ECOLOGY OF ISLE ROYALE. 129 



ing attention upon the importance of recognizing these dominant en- 

 vironmental influences, we may hope to escape some of the confusion 

 which appals those who are keenly impressed with the chaos and com- 

 plexity of the problem. These dominant factors are usually not single 

 isolated forces, but resultants of several or many influences. Thus, 

 as in the case of the vegetation, it is not one factor, but a complex, 

 which influences different birds in different ways. Nevertheless there 

 is what may be called a mass or dominant effect. 



A major habitat unit may be considered as a combination of condi- 

 tions which are dominant in a certain area. The very dominance means 

 that (I relatively limited number of forces or complexes are operative. 

 With departure from such a center of influence the dominance changes, 

 as other influences are encountered and other dominants are established. 



When we consider that certain ecological groups of birds are world- 

 wide in their environmental relations, it becomes evident that such 

 charactera are of fundamental importance. Thus water birds may 

 occur in any part of the world where water is a dominant environ- 

 mental factor. This is not a, simple ecological group of birds, but one 

 of the greater units of association which may be subdivided into many 

 minor classes; as those which frequent the sea, and others the inland 

 bodies of water. The shore birds form another natural ecological 

 group, and also the inland birds a third. There may thus be considered 

 to be three primary ecological groups of birds which ai'e closely correl- 

 ated with definite and dominant environmental influences : Thus : — 



1. Water birds. 



Those frequenting the sea and the adjacent rooks on which they nest, 

 and inland waters. 



2. Shore and Marsh birds. 



Those frequenting shores of all kinds, seas, lakes, swamps and rivers. 



3. Inland birds. 



Those frequenting deserts, grass lands and forests. 



Of course these ecological classes are not sharply defined, and yet 

 they are so distinct that they can be easily recognized. It should be 

 noted that the above groups are closely correlated with certain domin- 

 ant physical features of the earth — the sea, the shore and the inland 

 environments. 



The relative abundance and dominance of these classes of birds will 

 be determined largely by the dominance of such physical conditions 

 as most distinctly favor a particular ecological group. Thus at sea 

 the water birds are dominant; on shore, the shore birds; and inland, 

 still other kinds. The linear character of the shore habitat and the 

 adjacent breeding grounds gives it a rather unique character, as the 

 two other habitats occupy large expanses. However, the swampy, some- 

 what shore-like conditions of the far north most nearly approach, for 

 the shore birds, the expansive character so usual for water bodies and 

 inland areas. 



In the present discussion the emphasis placed upon the inland vegeta- 

 tion does not mean that the dominance of other influences is not recog- 

 nized, but simply that it makes a convenient and fairly reliable index 

 to many other enrironmental influences, as, for example, the climate 

 and topography. A further important advantage of th'e plant index is 

 17 



