Productivity and Populations 311 



Before we examine the evidence relating to grouse and cyclic be- 

 havior, let us pause a moment to cite clear-cut examples of the two 

 types of cycHc effects on living things. 



The case of the apple tent caterpillar {Clisiocampa americana) is 

 a good example of the cyclic eJBFect brought about by a population 

 increase to an untenable density. The caterpillars find an abundance 

 of food on wild and cultivated apples, cherries, and other food plants 

 and, with their inherently high reproductive ability, increase their 

 numbers rapidly over a period of years. The parasites that live on 

 these larvae have a progressively easier task of locating their hosts 

 as the population of the host grows. The curve of parasitic infesta- 

 tion heightens even faster than the curve of tent caterpillar abun- 

 dance and finally, in one opportune year, catches up and causes 

 the abrupt loss of most of the worms. The next year the population 

 of the caterpillars is at the low of the cycle. Likewise the parasitic 

 infestation is lowest, since the parasites have nearly ruined their 

 own food supply. The upward trend of the cycle begins, slowly at 

 first, but gathering momentum each year. And once more the curve 

 of infestation sets out to catch the curve of host abundance. Thus 

 we have a simple cause-and-effect cycle of abundance in two ani- 

 mal species brought about by the specific relationship of parasite and 

 liost. True, other factors such as the weather conditions will affect 

 this picture so that it never appears in actuality quite as smooth in 

 action as we have pictured it. Nevertheless, these populations do 

 follow this basic cyclic pattern. 



Cycles in some forms of plant and animal life caused by physical 

 phenomena that themselves occur in cycles are fairly well estab- 

 lished. Best knov^m are the cycles in "sunspots" that repeat at about 

 ten- to eleven-year intervals (see writings of R. E. DeLury, Harlan 

 Stetson, and others ) . These solar changes apparently are related to 

 the well-established weather cycles ^ that occur in periods bearing 

 multiples of about eleven years (Abbot, 19-35). It has been shown 



^ Authorities do not yet agree as to the effects of sunspots on our weather. In 

 Science (Vol. 95, No. 2473, May 22, 1942) it was reported: "Sunspots have their own 

 effect on the earth's weather. They give off vast streams of electrically charged parti- 

 cles that shoot through space. Some of them, entering the earth's atmosphere, serve as 

 nuclei for tlie condensation of water vapor in the upper atmosphere and thus lead to 

 the increase of cloudiness and of rainfall." Nine months later, in Science News Letter 

 (February 27, 1943, p. 137), Dr. Seth B. Nicholson of the Mt. Wilson Observatory 

 was reported to have concluded that "Vague correlations between sunspots and the 

 weather probably exist, but the weather is affected so much by other factors that tlie 

 influence of sunspots is insignificant." 



