142 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



the ground beetle Carabus silvestris, in the snowy zone of the Alps; 

 the wolf, in the tundra; and all migratory birds. Tychocoen animals 

 may be of greater importance in the animal community than are the 

 autochthonous eucoen forms. On the basis of relative importance in 

 the community, animals are classified as dominants, influents, and sub- 

 influents. Dominant animals are those of outstanding abundance or 

 conspicuous influence which are present at least throughout the entire 

 active or open season. Influents are common animals which are of less 

 importance, and the subinfluents are regular members of the commu- 

 nity but with relatively slight influence. 6 



In addition to these regular elements of a biocoenosis, a few forms 

 may be present which are more or less accidental. These are guests, 

 tramps, heterotopous animals which live for a time in a biotope into 

 which they have wandered, but inevitably perish if they do not find 

 their way in time to more favorable environment. A fish in a spray 

 pool on a rocky coast, a bird of the high seas blown inland, a butterfly 

 on a glacier, a dragonfly in the desert, are examples of such accidental 

 distribution. 



The biocoenosis of a given biotope is accordingly the more uniform, 

 the more severe the selection by the physical habitat conditions, and 

 in general it will be the more distinct the poorer it is in species. The 

 most distinct biocoenoses are those of deserts and ice waters, caves, 

 moss, and temporary pools. This is not a uniform rule, however, for 

 the sparse fauna of hot springs consists in the main of ubiquitous 

 forms which are sufficiently resistant to withstand the unfavorable 

 conditions. 



Both the biotopes and their accompanying biocoenoses are con- 

 stantly changing. Changes of the biotopes, such as the filling of lakes 

 and the increased aridity of the Arabian peninsula, have already been 

 discussed. Such changes are accompanied by obvious shifts in the 

 biota. Even when the climate and other physical conditions remain 

 constant, the biocoenosis may still undergo evolution from pioneer 

 to mature, so-called climatic climax communities. Such succession is 

 effectively illustrated in dunes where, with the passage of time, largely 

 as a result of the activity of plants, the communities of the bare dunes 

 evolve through various stages until the climax community of the 

 region is reached. The biotic communities in the dunes along Lake 

 Michigan evolve towards the beech and maple forest community as a 

 climax. This climax is insensitive to its own effects and, other things 

 being equal, will remain as long as the present climate persists. This 

 is the climax community for much of eastern United States; in it the 



