164 ARTHUR G. VESTAL. 



Estimation of Relative Numbers in Different Habitats. 



The Distinction Between Frequency and Abtindance.Many 

 collectors do not distinguish between frequency of occurrence 

 and abundance of individuals. To them a species is common if 

 scattered individuals are seen frequently, or if numerous indi- 

 viduals are seen infrequently. Frequency refers to regularity of 

 occurrence in one or more habitats ; abundance is concerned with 

 numbers of individuals per unit of area. The distinction is 

 indispensable if we are to estimate numbers of individuals in 

 different habitats. Thus many of the conspicuous (Edipodince, 

 a number of which can be seen in flight at one time, but over a 

 considerable extent, as far as one can see, are frequent species, 

 but are not numerous as compared with certain inconspicuous 

 Melanopli, a larger number of which may be found in any area 

 within the habitat as large as a few meters square. These last 

 are abundant and frequent. 



Difficulties in the Way of Numerical Estimation. The writer is 

 not acquainted with any very practicable method of estimating 

 absolute numbers of insects in different habitats of a region. 

 Absolute numbers of plants may be estimated under favorable 

 circumstances, as may also the numbers of the larger or more 

 sedentary animals. Following are some of the difficulties en- 

 countered in estimating absolute numbers of terrestrial animals: 

 In any one species, there are differences in numbers of individuals, 

 in degree and kind of activity, in readiness with which it is per- 

 ceived, in ease with which it is captured, or recognized if it is not 

 taken, in time and in space. These conditions vary with the 

 year, with the season, with weather conditions, and with the time 

 of day; they vary in different habitats, in different strata, with 

 different kinds of soil and against different backgrounds, and in 

 different kinds of vegetation. There are many difficulties in the 

 way of estimating even relative numbers of different species. 

 Less abundant species are likely to be confused with more abun- 

 dant kinds. There are differences in appearance, in conspicuous- 

 ness, in degree of activity, in ordinary behavior and behavior 

 upon being disturbed, with different species, which make some 

 of these much more readily perceived, or recognized, or captured, 

 than others. Differences in time of appearance and activity, and 



