86 



ECOLOGY 



Part II 



Fig. 5.19. Social hibernation of ladybird beetles. With the first frosts the beetles 

 fly to the ground and then to trees searching for holes in which they gather by 

 hundreds. Animals that are solitary in summer may be social in winter. (Photo- 

 graph by Carl Welty.) 



though they are solitary at other seasons (Fig. 5.19). Cold as well as sex 

 encourages sociability. 



Light. Light is necessary for vision but there are other ways in which it con- 

 cerns animals. Like plants they are deeply affected by longer or shorter days. 

 This shows in their breeding seasons, in the migrations and seasonal changes 

 of color in birds, and in the color changes of snowshoe rabbits, and other 

 northern animals. In general, animals are responsive to light whether they have 

 light-perceptive organs or not, but lenses are present even in certain proto- 

 zoans. The majority of higher animals probably find their way chiefly by 

 vision, but by no means entirely. 



The amount of Hght that enters water depends upon the direction of the 

 rays, which differs with the time of day and year, the amount and clearness of 

 the water through which the rays pass, and the intensity of the light. In rela- 

 tively clear water, one-third of the light is generally lost in about three feet 

 and three-quarters of it in 16 feet. At depths of 2,000 feet or more the ocean 

 is completely dark except for the luminescent animals, mainly fishes. 



Biological Environment 



The neighboring plants and animals compose an organism's biological envi- 

 ronment. Whether the organism is a crocus in a mountain meadow, a parasite 



