ECOLOGY OF ISLE ROYALE. 



69 



or two species of flies (No. 31) (Cynomyia caclavcrlna Desv.) were also 

 ■common. 



Crawling over the sand wei'e ants (No. 30), spiders with eggs cases, 

 Pardosa groenlandica Thor. (No. 30), and beetles, Bemhiditmi cannula 

 •Chaud. (No. 30). The latter were very numerous, and included two 

 species. They ran rapidly and irregularly over the sand, and especially 

 the fine gravel just back of the wet margin. When alarmed they try to 

 hide under small pebbles, or sometimes fly a short distance. 



A dead shell of Limnaea stagnalis (No. 32) was found on the beach, 

 and a dead Polygyra alholabris in the small stream (32). 



VI. The Rode Beach. 



Where the slope of the shore is steep or the action of the waves severe, 

 gravel or sand cannot accumulate, and the bare rock is left exposed. 

 The ecological conditions affecting animal life here are so different from 

 those of the gravel beaches that they require especial mention. 



Rising directly from the water they are naturally exposed to the full 

 force of the waves, .{Fig. 3), which dash upon them to a considerable 

 beight, washing away all loose particles and effectually preventing even 

 the most meager formation of soil. Beyond the reach of the waves, rains 

 and drainage water act with greater or less effect in the same way. The 

 vegetation is therefore limited to various species of crustaceous or 

 foliaceous lichens, which are true lithophytes. Even they are absent 

 from the lower portions where the wave action is more continued^ and 

 •especially where the ice may scrape them off. Higher up the procumbent 

 juniper and Cladonia appear and the whole eventually merges into the 

 Gladoma clearing to be described next. Some idea of the zonal succes- 

 sion of the different jjlants may be gained from the following table, 

 showing the heights of the different zones on a rock beach near the 

 Hock Harbor light-house, Figs. 6 and 7. 



The first two zones, to the height of twelve feet above the lake, are 

 included here in the rock beach. Naturally these levels may vary with 

 different localities, being lower in more sheltered places. 



Over the lower portion of the beach the fauna is practically without 

 shelter or protection, and in the zone of foliaceous lichens shelter is 

 afforded only to very minute species. There are sometimes small fis- 

 sures in the rock, but only two species were observed to enter them. 

 During all or part of the day the beaches are exposed to the direct rays 

 of the sun, and the rock consequently reaches a temperature ■ far above 

 that ever reached by the air. 



