ESTABLISHMENT OR ECESIS 65 



fraction of the initial seed production and would be practically negli- 

 gible were it not for the recurrence of bare or denuded areas in which 

 the barrier of competition is absent. So complete is the control of 

 dominants, in climax communities especially, that the ecesis of in- 

 vaders is all but completely inhibited, becoming possible only in con- 

 sequence of some marked disturbance or climatic change. From this 

 standpoint, the earlier views of more or less constant and widespread 

 migration or of gradual advance, such as that of woodland along val- 

 leys, become entirely untenable, and the great movements of climaxes 

 and subclimaxes are to be explained only on the basis of climatic 

 compulsion, with ecesis as the decisive function of the community 

 (Clements, 1922). 



Ecesis in Animals. The motility of essentially all land and many 

 aquatic animals leads to an invasion of new territory in their daily 

 rounds for food, in consequence of fright due to enemies or other inci- 

 dents. In addition, many of the smaller forms are borne out of their 

 natural area by currents of wind or water. Thus, there are thousands 

 of temporary invasions of new territory by many species to one inva- 

 sion of significance as biotic migration. The sessile and sedentary 

 animals show a series of phenomena so nearly parallel to those of 

 plants that the essential principles may be found in the preceding 

 section. 



Motile animals with power of flight, and one or more generations 

 per year, may invade new territory under agricultural conditions and 

 establish themselves until a cold winter or a dry season kills them 

 off. The boxelder bug (see p. 63), harlequin, and cabbage bug are 

 examples. The birds of forest edge and meadow have doubtless in- 

 creased greatly over eastern North America with the increase of such 

 habitats about farms and villages. The introduction of the English 

 sparrow and starling is largely a transfer of a European species of the 

 deciduous forest to similar conditions in America. However, Wetmore 

 (1926:211) states that it took ten years for the starling to establish 

 itself firmly in Central Park, though the responses and coactions in- 

 volved are not known. His account indicates that the starling ranges 

 considerably beyond its breeding area, which is a frequent phenome- 

 non. The instances of the ecesis of animals outside their original 

 ranges in North America have been very numerous under the influence 

 of the rapid modification of the original biomes, but there is scarcely 

 a case in which the detailed causes or processes have been studied. 

 In fact, at present little more can be said of ecesis among animals 

 than that it involves maintaining a population over an adverse year 

 or series of years. 



