ECOLOGY OF ISLE ROYALE. 57 



THE ECOLOGICAL RELATIONS OP THE INVERTEBRATE FAUNA 

 OP ISLE ROYALE, MICHIGAN. 



BY DR. HENEY ALLAN GLEASON. 



I. Introduction. 



The most recently emerged portions of Isle Royale are the rock and 

 gravel beaches which together constitute virtually the entire shore of 

 the island. Animal life is found upon them almost to the edge of the 

 water, and well within the limits of wave action. The physiographic 

 succession in the island is such that the areas originally occupied by 

 beach pass through a series of changes in the physical factors, a series 

 which is accompanied, sometimes hastened, sometimes retarded, by cor- 

 responding vegetational successions, and which culminates in the final 

 pr climax plant association of balsam and sjiruce forest. The detail of 

 this physiographic and vegetational succession is by no means uniform ; 

 it may proceed along either of two well-marked lines, depending on the 

 immediate physical and biotic conditions, certain intermediate stages 

 may ibe prolonged or omitted entirely, and various other deviations may 

 occur. Nevertheless the final stage is always the same. Accompanying 

 the changes in physiography and vegetation is a similar and dependent 

 change in the fauna, so that there is a corresponding series of animal 

 associations, beginning on the beaches and developing in the same di- 

 rection, with the same deviations or omissions, to the, final or climax 

 association in the balsam-spruce forest. 



The preceding general statement rests on the assumption that the 

 areas now occupied by the climax biotic associations have developed from 

 the, beach associations through a series of stages intermediate in time 

 corresponding to those associations which now stand intermediate in 

 space between the two extremes. Or briefly, as some ecologists have 

 expressed it, the lateral distribution in space recapitulates the vertical 

 distribution in time. Such an assumption is evidently closely akin to 

 the recapitulation theory of the evolutionists, and just as that so-called 

 biogenetic law has been accredited with more than its true value, so 

 has this ecological dictum possibly much less importance than has been 

 usually supposed. The weakness lies in too little consideration of the 

 time element. It is certain that the higher land in Isle Royale has 

 been submerged. This is shown by the old beach marks now many feet 

 above the present level of the lake. Consequently by the gradual emer- 

 gence all of the island has passed through a beach stage. But it is un- 

 warranted to conclude from this that the faunal or floral associations 

 of the former beach were similar to those of the present, or that in the 

 intermediate stages the biota resembled that which now occupies the 

 area between the ancient beach and the present shore. While it is like- 

 wise certain that with a continued subsidence of the lake level the pre- 

 sent beaches will eventually be left far above the water, it must not 

 therefore be assumed that their biota will show the same successions 

 or reach the same climax as those of the past. Changes in the tempera- 

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