730 TEXTBOOK OF ZOOLOGY 



are added. Soil and sand continue to be deposited, the trees in- 

 crease in size, and the ground becomes more thickly covered by 

 vegetation. Finally, after a hundred years or so a still higher level 

 is attained and the willows and cottonwoods give way to oaks and 

 elms. The accompanying table indicates some of the insects to be 

 found in each stage of this developmental series. After the name 

 of each insect is given, in parentheses, its principal food. 



Animal Populations 



A great deal can be learned about the relations of organisms by 

 quantitative methods making possible an estimate of the numbers 

 of various species present in a given community at any one time. 

 Comparisons may then be made between the populations of com- 

 munities differing in some observable respect, or between the popu- 

 lation of the same community at different times of the year. It is 

 comparatively easy, although somewhat tedious, to determine the 

 relative numbers of various plants in a given region by blocking 

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

 will 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 

 relative 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 

 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 accompanying table. The figures indicate their relative 



