136 MICHIGAN SURVEY, 1908. 



island has been materially elevated) became exposed as a wave-washed 

 beach. These conditions are approximated to-day by the low outly- 

 ing islands. The beach or shore is thtas the original habitat upon Isle 

 Royale, and in general, all others have been derived or developed from 

 it. To discuss these as a truly genetic series would require that these 

 be described simultaneously, as the differentiation took place. These 

 habitats did not develop as isolated phenomena, but several developed 

 at the same time, or abreast. Tlius as soon as enough of the land sur- 

 face had become exposed so that its inequalities began to have an in- 

 fluence, the ridges would be the parts best drained, and certain depres- 

 sions would tend to accumulate the drainage. This process would lead 

 to a simultaneous development or differentiation of the well, moderately, 

 and poorly drained habitats. Almost all of the residual soil formed 

 as the region was baseleveled Avas probably cleared away by the glaciers ; 

 or later, as the waves fell from the island, by the pounding of the waves. 

 Thus the relative absence of a soil must characterize all habitats. At 

 what period life first reached the island in post-Glacial time is not 

 definitely known ; but it is likely that the pioneer vegetation of lichens, 

 mosses and low herbaceous vegetation reached it soon after its exposure. 

 If the biota reached the island about the time of the formation of the 

 Algonquin beach, which, roughly speaking, may have been at about 

 the present elevation of 475 feet above the Lake surface, it has since 

 spread upward and downward from that level. The composition of 

 the initial societies is not liable to as much variation at the later ones. 

 Thus if the Hen'ing Gulls returned to the region at this early period 

 of the exposure, they were probably the pioneer birds; but if only 

 at a much later date, still other species might have accompanied them. 

 While such variations as this may be expected, and due allowance must 

 be made for them, yet there can be little reasonable doubt but that 

 water liirds and those frequenting open places tended to become the 

 pioneers, and that later, with the development of a soil and forests, 

 other associations of birds became established. 



There are at least five important factors which enter into the com- 

 position of the past and present conditions which have moulded and 

 are even now moulding the formation of the habitats upon Isle Royale. 

 These five are: — first, past climatic changes; second, the local topogra- 

 phy; third, the falling lake surface; fourth, dynamic tendency of the 

 vegetation ; and fifth , the habits and structure of the birds. Viith these 

 guiding principles, let us now turn to certain details of the resultant 

 succession. 



a. The Aquatic Association and Hahitat. 



The expanse of Lake Superior, the irregular shore line producing 

 coves, the inland water bodies and streams, together furnish an ex- 

 tensive and expansive area of habitat. The cutting of the Lake waves 

 encroaches upon the land habitat, and the deposition by them elsewhere 

 causes minor extensions of the land habitat (as at Rock Harbor where 

 a sand spit furnishes a nesting site for a Kingfisher). Inland the 

 encroachment of the vegetation tends to restrict the water areas, as the 

 falling Lake level has, in the past, tended to increase the land habitat. 

 These processes must be recognized in order to grasp the dynamic ten- 

 dencies of the habitat. 



