ECOLOGY OF ISLE ROYALE. 11 



ridge, this habitat may be rather extensive and ill defined, as at the ridge 

 south of the light-house (I, 2), but when it borders a depression, as at 

 the head of the rockbound coves, or where a beach is well developed, 

 this transitional zone is more clearly defined and limited. When this 

 beach is wide and grades into the rock openings, as in F'if/arrs 6 and 7. 

 the crustaceous and foliaceous lichens grow upon the rocks; but if 

 soil accumulates, as is shown in Fig. 6, the Cladonia — Bearberry society 

 becomes established, and includes some annuals, svich as Solidago. A 

 limited variety of insects, especially ants, characterize such conditions. 

 When adjacent to the forests, in depressions, this beach is generally bor- 

 dered by alders, some aspens and young trees. 



The fauna consists largely of insects, such as butterflies, certain dra- 

 gonfiies and Hymenoptera, which freqvient the open places on wing. 



Station I, Substation 2. Natural Rode Clearings. This Station con- 

 sists of two small rock openings, one just north of the light-house, and 

 the other south of it, on the north side of the entrance to Tonkin Bay. 

 only a short distance from the light-house. They were both park-like ave- 

 nues extending along the ridges, largely bordered hy the Balsam-Spruce 

 forest. 



The north ridge will first be considered. The general character of the 

 opening is well shown in Fig. S. The White Spruce, Balsam, Paper 

 Birch and Arbor Vitae bound the ridge on either side, within which 

 there is a distinct heath zone of Bearberry and patches of Cladonia, while 

 along the central aisle there is a shallow residual and humic soil on the 

 almost bare rock. The south slope is rather gradual, but the north 

 slope and the end of the ridge at the shore form a cliff. 



The fauna of this location was limited. Snails were found among 

 the Cladonia, such as Vertigo, Zonitoides arhorea and Pyramidula 

 crorikheitei anthonyi. This was also a runway for Hares. 



The south opening or clearing is situated on a low sandstone ridge 

 which slopes down to the beach, and is thus in marked contrast to the 

 north clearing, which ended in a cliff. This gradual slope beautifully 

 illustrates the transition from the bare rock beach, through the moss 

 and lichen zone, to the Cladonia, Bearberry and Solidago flora, (Figs. 

 6 and 7), and on to the crest of the ridge, Fig. 9, with its dominance 

 of Cladonia and Bearberry. The severity of the conditions is furthered 

 by the weathering of the sandstone into thin scale like layers, about % 

 of an inch thick, which become loosened and slide down the slope. Thvis 

 a vegetation may become fixed to the rock surface,, but not permanently 

 to the slope. These scale like fragments are shown in Fig. 6. That a 

 greater amount of vegetation would grow here, if the soil were allowed . 

 to accumulate, is shown in Fig. 6, where such conditions have been 

 produced by the presence of a larger boulder. The Cladonda-BeaTbevry 

 avenue extends along the crest of the ridge. Fig. 9. This is bounded 

 by large Jack Pines near the beach, and farther from the shore by the 

 Balsam-Birch forest. 



The zonal distribution on the ridges is quite marked ; the central strip 

 is composed of Cladonia, Bearberry, Solidago, and Linne-a iorealis; 

 while this is bordered by a shrub zone composed of Juniperus nana, 

 alder, Arbor Vitae and young Balsams, and a bordering tree zone is com- 

 posed primarily of Balsam. When once the shade of the forest, es- 



