POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



381 



by populations is unknown among animals 

 with the possible exception of certain ter- 

 ritorial species, the concept of random 

 search can be considered "a true fact of 

 nature" and that the competitive pressures 

 resulting from such search can be depicted 

 by a curve of general application. Nichol- 

 son and Bailey differentiate carefully be- 

 tween the search by individuals and that 

 by populations. They assert that even 

 though the former may be systematic, the 

 latter is random and follows a so-called 

 "competition curve." This theoretical curve, 

 as applied to population groups, is repro- 

 duced as Figure 133 and graphs "area 



the theory of random searching is incon- 

 sistent with events as they occur in nature. 

 It will be remembered that Nicholson 

 and Bailey concluded that even if individ- 

 ual population members searched system- 

 atically, the total searching effort ex- 

 hibited by the population would still be 

 random and would lead to the expecta- 

 tions shown in their competition curve 

 (Fig. 133). In discussing this point Thomp- 

 son says: 



"This argument may apply to the searching 

 of areas in the sense that though one animal 

 Qiay take care never to retrace his steps, or 

 cross his own track, he may cross the track of 



0.2 



AREA TRAVERSED 



Fig. 133. The Nicholson-Bailey "competition curve." (After Nicholson.) 



covered" on the ordinate against "area trav- 

 ersed" on the abscissa. By a further exten- 

 sion of this argument Nicholson and Bailey 

 reach the reasonable opinion that as the in- 

 tensity of competition increases, the success 

 of an individual finding the things it seeks 

 decreases. In other words, the amount of 

 new area discovered as time goes on dimin- 

 ishes progressively according to the law for 

 random distributions. 



On the basis of an extended review of a 

 hterature largely concerned with the find- 

 ing and exploitation of hosts by parasites, 

 and on the basis of a lengthy theoretical 

 argument, Thompson observes that ani- 

 mals "do not in general search the environ- 

 ment at random for things they require." 

 This, he asserts, follows in part because 

 nature is organized according to a system 

 of "sign-posts" which correspond to the 

 perceptive powers of the animal in ques- 

 tion, and in part because these perceptive 

 powers establish a definite connection be- 

 tween the animal and whatever it seeks 

 that is, at least to some extent, independent 

 of distance. In sum, Thompson feels that 



others of the same species. It may apply also to 

 the searching of suitable environments, because 

 the visit of one animal to an environment may 

 not prevent the visit of another animal to it at 

 a later date. It does not, however, apply to the 

 searching for environments. It is evident that 

 if individuals do not search for suitable environ- 

 ments at random, then populations do not 

 search for them at random either. The general 

 property of non-random action belongs to the 

 population, just as it belongs to the individual" 

 (pp. 358 and 359). 



This issue has been clarified and given 

 more precise definition by Varley (1941). 



Returning now more specifically to the 

 topic of host-parasite interactions, we may 

 develop this cursorily by presenting certain 

 formal considerations that are utihzed by 

 the epidemiologists and then by reviewing 

 several experimental studies that deal with 

 host-parasite relations among insect popu- 

 lations. 



Jordan and Burrows (1945) discuss host- 

 parasite interactions from a popula- 

 tion viewpoint. They say: "The infectious 

 diseases of man constitute a series of spe- 

 cial cases of the host-parasite relationship, 



