ECOLOGY OF ISLE ROYALE. 3'Jl 



a forest. Yet the fact that there must be similar animal surcessions 

 has attracted but little attention and, so far as linown to the writer, 

 no definite attempt has even been made to determine mammal succes- 

 sions, much less to recognize the need of formulating its laws. Suc- 

 cessions of vegetation initiated by man were recognized long before 

 those in nature, but it seems that the students of animals have not onlj' 

 neglected "natural" successions but also even those influenced by man. 

 A priori no one can doubt but that there must be mammal successions 

 correlated with environmental changes upon which mammals are de- 

 pendent. To resolve such a problem as this demands more than a 

 recognition of the species involved and needs a knowledge of their 

 life history, habits and their environmental relations. On account of 

 the preliminary character of this work only a few suggestions will be 

 attempted at this place. 



As the level of the Glacial and post-Glacial antecedents of Lake Su- 

 perior were lowered, Isle Royale began a new biotic cycle; from a reef 

 in the lake it became transformed into an island. But the history of 

 the island even prior to its emergence must be considered because the 

 pre-Glacial topography and the overriding ice both left a record of their 

 influence upon its surface in the form of parallel ridges and depressions. 

 Thus the Isle inherited from the past certain characters which are 

 conspicuous features of the animal environment even today. These 

 irregularities of the surface produced rocky flats and ridges, or rock 

 bound basins, which in all probability were thoroughly wave washed 

 and cleared of soil as the waves fell from them. The inheritance of 

 these depressions, rock surfaces and ridges, allows us to consider two 

 sets of original conditions. That of the depressions with their lakes, 

 nonds and swamps, and that of the ridges or rock surfaces with open- 

 ings or "rock clearings." The first will be called the Lake-Pond- 

 Swamp series. 



1. Lake-Pond-Sivaiiip Scries. — Prom the large lakes upon the island 

 nil gradations of conditions are found leading to the forested swamps. 

 The shore line of the island itself should also be mentioned in th|is 

 connection as its conditions and mammal fauna in protected parts must 

 be much like that of the larger lakes upon the island. To these mar- 

 ginal conditions must be related the Muskrat, Mink, and perhaps the 

 Otter and the Beaver. All of these animals will traverse the open water 

 but are more truly amphibious or frequenters of the margin. The dryer 

 shrub or Cassandra zone is likely to be invaded by Hares, as is clearly 

 shown by their numerous run-ways, while Avandering Lynx, Mink and 

 Weasels may also be expected here in search of their food, while the 

 open area over the water and marsh are likely to furnish a flight area 

 for bats. It should not be inferred, however, that these mammals do 

 not occur in other conditions, but rather that they are representative 

 or dominant forms in such an environment. 



The dynamical relations of such conditions should be considered 

 for their bearing upon the laws of environmental changes. With the 

 falling of the Lake level the beach zone moves downward and is invaded 

 by ii land flora and fauna. This same change of level, supplemented by 

 inwash, vegetable and animal debris, and possibly the down-cutting 

 of outlets tends tO' drain basins and allow the encroachment of the open 



