FIG. 16-8 Relation between population size of house ml< 

 expressed by average amount of food consumed per day 

 emigration of mice away from the colony. The colony was si 

 with five pairs of mice in January (Strecker 1954). 



Emigration 



The pressure of overpopulations can be re- 

 lieved by mass emigrations of individuals from par- 

 ticular localities as well as by their death. It has been 

 shown experimentally that such emigrations will ac- 

 tually occur under conditions of crowding in a mouse 

 population. It is of interest that those individuals 

 which remained continued their normal rates of re- 

 production. This contrasts with the drastic reduction, 

 even cessation, of reproduction in other colonies from 

 which emigrations were prevented. 



Two species of aphids placed in their optimum 

 niches, one at the top. the other at the bottom of a 

 single barley plant, multiplied to saturation and dis- 

 persed downward and upward on the plant until both 

 species came to exist side by side. Continued repro- 

 duction and overcrowding forced surplus individuals 

 to emigrate to surrounding plants over 7.S cm away, 

 leaving the two populations in equilibrium on the 

 original plant. In another experiment where plants 

 were within 3.0 cm of each other, the aphids spread 

 to the preferred sites on the second plant rather than 

 to less favorable spots on the first plant (Ito 1954). 



Emigrations under natural conditions occur when 

 there is overcrowding in the migratory locust, lem- 

 ming, grouse, snowy owl (Gross 1947), snowshoe 

 rabbit (Cox 1936), Arctic fox (Braestrup 1941), 

 gray squirrel, and occasionally in other species 

 (Heape 1931, Dymond 1947). The emigrations of 

 the European lemming in the Scandinavian countries 

 are spectacular (Elton 1942). Emigrations on a re- 

 duced scale are known to occur also with lemmings 

 in North America (Thompson 1955). 



Lemming emigrations do not invariably lead to 

 death of whole armies as popularly believed, but to 

 settlement of new areas, leaving the original area 

 populated with reduced numbers. Emigration must 

 have survival value for the species or otherwise the 



tendency for emigration would have been a weaken- 

 ing factor and disappeared in the course of evolution. 

 It is, of course, population pressure that is re- 

 sponsible in large part for the dispersal of young and 

 extension of ranges into new areas. Under normal 

 conditions adult animals, especially among the higher 

 vertebrates, are well established on their territories 

 and the young are forced to seek homes elsewhere. 

 Among insects, there is a relation between emigration 

 and inherited behavior tendencies. Individual tent 

 caterpillars, both larvae and adults, differ innately in 

 the extent to which they show activity even within 

 the same colony. In the development of populations 

 of excessive size, spread of infestations of the insect 

 into new regions is largely by the more active indi- 

 viduals. The outbreak finally terminates when the 

 proportion of sluggish individuals comes to predomi- 

 nate in the population (Wellington 1960). 



Disease 



Although infectious disease in some form is a 

 common cause of mortality, it is less important as a 

 stabilizing factor than the others already considered, 

 because it reduces the population size in an important 

 manner only when epidemics, or more accurately 

 epizootics, occur. The mortality may then be ex- 

 treme so that the population falls way below the level 

 of stabilization, and a period of recovery follows. 



Whether or not epizootics occur, depends on the 

 virulence of the disease-producing organism, the 

 rapidity with which it is transmitted from individual 

 to individual, and the resistance of the hosts. Worm 

 and protozoan parasites, bacteria, and viruses may 

 be transmitted through body contact of host indi- 

 viduals, by the host ingesting contaminated food or 

 water, or by vectors which are commonly external 

 parasites themselves. It is obvious that ease and ra- 

 pidity of transmission increase with the size of host 

 populations. Overcrowding often also lowers the 

 vigor of the hosts so that they become more sus- 

 ceptible. 



In the course of time, natural selection tends to 

 evolve tolerable relations between hosts and the dis- 

 ease organisms that they harbor. Mutations of dis- 

 ease organisms to greater virulence result in more 

 rapid or extensive die-oflfs of the host with the con- 

 sequence that the mutant strains disappear. During 

 upswings in host populations, extra virulent muta- 

 tions may persist for a time, because of the abundance 

 of host individuals to which they can spread, but 

 when the host population declines, only those host 

 individuals will survive that are not infected with 

 the virulent strain or that develop immunity to it. 



Epizootics among wild animals are often severe, 

 and they usually break out when population densities 



228 Ecological processes and dynamics 



