12 NORTH AMERICAN FAUNA 61, FISH AND WILDLIFE SERVICE 



GEOGRAPHIC AND GEOLOGIC INFLUENCES 



The Aleutian district lies within the Boreal region, and it may 

 be identified as the southern fringe of this great circumpolar 

 area throughout which life has so much in common. It is 

 significant that the Alaska Peninsula and Aleutian Islands lie, 

 in a sense, almost isolated from the mainland of Alaska and 

 extend westward a tremendous distance toward Kamchatka, 

 thus, in some respects, serving as a "bridge" between Asia and 

 North America. It is also significant that they are near the 

 other intercontinental bridge at Bering Strait, which is recognized 

 as having an important influence on our biota. 



The Aleutian Islands and Alaska Peninsula are of comparatively 

 recent geologic origin. Volcanic activity is still prevalent, and 

 changes in the surface of the land are still taking place. For 

 this reason, and for reasons mentioned later, the area has a 

 new and changing environment that has not yet been fully 

 occupied by flora or fauna to the extent of its potential capacity. 

 Thus, the area presents an opportunity to see immigration still 

 taking place and to note the changes imposed on the newcomers 

 by an unusual environment. 



As the following sections show, the Aleutian biota is drawing 

 its members from several directions. As would be expected, some 

 have come directly from Asia ; some have come from the north 

 on the Alaskan side; others have come from the southeast along 

 the Pacific coast; and still others are part of the fauna that 

 appears to have developed in the Bering Sea region — an area 

 roughly bounded by Siberia, mainland Alaska, and the Aleutian 

 Islands. Many others are drawn from biotic populations that 

 at present are so widely distributed in the Palaearctic region 

 that it is impossible to judge the direction from which they 

 entered the Aleutian area. Following, are some of these groups 

 that have contributed to the Aleutian biota : 



ASIATIC IMMIGRANTS 



BIRDS 



SCIENTIFIC NAME COMMON NAME 



Branta nigricans Black brant 



Mareca penelope European widgeon 



Anas crecca ssp Common teal 



Haliaetus albiciUa White-tailed sea eagle 



Haliaetus pelagicus Steller's sea eagle 



Falco rusticolus uralensis Asiatic gyrfalcon 



Charadrius dubius curonicus Little ringed plover 



