188 AGGREGATION, COMPETITION AND CYCLES 



(cf. Napier, 1914; Clemens and Clemens, 1926; Cobb, 1922; F. A. 

 Davidson, 1934; Gilbert, 1914, a, b; Gilbert and Rich, 1927). 



Cannibalism. The black crappie population of the bottomland 

 lakes along the Illinois River varies in density through a five-year 

 cycle. During this time the average weight of all black crappies 

 taken with 1-inch tackle ranged from a minimum of 0.056 pound each, 

 at the beginning of the cycle, up to a maximum of 0.458 pound each, 

 at the end of the cycle. During the increase in size of the fish, the 

 population density decreased from 852 black crappies per acre down 

 to 118 per acre. The black crappie is the largest and most numerous 

 piscivorous fish in these waters. The adults apparently devour nearly 

 all newly hatched young crappies before they are one year of age, 

 until the number of these adults is so reduced that they are unable 

 to devour their progency, when the cycle again repeats (Illinois Natu- 

 ral History Survey, unpublished results, personal communication from 

 Dr. D. H. Thompson). 



Irregular Fluctuations. Irregular fluctuations must be separated 

 from the larger cyclic phenomena. Fluctuations in an abundance of 

 organisms are continuously going on. One may best conceive of the 

 great mass of life in any area as comparable to a lake in which there 

 are fluctuations in the general level or quantity of water and, in addi- 

 tion, a considerable rising and falling of the surface in the form of 

 waves, ripples, etc. The truly cyclic phenomena are comparable to 

 changes in the volume of water in the lake, while the waves and 

 ripples represent more or less erratic changes of abundance in a par- 

 ticular species which must be prevented from obscuring the investi- 

 gator's view of the truly cyclic phenomena. This calls for compari- 

 sons over a considerable range of territory so that differences due to 

 local phenomena may be eliminated. These erratic changes in abun- 

 dance are usually determined in some brief but very sensitive period 

 in the life cycle of species with short developmental periods and life 

 spans. 



An example of this erratic change and difference in abundance is 

 found in the bottom fauna of Danish waters, where a particular species 

 is lacking in certain spots, although it is present throughout the gen- 

 eral region. This was described by Blegvad as being due to local 

 unfavorable physical conditions in connection with the transition from 

 free swimming to the bottom stage; the change at any one point in 

 such a set of faciations is entirely comparable to the larger cycles. 

 Several maps (Fig. 45) show erratic shifting of the areas of great 

 abundance of chinchbugs over the state of Illinois in the years 1922 

 to 1925. In the study of cycles, such local phenomena make neces- 



