to it. Often the two processes are 

 simultaneous, especially in denuded habitats. The relation is as simple as it 

 is intimate. Water as a migration agent brings to new water or soil areas 

 chiefly those germules which can be gathered along its course. Thus it is self- 

 evident that a new area with an excess of water will be provided for the most 

 part with water-borne migrules, and that the viable ones will practically all 

 be of this kind. The action of wind is broader, but it is clear that initial areas 

 due to wind are found only in wind-swept places, which are of course where the 

 wind will carry the largest load of migrules. An extremely close connection is 

 found also in the talus slopes due to gravity, for the majority of the species are 

 derived from above. The universal prevalence of ruderal plants in denuded 

 areas due to man's activities is sufficient evidence of the direct relation here. 



Destructive action of agents. — ^The action of water upon seeds practically 

 eliminates all but hydrophytie or ruderal species as pioneers (Shull, 1914:333) 

 in water or wet areas, though this effect is doubly insured by the difficulties of 

 ecesis. Large quantities of seeds are also destroyed in all areas produced by 

 deposit, and especially in talus. The action of seed-eating agents, particularly 

 birds and rodents, is often completely decisive. This is seen most strikingly in 

 secondary areas, but it occurs in all places where seeds are exposed. So com- 

 plete is the destruction of seeds in certain instances, notably in forests of lodge- 

 pole pine, that the reappearance of certain species is possible only where the 

 rodent population is driven out or destroyed. This is confirmed by the almost 

 uniform failure of broadcast sowing in reforestation, as well as in other methods 

 of sowing when the birds and rodents are not destroyed. No other factor in 

 invasion has been so often overlooked, and its exact value is consequently 

 hard to determine. If the few quantitative results so far obtained are rep- 

 resentative, it must be regarded as of great and often of critical importance 

 (plate 15 b). 



Direction of migration. — ^While migration tends to radiate in all directions 

 from the parent group, it often comes to be more or less determinate. In 

 general, it is radial or indeterminate when it is local, and unilateral or deter- 

 minate when more general. The local movement due to wind, man, or animals 

 may be in any and all directions, while distant migration by either agent will 

 usually be in one direction^ This is peculiarly true of carriage by streams, 

 in which the regular movement is always down the valley. In the floristic 



