ECOLOGY OF ISLE ROYALE. 143 



sufficient room for — }et us say — two pairs of Yellow Warblers, two 

 pairs of orioles, three or four pairs of Chippies and four or five pairs 

 of Eobins, seldom or never harbors more than a single pair of King- 

 birds or crested Flycatchers. * * * As a rule, the species which 

 roam over the most ground in the coiu'se of their daily wanderings 

 9laini and maintain the broadest preserves, while tliose of sedentary 

 habits often content themsehes with very modest freeholds. Whatever 

 the extent of the domain, the birds who occupy it as a summer home 

 evidently regard it as exclusively their own. The readiness and celerity 

 with which trespassing birds are accustomed to retire when attacked 

 or even merely threatened by the established tenants, has seemed to me 

 to indicate that the claims of tempor^ary ownership are respected by 

 all right-minded birds. * * * In my opinion the desire for exclu- 

 sive possession so conspicuously shown by the male, and often by him 

 alone, is usually the direct i^esult of sexual jealousy. This, as is natural, 

 makes him intolerant, during the breeding season, of the near presence 

 of rival males. If his concern were chiefly in respect to the food supply, 

 it would be equally manifested at every season and towards all birds 

 who subsist on the same food that he and his mate require — which iis 

 certainly not the case." 



The tendency of pairs and species to space themselves and to become 

 relatively sedentary is thus a characteristic condition in an association,, 

 and is an important element in an understanding of succession because 

 it shows the internal organization and habit with which an invader 

 or pioneer from another association has to contend. As Dixon ('97, 

 p. 91) has pointed out, this spacing tendency is an important factor in 

 the extension of range of species and is intimately related to the loca- 

 tion of nesting sites. These facts clearly show that both these internal 

 influences and the environmental ones must be distinguished if Ave wish 

 to deteiTuine the relative influence o^ each and their bearing on suc- 

 cession. The above quotation from Bi-ewster clearly shows that in 

 general not only a greater number of birds can live in a given area, 

 but also that they can live closer together, if they vary in kind. Then 

 again, within the association there are marked differences in habitat 

 preference. Thus in tlie forest there are those birds which nest in 

 the trunks or among the topmost branches of the trees, or even upon 

 the ground; and these are differences largely distinct from the spacing 

 of the pairs of the same species. These influences must be recognized 

 among the dominant influences within the association, and upon which 

 much emphasis must be placed. 



4. Environmental Factors. Then in addition to these internal fac- 

 tors, there are the dominant physical factors. In the following discus- 

 sion primary emphasis will be placed upon succession as found in the 

 Northeastern Biotic or Conifer Center, because successions at other 

 centers with different biotic components and other dominant physical 

 conditions must possess a certain amount of individuality, in addition 

 to those features common to succession in generai. The dominant biotic 

 tendency or dynamic trend of this center, as a resultant of all internal 

 and environmental influences, is for the conifer biotic association to 

 enot^oach upon all other societies and habitats and to become the domi- 

 nant or universally distrihiited association. Thus, in general, all. 



