366 



POPULATIONS 



stinct' on the part of the animals would 

 lead to contagious distributions of the type 

 here observed." 



This aspect of dispersion has been 

 briefly reviewed at this place primarily to 

 indicate something about the quantitative 

 pattern of distribution of terrestrial inverte- 

 brates within their population configura- 

 tion. Obviously, the discussion could be 

 greatly extended to include more examples 



OBSERVED DISTRIBUTION 



\ POISSON DISTRIBUTION 



-I I I I __l L. 



12 3 4 5 



NUMBER OF SCYTONOTUS PER STATION 



Fig. 130. Nonrandom or "contagious" distri- 

 bution of the diplopod, Scytonotus granulatus. 

 (After Cole.) 



such as those just treated: examples of 

 forms displaying difiFerent, and perhaps 

 more intricate, patterns of dispersion, and 

 examples of ecologically difiFerent species 

 such as aquatic, aerial, infrasocial, and 

 social forms. Limitations of space prevent 

 further discussion of this interesting and 

 highly important topic. The reader should 

 be aware of the considerable literature 

 that deals with these problems from several 

 points of view, but always stresses the 

 quantitative or census approach.* 



* Some data concerned with marine popula- 

 tions are to be found in Sverdrup, Johnson, and 

 Fleming (1942) and in Redfield (1939). From 

 the limnological point of view something of 

 general character appears in Welch ( 1935 ) and 

 in papers of Edmondson (1944, 1945) on 

 rotifers. Particularly stimulating publications 

 about insect populations that contain both data 

 and imaginative, statistical analyses are the fol- 

 lowing: for Lepidoptera, Williams, Cockbill, 

 Gibbs, and Downes (1942), Beall (1941), and 

 Dowdeswell, Fisher, and Ford (1940); for 

 Coleoptera, Beall (1941a), and Bliss (1941); 

 for Diptera, Jackson (1933, 1936, 1939, tsetse 

 flies); Patterson (1943), and Dobzhansky and 



THE ANALYSIS OF POPULATION CYCLES 



The population ecologist is primarily in- 

 terested in these aspects of population 

 cycles: 



1. Statistical description of the cycles of 

 a certain species. 



2. Statistical description of those cycles 

 exhibited by a predator population that ex- 

 ploits a particular, and cyclic, prey popu- 

 lation. 



3. The identification of causes underly- 

 ing the cycles. Do the causes lie outside 

 the population system, as, for example, 

 weather and sunspot activity, or do they 

 originate within it, or both? 



4. The relation of cycles to the partic- 

 ular community in which the cyclic spe- 

 cies live. 



The study of population cycles, of 

 course, is merely a special instance of 

 population fluctuation (p. 318) in which 

 the interval between population maxima 

 (or between minima) is relatively fixed 

 over a considerable period of time. Such 

 cycles have been actively studied, partic- 

 ularly of mammals and, to a lesser extent, 

 of birds and insects, for a number of rea- 

 sons, some of which follow: 



1. The cycles may constitute, or be re- 

 lated to, plagues and thus be imnortant in 

 their own right, in terms of the public 

 health, or in relation to certain human 

 enterprises such as agriculture, aquiculture, 

 and fur trapping. 



2. The cycles may be related to con- 

 servation of natural resources. 



3. The cyclic character of the popula- 

 tion growth form may be, or alleged to 

 be, related to such extra biologic, control- 

 ling periodicities as weather, sunsDOt activ- 

 ity, and the like. Explanations of this sort 

 have seemed peculiarly satisfying intel- 

 lectually to certain workers, and, in our 

 opinion, enthusiasms have too frequently 

 tended to outdistance evidence. 



4. The cycles may be studied because 

 of their impact on other populations or 

 upon the community. 



5. The cycles may be studied because 

 of their spectacular nature as a result of 

 great variation in numbers or their regu- 

 larity, or both. 



Wright ( 1943 ) on natural populations of the 

 Drosophilidae; and Gilmour, Waterhouse, and 

 Mclntyre (1946) on the sheep blowfly. 



