violet are important, of course, to photosynthesis 

 and the trophic cycle, and for heat. 



Continuous ultraviolet radiation of the sun has 

 been measured only haphazardly in the past, and in 

 only a few localities, but the data available indicate 

 only a general relation to the sunspot cycle. Varia- 

 tions in monthly averages of ultraviolet intensity and 

 sunspot numbers may show an inverse rather than a 

 direct relation for a year at a time (Pettit 1932), and 

 variations in yearly averages do not closely corre- 

 spond (Thomson 1936). 



In order to establish any correlation between 

 ultraviolet and variations in the size of animal popu- 

 lations, it is necessary first to determine what stage 

 in the yearly cycle of the animal's activities is sensi- 

 tive to its effect and then to use quantitative data on 

 ultraviolet intensity for only those critical periods. 

 Likewise, other factors, such as rainfall, may mask 

 the effect at certain intensities. Such analyses are 

 complicated, but a few such studies indicate that 

 medium intensities of ultraviolet radiation combined 

 with optimum conditions of rainfall or weather cor- 

 relate with highest populations of chinch bugs, bob- 

 white quail, prairie chickens, pheasants, cottontail 

 rabbits, pronghorns, and the amount of butterfat in 

 cow's milk (Shelford 1951a, 1952, 1954a). 



Solar radition is not received in equal intensities 

 in all parts of the world. Its intensity varies because 

 of inclination of the earth's axis relative to the sun, 

 differences in terrain, amount of cloudiness, and so 

 forth. Solar radiation likewise does not have an iden- 

 tical effect on all species because of differences in 



their sensitivity to it and because critical periods in 

 their life cycles come at different times of the year. 

 The lack of agreement among scientists as to the 

 cause of oscillations in population size is evident from 

 the number of explanations that are offered. It is 

 clear that much more information on animal popula- 

 tions under both experimental and natural conditions 

 is required to obtain full understanding of the dy- 

 namic forces involved. 



SUMMARY 



Populations may increase or decrease in 

 size progressively through a period of time ; they may 

 suddenly irrupt when conditions become favorable, 

 or decline precipitately with unfavorable weather, or 

 they may vary cyclically. The most apparent cyclic 

 phenomena are the 3^ year rodent cycle and the 

 9-10 year grouse-rabbit cycle. Populations of several 

 predators display correlated patterns. Cycles are most 

 clearly developed in the far North. 



Many theories have been advanced as to the im- 

 mediate cause of population cycles : coactions be- 

 tween prey and predator, disease, depletion of food 

 supplies, and changes in nutrient value of foods, phys- 

 iological stress, and physiological vigor, but a com- 

 plete and satisfactory explanation is not yet at hand. 

 There is enough synchrony in the timing of cycles 

 over the world as to suggest that variations in wea- 

 ther or solar radiation may be involved. 



244 Ecological processes and dynamics 



