THE ANIMAL AND ITS ENVIRONMENT 733 



Relative Numbers of Ten Most Abundant Insects From Overgrazed and 

 Normal Grassland. (Wichita Mountains Wildlife Refuge.) 



abundance in ten collections of fifty sweeps of the insect net, each. 

 These collections were taken over a period of a month and indicate 

 a very real difference in the insect populations of the two areas. 



Seasonal Changes 



It is a matter of common observation that the animals observed 

 in any one place vary greatly from season to season during the 

 year. This is not only true of those animals which, like many birds, 

 migrate southward on the approach of winter and return to north- 

 ern climates for nesting. Some animals hibernate or aestivate, and 

 others spend a part of the year in an inactive stage, such as the egg, 

 or the pupa. The life cycle of an organism must be adjusted to the 

 annual climatic cycle of the climate in which it lives. Some idea 

 of the variations in the number of insects during the year may be 

 gained from a study of the accompanying charts (Figs. 381 and 382) 

 giving the average catch with one hundred sweeps of an insect net 

 at different seasons. 



A study of the abundance of Protozoa in an artificial lake shows 

 a similar difference in the time of abundance of the various species. 



Summary 



This chapter has considered very briefly the distribution of the 

 biotic communities of North America in relation to climate. The 

 phenomena of succession and seasonal fluctuation of populations 

 have been discussed, with examples. Attention has been directed 

 toward the community rather than toAvard the individual organism 



