342 



POPULATION ECOLOGY: 



than average, why more litters are produced per year, 

 and why other phenomena generally elevate the 

 biotic potential. 



Some recent studies lead to a hypothesis that 

 might well be the answer to the mystery of cycles. 

 This is the only hypothesis that accounts for both 

 increase in vigor and vitality during population 

 growth and decrease during population decline. The 

 hypothesis uses the physiology of stress. Stress is 

 strain, pressure, urgency, effort, exertion — the many 

 phases of environmental resistance. The physiology 

 of stress encompasses an organism's functional re- 

 actions to stress. Unfortunately, the effects of stress 

 upon internal functions are poorly known. However, 

 there is evidence that many human disorders, such 

 as colds, arthritis, and mental illness, are either a 

 direct consequence of stress or an indirect one in 

 which stress creates the susceptibility to these dis- 

 orders. Therefore, it seems that when great stress 

 occurs, man becomes less vigorous and has less vital- 

 ity. 



Assumptions about the physiology of stress can be 

 applied to rodent cycles. The general hypothesis is 

 as follows: When a population is small, there are less 

 contacts among individuals. This would mean less 

 competition, less strife for food, shelter, mates, and 

 other life activities. The reduced competition would 

 in essence mean less stress placed upon individuals of 

 the species, so the individuals would display high 

 vigor and vitality and there would be more young per 

 litter and more litters per year. Naturally, if young 

 also were subject to less stress, more of them would 

 survive. This, then, might explain why cyclic popu- 

 lations periodically have an eruption in numbers. 

 The next phase of the cycle would come from 

 much greater stress resulting from the competition 

 created by increased individuals in the population. 

 With ever-increasing stress there would be a parallel 

 reduction in vigor and vitality; and at the height of 

 stress, there would be conditions already observed in 

 nature. For example, litter size is smaller, fewer 

 litters are born each year, all animals are more sus- 

 ceptible to parasites and disease, oflfspring reach 

 adult size but are not sexually mature, and animals 

 captured alive seem to die from "shock" alone. 



CARRYING CAPACITY 



The subject of population variations is conveniently 

 summarized by the concept of carrying capacity, the 



maximum number of individuals in each population 

 or the entire community that a particular geographic 

 unit can support (Figure 18.1). The two major fac- 

 tors, biotic potential and environmental resistance, 

 are the antagonistic forces that affect carrying ca- 

 pacity. To summarize, biotic potential is the innate 

 tendency for organisms to increase, that is, all aspects 

 of potential increase within the general framework of 

 ecological amplitude. Environmental resistance is the 

 biotic and physical complex of factors which curtail 

 increase and place an absolute maximum of num- 

 bers (carrying capacity) for each population within 

 an area. 



biotic potential 



environmental resistance 



periodic cycle 



senescence 



Figure 18.1 Populotion numbers under particular conditions, from 

 origin and original population growth to cyclic behavior, senescence, 

 and extinction. Minor fluctuations in numbers and seosonol fluctuations 

 during the periodic cycle ore not shown. 



SPATIAL RELATIONS 



Organisms require a definite amount of space in 

 which to live. This need causes animals to display 

 many reactions that are summarized in the concepts 

 of home ranges, territories, migration, emigration, 

 and immigration. A home range is the area occupied 

 by an individual animal; a territory is the portion of 

 the home range that is actively defended by the ani- 

 mal. Migration is the cyclic movement of populations 

 between two or more places, emigration is the perma- 



