ECOLOGY OF ISLE ROYALE. 65 



different fauna and their genetic development is along a different line, 

 culminating however in the climax type or balsam-spruce forest. The 

 only intervening stage is the tamarack swamp. 



IV. The Tamarack and Arhor Vitae Swamps. 



Nearly every inland lake in the Isle Royale region is wholly or partly 

 surrounded by tamarack swamps, (Figs. 14, 19, 22, 41, 41, 48). It is 

 not necessary to discuss the general structure of the vegetation, since 

 that is described elsewhere in this report, but it may be indi- 

 cated here that the ground cover is a spongy mass of sphagnum covered 

 with a dense growth of ericaceous shrubs, such as Cassandra and Ledum, 

 and that the trees are almost entirely tamarack and black spruce. The 

 forest cover is open enough to allow ample illumination. Tamarack 

 swamps may be found of all ages, from those developing at the edge of 

 a lake to those which have completely covered the lake and are now 

 dying as an association. Their surface is generally level, the older parts 

 being successively somewhat higher as they are built up by the accumu- 

 lations of peat. 



When the level is nearly that of the lake the beds of sphagnum are 

 interspersed by little streams or pools of water, some of them being 

 merely extensions of the lake itself, or some of them serving as inlet 

 or outlet. The smaller ones have no bottom except the sphagnum itself, 

 while the larger have a loose incoherent bottom of slime. In the larger 

 of these streams are found small bivalve shells, Pisidium sp., embedded 

 in the slime at the bottom (No. 230; V-5), and other material; and the 

 beetles Haliplus rtificolUs DeG., Hydroporus tristis Payk, and Agahus 

 congener Payk. (No. 230, V-5). In the smaller ones, which are fre- 

 quently only a decimeter or two wide and half as deep, there is no 

 difference in the vegetation except for a little Utricularia in the bottom. 

 Animal life is there very scarce (No. 237, V-5), but included Pisidium 

 sp. 



As the swamps become older the water is limited to small shallow 

 pools, seldom more than one decimeter deep or three or four decimeters 

 wide. Their bottoms are covered with dead leaves and sphagnum, and 

 they are usually densely shaded by the forest growth above. In them are 

 found small bivalves, Pisidium affine Sterki (11 A, 79A), P. suhrotundumi 

 Sterki (116, 181, 182, 237), P. suhrotundnm Prime (116, 237), and water 

 beetles, Haliplus ruficollis Deg. (No. 116, 1-4) and Scutopterus hornii 

 Cr. (No. 181, 144). The latter is restricted, so far as observed, to this 

 single habitat in the pools in tamarack and arbor vitae swamps. Dragon- 

 flies are the principal aerial insects, but are not abundant. A fly (No. 

 240, V-5) was taken on the flowers of Solidaga neglecta. 



In still drier swamps, where there is no longer any standing water, 

 (Fig. 14). ants are a characteristic feature of the fauna. They 

 build huge dome-shaped nests, 4 to 7 dm. high, composed within of 

 sphagnum and other vegetable debris, and smoothly covered on the 

 outside with leaves of Cassandra, doubtless to prevent drying. Formica 

 adamsii Wheeler (No. 115, 1-6) seems to be the only species concerned, 

 and a nest from which the collection was made was photographed. No. 

 114, taken at the same time from a similar nest, has been identified 

 9 



