376 POPULATIONS 



Table 28. Horned Owl Pressure upon Wintering Bobwhite Populations (From Errington, 1936) 



to densities that the local environment can 

 support, the predation drops ofiE. 



During periods of overpopulation when 

 the carrying capacity is exceeded, certain 

 individual quail appear to be weakened 

 from hunger, injuries, and disease, and ex- 

 hibit an increased vulnerability to preda- 

 tion. Under such conditions of overcrowd- 

 ing some coveys are forced into niches that 

 are decidedly unfavorable, while still 

 others wander from one occupied or unin- 

 habitable covert to another. "Badly sit- 

 uated coveys, whether they keep moving 

 or attempt to station themselves in inferior 

 environment, bear the brunt of pressure 

 from enemies" (p. 249).* 



Errington's report on owl and quail 

 populations also furnishes an illustration of 

 predation patterns that are somewhat more 

 complex. This complexity arises primarily 

 because of the organization of the prey 

 population as a territory-inhabiting species. 

 Such organization reduces predation to a 

 point where, in terms of the prospective 

 history of the prey, it is incidental rather 

 than controlling. The quail possess a series 

 of territories which support for any length 

 of time a limited number of individuals- 

 some sixty to sixty-five per square mile for 

 those discussed here. This upper limit is 



* It is to be noted in Table 28 that during 

 the winter 1932-1933 the quail population was 

 "insecure," but owl predation as shown in the 

 right-hand column was also low. This is not an 

 inconsistency (says Errington), but rather 

 reflects the fact that a predator (or predators), 

 in addition to the owls, is feeding on the quail. 

 "One thing that seems characteristic of in- 

 secure populations is their common vulner- 

 ability to a number of different predators, even 

 predators differing greatly in prowess and hunt- 

 ing tactics" ( p. 249 ) . 



set by agencies other than predation. 

 When it is exceeded, the predators, regard- 

 less of kind, seem to remove the excess. It 

 appears reasonable to assume that these su- 

 pernumerary quail would be removed by 

 one means or another in the absence of all 

 predators. For this case, then, predation is 

 proportional to overpopulation rather than 

 to population, as is frequently the case in 

 "simpler" population systems. Studies on 

 this subject and tending to this conclusion 

 form a major contribution of ecology to 

 practical "game management." 



Perhaps the final point to be made about 

 predation is that it can have obvious sur- 

 vival value in terms of the prey popula- 

 tion. Cartwright (1944) definitely suggests 

 this for the sharp-tailed grouse and Hun- 

 garian partridge in Canada. His conception 

 is stated in his own words as follows: 



"This brings us to the role of the predator. 

 It is obvious that a species with a 3-year life 

 span which produced all its young uniformly in 

 June would become extinct if three successive 

 adverse seasons destroyed the hatch. Hence, 

 predation, by destroying a substantial propor- 

 tion of the first and second nestings, daggers 

 the nesting attempts and thus becomes a major 

 factor in the survival of the species. To be 

 effective, the predation must be substantial. 

 Recorded observations show that approximately 

 60 per cent of the first nestings are destroyed 

 by various predators and other causes. Let's 

 just apply this to a theoretical 100 females. 



100 - 60 = 40 X 6 = 240 

 60 - 36 = 24 X 6 = 144 



384 



Thus 100 females suffering 60 per cent preda- 

 tion on two successive nestings produce 384 

 young. Add 100 males and you have 200 adults 



