10 



In order to uiKk-rstand tlie gcograpliic distribu- 

 tion local and otherwise of animals, the eventual suc- 

 cession or replacement of one community by another, 

 and the stabilization of different S])ecies at different 

 levels of abundance, we need to know several things. 

 W'e nnist discover how organisms invade new habitats 

 or geograjihic areas : how they react on the habitat ; 

 the manner in which they compete with or otherwise 

 affect each other : and the factors that determine the 

 success or failure of organisms in the struggle for 

 existence. The processes of dispersal, migration, com- 

 petition, speciation. reaction, coaction, and ecesis con- 

 stitute the dynamics of the community, the under- 

 standing of which is a prime objective of ecological 

 tiiougiit and research. 



David Starr Jordan (1928) once stated tiiat the 

 general laws governing the distribution of animals 

 can be reduced to three simple propositions. Accord- 

 ing to Jordan, a species of animal will be found in 

 any part of the earth having conditions suitable for its 

 maintenance unless ( 1 ) its individuals have been un- 

 able to reach this region because of barriers; or (2), 

 iiaving reached it, the species has been unable to main- 

 tain itself because of inability to adapt to the region or 

 to compete with other forms; or (3), having arrived 

 and survived, it has subsequently so evolved in the 

 process of adaptation as to have become a species 

 distinct from the original type. 



DISPERSAL DYNAMICS 



Ecological Processes 



and Community 



Dynamics: 



Dispersal, 



Migration, and 



Ecesis 



Dispersal is the spread of individuals away 

 from their homesites. Dispersal movements are usu- 

 ally slow, and cover relatively short distances in the 

 life time of an individual. The cumulative result of 

 short dispersions by successive generations, however, 

 may become conspicuous in the course of years, dec- 

 ades, or centuries, especially when it amounts to range 

 expansion of a species into a new habitat or area. 

 Some remarkable instances of range expansion have 

 resulted from the purposeful introduction of the house 

 sparrow into North America in 1832-53 and the Eu- 

 ropean starling in 1890-91 ; and the accidental intro- 

 ductions of the black rat, Norway rat, and numerous 

 insect pests, at various times. In similar fashion the 

 gray squirrel and muskrat have been introduced into 

 Europe (Elton 1958). Once man had helped them to 

 overcome the ocean barrier that previously held them 

 in check, these species spread unusually rapidly be- 

 cause of the optimum environment on the continent. 

 There is reason to believe, however, that under en- 

 tirely natural conditions the rates of dispersal of all 

 species, once a barrier is passed, would be similar, 

 varying only with respective reproductive potentials, 

 characteristic speeds of locomotion, and relative abil- 

 ities to find unoccupied niches, overcome competition, 



145 



