THE PHENOMENON OF PREDATION — ERRINGTON 511 



would be likely to encounter each other with great frequency in their 

 daily lives. It has always seemed to me that excess minks tend to 

 withdraw from the mink-crowded places, though this might mean 

 wandering or trying to live in ecologically inferior environment. 



If North American minks have any one favorite food, I should say 

 that it is the muskrat. Minks may at times subsist upon muskrat flesh 

 almost as exclusively as wolves may upon venison — with the outstand- 

 ing difference that the minks may not find the presence of large num- 

 bers of muskrats synonymous with availability of large numbers of 

 muskrats as food. Our Iowa data show a peak fall population of about 

 9,000 muskrats living securely on a 935-acre marsh, despite the activi- 

 ties of about 30 muskrat-hungry minks. The distinction between 

 availability to predators and mere presence of prey animals should be 

 emphasized. In the case of our Iowa muskrats, the predation is 

 centered upon overproduced young; upon the restless, the strangers, 

 and those physically handicapped by injuries or weakness; upon ani- 

 mals evicted by droughts, floods, or social tensions ; and upon what is 

 identifiable as the more biologically expendable parts of the popula- 

 tions. 



I do not think that predation should be regarded as a true limiting 

 factor of these muskrat populations. To the extent that predation 

 operates only incidentally, removing little except the wastage parts of 

 populations that are more or less destined to be frittered away somehow 

 through one agency or another, it may make little difference to the 

 population levels reached or maintained if the predation losses are lifht 

 or heavy. I should say that the dominant limiting factor of a muskrat 

 population is still its own sociology, within the frame of reference im- 

 posed by the material features of its environment. 



Another predator-prey relationship in which severity of the preda- 

 tion suffered by the prey may be most misleading in off-hand appraisals 

 of population effects is that of the great homed owl and the bobwhite 

 quail in north-central United States. Our year-after-year popula- 

 tion case histories show heavy predation by low populations of owls 

 upon either high or low populations of quail ; light predation by high 

 populations of owls upon either high or low populations of quail ; and 

 much variation in between. ^Vliat counts in determining the popula- 

 tions reached or maintained is not that the owls have quail to eat or that 

 the quail have owls to eat them. Both species are highly territorial 

 and show a strong degree of self-limitation independently of each 

 other. Big owl or small quail, neither mider normal conditions per- 

 mits itself to mcrease up to levels that are biologically top-heavy. 

 Each of these two species has in this way much in common, though 

 one is subject to very little predation and the other is subject to much. 



In its workings, territoriality tends to separate the haves from the 

 have-nots in a population, with the holders of "property rights" having 



