TABLE 16-4 Evaluation of horned owl winter predation on bob- 

 white (from Errington 1937). 



Number of Intensity of predation 



locality- Density of prey in in per cent of owl 



winter per cent of carrying pellets containing 



capacity quail remains 



records 



come reduced, the owl will prey to a greater extent 

 on other species if they are available (Table 16-5). 

 Other species that take the brunt of predation when 

 a perferred species becomes reduced in availability 

 are called buffer sfiecies (Bump ct al. 1947). 



Not only is there variation in the food of preda- 

 tors dependent on the availability of several potential 

 prey species, but with any one prey species there is 

 variation in the number and kinds of predators affect- 

 ing it, dependent on its density and vulnerability. 

 During outbreaks of insect or mouse plagues, preda- 

 tor species of many kinds converge on the easily ob- 

 tainable food supply (Piper 1928, McAtee 1922, 

 Kendeigh 1947). Predatory pressure is therefore 

 very flexible, shifting its major impact from species 

 to species and from one locality to another. This 

 makes the evaluation of predation as a density- 

 stabilizing factor particularly difficult, but there can 

 be no doubt that the application of predation pres- 

 sure from all sources often exerts an important regu- 

 latory influence on prey populations. There are too 

 many cases on record of prey species developing dis- 

 astrous overpopulations when predatory species are 

 artificially eliminated to think otherwise (Ball and 

 Hayne 1952). 



When only a small number of species are involved 

 in the food web, as in arctic communities or with in- 



TABLE 16-5 Percentage of different prey species taken by wood 

 owls, during a winter (1930-1931) in which populations of 

 European meadow voles were high, compared with the following 



winter (1931-1932) 

 Tinbergen 1933). 



Species 



Winter 

 1930-1931 



educed (fr^ 



Winter 

 1931-1932 



Rodents: 



European meadow-mouse 

 House mouse, European 



woodland mouse 

 European red-backed 



mouse 

 Norway rat 

 Birds 

 Miscellaneous 



52.0 



14.2 



2.1 



1.9 



27.1 



2.7 



sect pests infecting cultivated crops, stability of popu- 

 lation levels is difficult to attain. An increase or de- 

 crease in the abundance of any one species produces 

 changes in all other species. On the other hand, 

 when a large number of species are involved, as in 

 tropical communities or in complex stands of tem- 

 perate zone vegetation, each predator has so much 

 choice of prey and each prey species is subjected to 

 attacks from such a variety of predators that a sudden 

 change in the population level of any one species is 

 absorbed without greatly affecting the stability of the 

 community as a whole (Voute 1946, Craighead and 

 Craighead 1956, MacArthur 1955). Cycles of popu- 

 lation are much more prevalent, therefore, in far 

 northern communities where the variety of species is 

 scanty than in the highly complex communities of 

 southern latitudes. 



The importance that predation may have to main- 

 tenance of health and vigor in prey populations, aside 

 from regulating their numbers, is of significance. In 

 careful observations of 688 attacks by hawks on other 

 birds, only 7.6 per cent were successful in the cap- 

 ture of the prey, but of these successful captures, 

 over 19 per cent of the victims had previously shown 

 injuries, abnormalities, or unusual behavior (Rude- 

 beck 1950-51). In another similar study, the ab- 

 normal individuals among the victims varied from 

 14 to 33 per cent (Burckhardt 1953). Wolves have 

 great difficulty catching healthy adult caribou, and 

 even calves are not often overtaken except in the con- 

 fusion of a large herd. Over 50 per cent of the kills 

 that wolves make are of crippled or sick caribou, 

 although the incidence of such individuals is less 

 than 2 per cent (Crisler 1956). Water boatmen with 

 one or more legs artificially amputated were de- 

 stroyed by fish in an experimental setup at a faster 

 rate than were normal individuals (Popham 1942). 

 Apparently predation exerts a selective force and less 

 fit individuals are eliminated in greater proportion 

 than are the fit. 



The ultimate result of parasitoids is the death of 

 their hosts. They function in curtailing over-popula- 

 tions of host species in nearly the same manner as do 

 true predators. The relations between host and para- 

 sitoid may, however, be more varied and complex 

 (Nicholson 1933) than between a true predator and 

 its prey. 



The normal relation between host and parasitoid 

 is one of equilibrium where neither becomes overly 

 abundant or overly scarce. This means that the abun- 

 dance of parasitoids must also be controlled by 

 density-stabilizing factors. Parasitoids may in turn 

 be infected with hyperparasitoids, and the relations 

 between the two are similar to those between the 

 pasasitoid and the original host (Nicholson and 

 Bailey 1935). 



At low host densities, the reproductivity of the 



226 Ecological processes and dynamics 



