TABLE 16 3 Roproductivlty of the grain weevil 



Grain Weevil 



Weevils per gram of grain 0.25 0. 



Number o( grains per weevil 100 50 



PopulaUon size after 64 days 69 95 



Progeny per weevil 17.2 11. 



stages induced liy intraspecific competition, preda- 

 tion, and conditioning of the habitat becomes exten- 

 sive w'nh. overcrowding and is usually much more 

 important. 



Prcttation 



It is well known that variations in the popula- 

 tion level of predators coincide or often follow closely 

 after variations in the population of prey species, but 

 it is not always certain whether the number of preda- 

 tors depends simply on the abundance of prey serving 

 as food, or whether the predators by their feeding 

 regulate the number of prey animals. Experimental 

 studies amply demonstrate that under certain condi- 

 tions, at least, both true predators and parasitoids 

 greatly affect the numbers of the species on which 

 they feed, and hence similar relationships may be 

 looked for under natural conditions. 



.■\ study made in California shows clearly that 

 while the long-tailed mealybug increases rapidly on 

 the citrus trees from March through May, their 

 populations are reduced by June or July by the ac- 

 tion of three insect predators, two lacewings and a 

 lady beetle. The predator populations are low com- 

 pared with the prev, but each predator destroys many 

 mealybugs (DeBach 1949). 



Quantitative determination of the significance of 

 predation in controlling vertebrate populations under 

 natural conditions is difficult to make, since it re- 

 quires accurate measurement of the number of prey 

 per unit area, the number of predators in the same 

 area, and the number of prey taken by the predators. 

 In one of the best such studies (Errington 1937a), 

 carried on in Wisconsin and Iowa between 1930 and 

 1935, the population of the prey, the bobwhite quail. 

 was expressed in percentage of saturation or carrying 

 capacity of the area, and the extent of predation by 

 the great horned owl was given in terms of percent- 

 age of owl pellets containing quail remains. .A.lthough 

 there is considerable variability evident, the general 

 trend is for the percentage of predation to increase 

 with the density of the prey population in the manner 

 of a density-stabilizing factor (Table 16-4). It ap- 



l)ears that at densities of prey below the carrying ca- 

 pacity of the area, when the bobwhite can find plenty 

 of cover and food close at hand, predation is very 

 low, but as soon as populations reach densities above 

 the carrying capacity so that surplus individuals are 

 forced to make use of inferior cover or go greater 

 distances in search of food, predation intensifies. 



Some excellent studies of predator-prey coactions, 

 especially in vertebrates, have been concerned with 

 the relations between a single predator and its vari- 

 ous prey species (Tinbergen 1933, Errington et al. 

 1940, Errington 1943, Murie 1944, Tinbergen 1946, 

 Fitch 1947, Dunnet 19.S.S, Craighead and Craighead 

 19.S6), or between a particular prey species and all 

 its predators (Errington 194.5, 1946, Bump et al. 

 1947, Koford 1958), but a complete understanding of 

 the role of predation in regulating population levels 

 requires a knowledge of coaction between all prey 

 species and all predator species within community 

 limits, since interrelations between any two species 

 are affected by the interrelations of each species with 

 others in the community. Thus when meadow voles 

 are abundant an owl will feed largely on that single 

 species, but when populations of meadow voles be- 



MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 



FIG. 16-7 Relation of changes in the populations of long-taili 

 mealybugs and their predators, both living on citrus trees 

 California during 1946 (DeBach 1949). 



Regulation of population size 225 



