ANIMAL AND ITS ENVIRONMENT 



469 



relative numbers of various plants in a given region by blocking 

 off sample areas and counting the plants. Most animals, however, 

 w^ill not remain stationary while a census is being taken, and vari- 

 ous less exact methods of obtaining population data must be em- 

 ployed. Some of these methods may be suggested. The number of 

 birds may be estimated by the number seen within a given area 

 or during a certain time, or a census of nesting pairs may be taken 

 by counting nests. Larger mammals may be counted by experienced 

 observers, burrows of rodents may be counted (but it is also neces- 

 sary to determine the average number occupying a burrow) or the 

 rel'ative frequency of tracks, fecal pellets, or other evidences of the 

 presence of the animals may give an idea of their numbers. The 

 relative number of rodents is often estimated by the frequency with 



Fig-. 202. — Total number of insects collected in average catch with 100 sweeps of 

 insect net In prairie ravine (Oklahoma). (Data from Carpenter.) The scale for 

 this g-raph is one-fourth of that in the preceding figure. 



which they are caught in traps. The most frequently used method 

 of estimating the number of insects is the use of the insect net. 

 A net is swept through vegetation a definite number of times and 

 all insects caught are counted. While all such methods are neces- 

 sarily inaccurate, a great deal of valuable information may be 

 obtained. 



A list of the ten most abundant groups of insects found in two 

 adjacent areas, one heavily overgrazed and the other lightly grazed, 

 is given in the following table. The figures indicate their relative 

 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. 



