ECOLOGY OF NORTHERN MICHIGAN. 31 



predominating. These forms cover more or less completely the surface 

 of the talus fragments. Farther clown Stereocaulon coralloides and Lecanora 

 conspersa are added to this society which is dominated on the lower part 

 of the slopes by the Reindeer lichens, Cladonia rangiferina and alpestris, 

 that often occur superimposed on the remains of the other lichens. These 

 are replaced toward the bottom of the slope by the Bearberry, New Jersey 

 Tea, Polypodiimi vulgare, Raspberry, Wild Red Cherry, Mountain Maple, 

 Paper Birch, Quaking and Large-toothed Aspens, Beaked Hazelnut, Black- 

 berry, Brake, and Bush Honeysuckle, that in turn give way to the typical 

 mesophytic forest forms of the river bottom. 



The environmental conditions as regards the fauna are more unfavorable 

 than on the mountain top. The only forms that are found here are a few 

 ants in the narrow zone of vegetation at the top, and occasional grass- 

 hoppers and dragon-flies on the bare slope below. Near the bottom of the 

 slope the Lake Superior Chipmunk was often seen running about over the 

 rocks and among the bushes that fringe the forest. Forms from the meso- 

 phytic forest of the river valley such as the Garter Snake and Northeastern 

 Chipmunk are also occasionally seen here. 



When the biota of the mountain top and north slope are listed by habitats, 

 the genetic explanation suggested for the forest of the north slope is enforced. 

 It is evident that certain groups of forms are, in a general way, dependent 

 on certain environmental conditions. The action of the forces which bring 

 about these conditions tend to modify those that exist at any one time, 

 so that the biota must adjust itself to the new conditions or be exterminated. 

 This is especially noticeable on an elevated area. It was shown in the dis- 

 cussion of the topography that the effect of physiographic processes on the 

 ridge in question was to reduce it to a base-leveled plain. The environ- 

 mental conditions are thus being changed and modified in the direction of 

 the conditions that prevail on the lowland at the foot of the mountain. 

 These conditions are brought about first on the lower parts of the ridge, so 

 that the habitats of the lowland biota are extended at the expense of cliff 

 habitats, and correlated with the changing conditions a succession of societies 

 occurs. This may be easily seen fi'om the top of the ridge. Fig. XL The 

 first plants to get a foothold on the bare rocks are those of the lichen society 

 such as Lecidea lactea and Parmelia conspersa, that form large patches over the 

 exposed rock surfaces. The wash and decay from these mats is at first 

 mostly washed away down the slopes. As soon, however, as the processes of 

 weathering have opened the edges of the joint planes, the soil accumulates 

 in them and with it a small amount of organic material from the lichen 

 mats. The conditions thus become favorable for a crevice vegetation 

 consisting of certain mosses, the Harebell, Cinquefoils, Goldenrods and 

 grasses. As the soil, held in the crevices by the plant roots, increases in 

 amount, the conditions become still more favorable, and the crevices are 

 invaded by the heaths from the extensive mat that surrounds the mountain 

 top on three sides, in front of the mesophytic forest. The dense mats formed 

 by the heaths do much to make the edaphic conditions more favorable by 

 holding the soil as it is formed, checking that which is washed from higher 

 areas, and by accumulating the humus formed by the decay of the vegetation. 



The first society to gain a foothold on the lichen mat is represented by the 

 White and Red Pines, and the next by the forms of the aspen zone. In many 

 instances, the Quaking Aspen, which is one of the hardiest plants of this zone, 

 follows the heaths along the crevices before the pines can obtain a foothold, 

 but in any case the pine stage is but poorly represented, probably o^Adng 



