650 Ecology 



Light. — Light has many complicated effects on animal activity. 

 One of the most striking of these is the migratory cycle of birds. Their 

 northward movement is apparently dependent upon the increasing day 

 length ; experimentally it has been shown that the testes increase in size 

 when birds are exposed to artificially increased daylight. The color 

 change of the fur of some northern animals, such as the varying hare, 

 apparently is controlled by changes in the length of day. In addition to 

 these more obvious effects, light also influences the movements of animals 

 w^ithin their habitat. Thus some animals remain under cover during the 

 day and emerge at night to search for food. Many small animals de- 

 liberately shun bright light to spend their lives in the dark recesses of leaf 

 mold and rotten logs. These include the salamanders, many insects, and 

 numerous other arthropods. 



A bright light often attracts insects from long distances. This has 

 been used as a method of both control and capture of many types of night- 

 flying forms. 



Water. — No animal can live without water; thus its ability to live 

 in any area is dependent upon the available water supply. The supply 

 of water varies in different habitats from that of the aquatic environment 

 to the dry desert. Animals are able to live in these drier regions by 

 developing special adaptations such as a heavy skin, utilization of water 

 produced by metabolism, the ability to secrete dry urine, and special 

 habits such as burrowing and being nocturnal. Aestivation such as that 

 of the lungfish is a response to the periodic drying of its swamp habitat. 



The life history of many animals is strongly influenced by available 

 moisture. The small crustaceans develop only during the spring months 

 when pools are available ; some toads have a developmental rate depend- 

 ent upon the concentration of salts in ponds; and birds of tropical rain 

 forests lay their eggs at the start of the rainy season when food is more 

 abundant. 



Presence of Various Salts. — In fresh waters dissolved salts are 

 one of the more important limiting factors. Salts of calcium, magnesium, 

 and phosphorus are probably the most important. Animals such as mol- 

 luscs and crustaceans are unable to live in calcium-poor water while 

 fresh-water sponges are more abundant in soft waters with a rich or- 

 ganic supply. 



Even among terrestrial animals the lack of certain elements in the 

 soil may be reflected in decreased vigor. 



