322 J- J' Christian 



increased physiologic adaptation to increased density. A causal relation- 

 ship between population density and parasitism in hares is suggested by 

 the studies of Erickson (1944). Parasites were scarce when population 

 densities were low and maximiun parasitism was found in populations of 

 hares at peak densities. The hare populations declined but the parasites 

 remained abundant in the surviving animals. There was no evident correlation 

 between shock disease and the parasites in these hares other than that 

 parasitism was at high levels when the population began to decline owing 

 to shock. Similarly, a close correspondence between the level of parasitism 

 of rabbits by Eimeria stiedae and population density was found in New 

 Zealand (Whittle, 1955) and it was suggested that the differences in the 

 incidence of severe infections were correlated with differences in host re- 

 sistence, which in turn may have been a function of host density (Bull, 

 1955, 1957, 1958). It is of interest that young rabbits were the ones pri- 

 marily affected in these populations (Tj^ndale-Biscoe and Williams, 1955; 

 Bull, 1958). In a study on parasitism by lice on populations of Microtus 

 penrisylvanicvs and Peromyscus maniculatus it was found that the level of 

 parasitism varied with season in both the voles and the deer mice, but that 

 the size of the louse population changed significantly with season only in 

 the male voles (Cook and Beer, 1958). It was suggested that the spring 

 maximum in the louse populations on the male voles came at the period of 

 greatest "stress" and therefore the increase in the louse population at this 

 time resulted from decreased host resistance. 



The evidence for a decrease in host resistance to specific diseases or 

 parasitism in natural populations is not conclusive, but is certainly sugges- 

 tive. However, there can be little (luestion that there are severe physiologi- 

 cal derangements associated with high population densities which result in 

 high mortality rates in voles, lemmings, and hares (Green and Evans, 

 1940b; Elton, 1942; Rausch, 1950; Chitty, 1952, 1954; Frank, 1953; God- 

 frey, 1955), and that this mortality cannot be accounted for by infectious 

 disease, even though the prevalence of infection and parasitism may be 

 increased. It is equally evident that young animals are more severely af- 

 fected than older animals and that the effects are prolonged and account 

 for continued declines in populations for appreciable periods following a 

 sharp reduction in population density (Chitty, 1952, 1954, 1955; Christian 

 and LeMunyan, 1958; Christian, 1959a, b). 



In summary, there is much indirect evidence indicating that a decline of 

 a population from peak densities may be caused by a density-dependent 

 activation of physiologic adaptive mechanisms and exerting their effects 

 in one or all of several ways: 



1. There may be direct mass mortality due to physiologic exhaustion 

 following prolonged, excessive stimulation of the adaptive mechanisms. 



