330 



POPULATIONS 



since we shall have more to say about cer- 

 tain of the points in Chapter 22. It seems 

 fair to conclude that many accentuated 

 changes in numbers represent primarily 

 selected extremes of the population curve, 

 although undoubtedly there are some such 

 events that occur so infrequently, or are 

 such a severe departure from the usual, 

 that they must be looked upon as special 

 cases. 



Carpenter (1940) reviews in interesting 

 fashion insect outbreaks in Europe. He de- 

 fines an outbreak as "the time during the 

 fluctuation cycle of an insect (or other ani- 

 mal) when it is abundant or injurious 

 enough (or both) over an appreciable area 

 to warrant a record being made by observ- 

 ers." We interpret this definition in our 

 terms as an excessive, maximal, fluctuation 

 peak. He goes on to study the reported out- 

 breaks of injurious insects for most of 

 Europe, including a considerable part of 

 European Russia. On the basis of this gross, 

 historical synthesis Carpenter concludes that 

 outbreaks tend to occur in the same groups 

 of years, or are closer together than would 

 be expected on the basis of chance alone. 

 Certain of the insects studied or mentioned 

 are somewhat periodic in their outbreaks. 

 For example, the cockchafer (Melolantha), 

 the cicadas, the May beetles, and migratory 

 locusts fall in this category. An interesting, 

 although admittedly sketchy, analysis of 

 the locusts in Eurasia is presented with 

 sporadic data going back to 300 A. D. The 

 conclusion is reached that the locust peaks 

 of abundance were probably attained in the 

 following years (The question marks refer 

 to questionable cases): 595, 695, 885, 

 935(?), 1095(?), 1165(?), 1205(?), 

 1245(?), 1335(?), 1405(?), 1475(?), 

 1545, 1635 (?), 1695, 1715, 1745, 1785 (?), 

 1815, 1855, and 1895. 



A key to Carpenter's thinking on the 

 matter of outbreaks is contained in the fol- 

 lowing quotations: "Since one of the hy- 

 potheses underlying this study is that the 

 biotic community reacts as a whole to its 

 environmental conditions, the analysis of 

 data is made with this in mind. According 

 to this idea we may expect a general re- 

 action of an ecological population on any 

 area to its environmental conditions. If, in- 

 stead, each species-population reacted in its 

 own way to its environment, one might well 

 expect a similar number of outbreaks (i.e., 

 peaks of fluctuation cycles) every year. 



but, as we shall see, this is rarely the case" 

 (p. 112). And again: "If the synchroniza- 

 tion of outbreaks is a reaUty, it can be ex- 

 plained in various ways. It is suggested that 

 the outbreak of one species may cause dis- 

 turbances in the community which may 

 generate similar outbreaks in other species" 

 (p. 144). 



Elton (1942) presents many pertinent 

 data on excessive changes in population 

 abundance and should be consulted for de- 

 tails. It is appropriate for our purposes to 

 note that many forms at one time or an- 

 other do attain unusually high densities, and 

 the case of the lemming comes immediately 

 to mind. Elton is particularly interested in 

 pointing out that periods of great mammal 

 abundance are not infrequently followed 

 by epidemics— a point we shall discuss later. 

 He reminds us that Charles Darwin had 

 this thought clearly in mind when he 

 wrote, in the Origin of Species, "When a 

 species, owing to highly favourable cir- 

 cumstances, increases inordinately in num- 

 bers in a small tract, epidemics— at least, 

 this seems generally to occur with our 

 game animals— often ensue." Elton also 

 stresses that such epidemics are reported 

 for a number of mammals, among which 

 can be enumerated voles, water-voles, lem- 

 mings, mice, rats, muskrats, beavers, ger- 

 billes, squirrels, marmots, ground squirrels, 

 rabbits, hares, capybaras, moles, hedgehogs, 

 foxes, weasels, deer, zebras, hippopotami, 

 kangaroos, opossums, and others. 



This concludes the chapter on popula- 

 tion growth form. We have tried to make, 

 by means of actual illustrations, the follow- 

 ing points: 



1. A population has a certain life history 

 roughly divisible into periods or phases. 



2. These periods vary in duration (ab- 

 scissal axis) and in numerical size (ordinate 

 axis) with the particular species; with the 

 stability or lability of the efi"ective environ- 

 ment both physical and biotic; and with 

 chance events. 



3. It is meaningful to study population 

 growth foiTn since it provides a numerical 

 measure of the population's past history up 

 to the time of most recent observation; 

 summarizes a wealth of knowledge about 

 the group in question; and, in so doing, 

 raises particular questions for analysis and 

 synthesis. 



