64 THE ECOLOGY OF ANIMALS 



in numbers from season to season or year to year. 

 This method is especially useful where a cyclical 

 change of some regularity is being studied. The 

 various techniques for following annual changes 

 differ for each group of animals. For larger forms 

 such as mammals and birds and fish, general sub- 

 jective estimates of abundance are often a useful 

 guide. The changes in numbers of the snowshoe 

 rabbit (varying hare) in Canada during its ten-year 

 cycle have been mapped by this means. Observers 

 record their opinion as to whether rabbits are more 

 or less abundant than in the previous year. Such 

 opinions are of value when given by people who spend 

 much of their life in contact with the animals con- 

 cerned or make their living by hunting and trapping. 

 A business man would remember whether prices 

 were higher one year than another without neces- 

 sarily remembering the exact figures. A trapper gets 

 similar subjective estimates from his trapping expe- 

 rience. If a sufficiently large number of observers 

 are taken, the method gives good results (Elton, 

 1933). The same methods can be adapted for 

 observing game-birds (Gross, 1929-32), or fish, or 

 crayfish (Duffield, 1933), or insects (Barnes, 1932). 

 By this means the general nature of the periodicity 

 of a species' fluctuations can be ascertained. The 

 records provide a rough framework, a sort of recon- 

 naissance of the problem, which is most valuable in 

 directing more intensive research. And for a long 

 time ecologists will have to employ these rough 

 methods if only because observers of trained scien- 

 tific experience are not numerous enough to obtain 

 better results. Such observations do not always 

 involve direct observation of animals, but may rely on 

 traces, snow-trails, or other indications of abundance. 

 Rather more accurate measures of relative changes 

 in numbers can be got by standard sampling of the 

 population, where the sample taken is about the 

 same percentage of the population each year, but 



