76 MICHIGAN SURVEY, 1908. 



clearings; sometimes slow, as upon the jack -pine ridges. Just as all 

 temporary plant associations are occupied by definite faunas of a com- 

 position largely dependent on the plant covering, so the climax associa- 

 tion of plants is also accompanied by a definite fauna, which must like- 

 wise be regarded as the climax animal association. 



The succession of the dense forest growth brings into play a number 

 of new ecological factors, which are not only of the highest importance 

 in controlling the animal life, but are also retroactive upon the plant 

 covering itself. In all the associations heretofore described physio- 

 graphic changes have l>een: proceeding with comparative rapidity. 

 They may be due to wave action, drainage, elevation, rock disintegra- 

 tion or soil formation as direct agents, or to changes in the soil com- 

 position, soil moisture, light, or heat through the indirect agency of 

 the vegetation. Corresponding to the wide diversity in physical con- 

 ditions there has been developed a fauna of many species adapted to 

 many different, modes of living. Through the agency of the forest 

 cover the light is reduced to a constant minimum, the temperature is 

 made more uniform, the soil becomes of uniform character throughout, 

 and the moisture is kept nearly constant. Indirectly the diffuse light 

 is normally too weak to allow the growth of a ground cover of herba- 

 ceous plants so that the variety of food supply is reduced. In short, 

 the change is from heterogeneity of ecological conditions to homo- 

 geneity, and the number of species varies directly with the heterogeneity 

 of the habitat. This is true not only for Isle Royale. but for any 

 biotic association. Here, however, the homogeneity is especially 

 marked, because two species alone, the balsam fir and the white spruce, 

 are dominant throughout. 



The soil in the balsam-spruce forest is a damp closely packed leaf 

 mold, sometimes deep, sometimes shallow over the rocks, and com- 

 posed of decaying balsam and spruce needles, mixed with decaying 

 sticks and interwoven with fungus myceliura,. 'S'S'hen the forest is not 

 so dense aspen and birch trees may be growing, and their leaves also 

 mix in the mould. In such places there may be a very thin 

 ground cover of Aster viacrophyllus, Liimaea amcricana and Pyrola 

 chlorantha; otherwise the soil is without cover. Above this rises 

 the dense growth of trees, the younger ones and the lower branches 

 stunted or dead from lack of sufficient light. 



The insect fauna is composed almost entirely of subterranean species, 

 all few in number, and mostly colorless. A few species of spiders are 

 seen, and a minute CoUembolan, Tomocerns nig^r Bourl. (No. 140). 

 Two species of myriapods (No. 140) are rather abundant in the mould, 

 one other larger species was seen once (No. 140), and an Enchi/traid 

 earthworm (No. 140). A few species of small shells are rarely found 

 at depths of about 5 cm. or sometimes on the surface. They are Pyra- 

 midula stritttvlki (Anth.), Zonitoidcs arboreus (Say), Yitrea l)inncyana 

 (Nise), and Ancijclus sp. (140). 



There are no ants except a large black species which forages singly 

 over the surface, Cainponotus hc.rciilcanus L. (No. 140). A single 

 black Carabid, Valthvs gregarimt^ Say (No. 140, 236), also runs over 

 the surface and hides under old balsam cones. 



A few species of flying insects occur, especially mosquitoes, and when 



