POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



339 



when muskrats leave their old ranges be- 

 cause of the drought and enter new, well- 

 populated areas. Thus, the primary "trig- 

 ger," drought, is density-independent and 

 sets oflF a chain of events starting with in- 

 creased emigration. The latter brings about 

 more stringent competition for and within 

 habitat niches— this is basically a density- 

 dependent operation. With increased emi- 

 gration goes an increased vulnerability to 

 predation. Also, some of the muskrats actu- 

 ally die from direct exposure during their 

 wanderings, particularly in winter, when 

 the food supply is inadequate and the tem- 

 perature low. 



Shelford (1943) reports that rainfall and 

 snowfall are important climatic factors af- 

 fecting the growth form of populations of 

 the collared lemming (Dicrostonyx groen- 

 landictis) in the Churchill, Manitoba, area 

 of Canada. He found that the local ecologi- 

 cal dispersal of lemmings "varies from year 

 to year with some correlation with the au- 

 tumn rainfall. Following wet autumns they 

 were limited to the sandy ridges and gen- 

 erally distributed in drier years" (p. 483). 

 From a series of weather analyses Shelford 

 tentatively concluded: (1) The capacity of 

 a lemming population to increase is favored 

 by the occurrence of average (or higher) 

 snowfall during the first three months of 

 winter, particularly if the snow stays on the 

 ground so as to provide protection all win- 

 ter. This holds when the snowfall is com- 

 bined with temperatures near normal (or 

 his;her) during the season. (2) Temnera- 

 tures of July and August that are above 

 normal probably also favor population 

 growth. (3) Two successively favorable 

 vears (or at least one average year followed 

 by a favorable year) seem to be required 

 before a maximum population of lemmings 

 can build up. The reader will here recog- 

 nize an operation functionally similar to the 

 development of locust plagues. 



In the chanter on Growth Form an ef- 

 fect of drought on salmon abundance was 

 discussed (p. 321). Here, the initial factor, 

 drought, apparently increased the hazard of 

 predation upon voung river salmon. This 

 affords another illustration of drought act- 

 ing primarilv as a densitv-independent fac- 

 tor, but followed by secondary, density-de- 

 pendent consequences. 



Excessive rainfall, in contradistinction to 

 average rainfall and drought, also can exert 

 a considerable effect on certain natural 



populations. Uvarov (1931, p. 141) sum- 

 marizes an illustration of this for bark- 

 beetle populations as follows: 



"Excessive rainfall during the flight of the 

 adults checks their activities and reduces their 

 progeny, while adults making burrows, as well 

 as eggs and larvae, are liable to be drowned in 

 the sap, which is more abundant when the 

 moisture content of the soil is high. This state- 

 ment is not based solely on theoretical con- 

 siderations, since the development of bark- 

 beetles in a tree can be checked to a con- 

 siderable extent by supplying water to its roots, 

 or by defoliating it and thus recording the loss 

 of water through transpiration." 



The reader is urged to reexamine certain 

 pertinent cases reviewed in Section II. Of 

 particular interest is the treatment of Boni- 

 tation (page 209). 



STORMS 



McClure (1943), in an extensive study 

 of natural populations of the mourning 

 dove, Zenaidura macroura, discusses among 

 other things the question of nesting losses. 

 By observing a group of these birds con- 

 sistently for three years in the vicinity of 

 Lewis, Iowa, he was able to catalogue and 

 evaluate certain agents that destroy the 

 nests and the birds and eggs in the nests. 

 Storms figure prominently among these 

 agents. Practically everv storm regardless of 

 kind, blew poorly placed nests to the 

 ground. Hail storms frequently killed many 

 adults and young when direct hits were 

 scored. Cloudbursts were also effective both 

 in dislodging nests and possibly by drown- 

 ing the birds. Sustained, high winds, in ad- 

 dition to blowing down the nests, snapped 

 off the supporting limbs and felled the 

 trees. The doves usually behaved so as to 

 afford some protection to the young, but 

 were not always successful. For example, 

 during heavy, wet snows in April the birds 

 remained constantly on the nest until well 

 after the snow stopped: during strong 

 winds the brooding parents protected the 

 nest by facing into the wind. "Both par- 

 ents were often seen sitting on the nest fac- 

 ing the cold, strong winds. One bird was 

 apparentlv on the eggs and voung. and the 

 other on the edge of the nest" (p. 394) . 



McClure concluded on the basis of three 

 years' observations that he could account 

 for 46 per cent of the total nesting losses, 

 but that the other 54 per cent resulted from 

 an unknown cause or causes. Of this 4Q 



