ECOLOGY OF ISLE ROYALE. 137 



The characteristic aquatic society is composed of the Herring Gull, 

 Loon, American and Hooded Mergansers, and the ried-billed Grebe; 

 mainly fish eaters and scavangers. Other species, of greater inland 

 tendencies, are attracted by the fish food, as the Eagle, Osprey and the 

 Kingfisher. The Gulls show a decided preference for the great Lake, 

 and the Loon for the inland waters. The presence of the Kingfisher 

 ■was, influenced by the harbor with its attendant sand banks and bars. 

 As all these water bodies near Isle Eoyale freeze over in winter, the 

 strictly aquatic birds must normally migrate to secure food. Of course 

 none of these birds, nest in the open waters, but on the island beaches 

 (Gulls), near the mouths of streams, and inland in marshy places; but 

 all, as a rule, nest near the water. The very young soon attend theii; 

 parents, and are thus in the water at an age when many land birds are 

 yet helpless in the nest, thus confirming their aquatic habits and habi- 

 tat. During migrations many other species frequent this habitat. 



Where Isle Royale now is, once rolled the open Lake; and it is not 

 improbable that as the island appeared the Herring Gull was one of the 

 first species to discover it. Such a bird might even reach the island under 

 climatic conditions of the Ice Age, for the species now ranges far north 

 along the shore of the Arctic Sea. A species of such extensive clu-onolog- 

 Ical and geographical range will tend to give much stability to suc- 

 cesion. The present range of the Mergansers and the Loon is not so far 

 north, and for this reason they may have arrived under milder climatic 

 conditions. But if the island became exposed under mild post-Glacial 

 conditions, all of these species may have arrived at much the same time. 

 But even with the chances for such variations the general succession 

 seeitiS to have been initiated with the aquatic association as the, pioneer 

 society. 



In following the genesis of the habitats and associations from this 

 point onward, divergence and differentiation becomes so marked that 

 it is impossible to develop all lines abreast. A linear treatment be- 

 comes necessary, and therefore certain general relations are liable to 

 become obscured unless specifically mentioned in advance. 



The aquatic and beach habitats possess a marked tendency toward a 

 zonal arrangement. From the Superior beach the transition is through 

 open or shrub zones into the climax forest. The topography of the 

 island with its longitudinal ridges and valleys form a dominant factor in 

 impressing this zonal structure upon the biotic associations. The series, 

 — from the water, through the beach, open and shrub marginal zone, 

 into the climax forest, — may be considered as the genetic vegetable suc- 

 cession. They change simultaneously and are due to the same general 

 «ause, — the falling Lake surface, which transforms the water area into 

 beach, the beach into forest margin, and forest margin into the climax 

 association. But as mentioned, it is manifestly impossible to discuss all 

 these transitions at once, and each ecological unit must therefore receive 

 separate genetic treatment. 



This tension line or marginal zone between the Lake and the forest 



shows such a wonderful aiversity and complexity in its conditions, 



that several plant and animal associations are formed within this zone. 



With its onward march there are simultaneous changes in several asso- 



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