462 THE DISTRIBUTION OF ANIMALS 



the other hand, the tropical heat and related factors form an impassable 

 barrier for arctic and cold-temperate animals. The arctic fox, the polar 

 bear, the snowshoe rabbit, and the reindeer are highly adapted for snow 

 and ice and could not long survive in tropical jungles. The amount of 

 rainfall is also important. Deserts, such as the Sahara which extends 

 nearly all the way across northern Africa, form a barrier for most ani- 

 mals which cannot be crossed. 



Ecological Barriers. An animal is limited in its range by the kind 

 of food that it eats. The American buffalo, or bison, was a plains ani- 

 mal and rarely entered the eastern forests. Squirrels and woodpeckers, 

 on the other hand, are confined to forested areas by the nature of their 

 feeding and nesting habits. Any parasitic animal is limited to the same 

 regions which are inhabited by its host, or by agents upon which it is 

 dependent for its spread. The parasite of malaria has recently had its 

 range in the United States greatly reduced by the control of the Anopheles 

 mosquito. It is often difficult to determine the ecological factors which 

 control range — a species may be common in one locality and rare or 

 absent in another that appears to be equally desirable. There are some 

 lungless salamanders which are confined to a very small range in the 

 cold, damp slopes of the Great Smoky Mountains where it rains almost 

 daily. Here their need for a very moist habitat is clearly the limiting 

 factor that prevents their spread into the surrounding lowlands, but 

 this hardly explains why they are not found on other slopes in the same 

 mountains where there is an equally heavy rainfall. The parula warbler 

 in Canada and northeastern United States nests almost exclusively in 

 the Usnea lichen. In the southeastern United States it nests in the 

 Spanish moss which is a seed plant in the pineapple family, but it re- 

 sembles the Usnea lichen in appearance. The cottonmouth water moc- 

 casin is confined to the rivers and swamps of southeastern United States 

 and occurs as far north as southern Illinois and western Kentucky. It 

 is difficult to assign any reason why this snake does not occur in the 

 numerous rivers and swamps farther north in Illinois and east in Ken- 

 tucky. There seem to be no barriers that could keep it from these 

 regions. A more thorough study will probably reveal some ecological 

 barrier. 



Aids to Dispersal 



Now that we have surveyed some of the barriers to dispersal, let us 

 now survey some of the aids to dispersal. Those animals which are 

 capable of rapid locomotion on land, in water, or in the air will spread 

 rapidly, but those that are more sluggish must depend upon other means 

 of dispersal, as a rule. Mud taken from the feet of migrating birds has 



