350 



POPULATIONS 



egg production is not further a£Fected to 

 any appreciable extent. 



In a stimulating paper, Robertson and 

 Sang (1944) reexamine Pearl's work and 

 extend it by a series of ingenious experi- 

 ments. They show that the fecundity of 

 Drosophila is highly sensitive to changes 

 both in the quantity and quahty of the 

 yeast food and advance the following con- 

 clusion pertinent to our present interest: 



"Crowding of adults leads to only a slight 

 lowering of fecundity when the flies are ade- 

 quately fed. There is also little evidence of 

 competition for oviposition space within the 

 limits tested. So the decrease of fecundity 

 demonstrated by Pearl (1932) can take place 

 only when there is competition for food and is 

 the direct result of this competition. This is 

 the correct explanation of his results" ( p. 258 ) . 



Thus Robertson and Sang also conclude 

 that fecundity may be reduced by crowd- 

 ing, but only when the food supply is in- 

 adequate: "If food is scarce then the suc- 

 cess with which it is found depends on the 

 number of flies in the culture." This sug- 

 gests that Pearl's work is a special rather 

 than a general case. The general case can 

 be stated in this fashion: When the food 

 supply, both qualitatively and quantita- 

 tively, is optimal, little in the way of den- 

 sity-dependent coactions affecting oviposi- 

 tion are operating in Drosophila cultures, 

 and egg production is not greatly impaired. 



The experiments of MacLagan and Dunn 

 (1936) with the grain weevil Sitophilus 

 orijzae afford another illustration of popu- 

 lation pressure resulting from coactions that 

 lead to a reduction in fecundity with in- 

 creased crowding. When ovipositing, the 

 female weevil lays a single egg in a small 

 cavity which she excavates in a grain of 

 wheat. For populations that are not 

 crowded— that is, where many grains are 

 available for each weevil to exploit— the fe- 

 male oviposits only in the hairy apex of the 

 grain and usually avoids damaged seeds. In 

 dense cultures oviposition seems to occur 

 indiscriminately over the entire surface of 

 practically all the grains. 



MacLagan and Dunn showed, as might 

 be anticipated, that as imago density in- 

 creased more eggs were laid per grain since 

 there were more reproducing beetles, but 

 the rate of egg production per individual 

 female declined. An interesting relation be- 

 tween maximum fecundity and the availa- 



bihty of wheat grains grew out of these 

 observations. The female weevils would not 

 oviposit at their maximum rate unless more 

 grains were present in the culture con- 

 tainers than were actually utihzed. The 

 authors comment on this point as follows: 

 "Tliis surprising result indicates that the 

 female S. oryzae will not lay this maximum 

 number of eggs unless the number of grains 

 available for oviposition is at least eleven 

 times that actually utihsed. Any reduction 

 in tliis number of grains is accompanied by 

 a reduction in the number of eggs laid per 

 female, despite the fact that she is utihsing 

 only a small proportion of the number of 

 available grains" (p. 133). This may fore- 

 shadow an ecological principle of broader 

 apphcation than is commonly appreciated 

 (see Chap. 25). 



In addition to these relationships, coac- 

 tions are also described between individual 

 weevils that stand in significant relation to 

 reproduction. For example, when consider- 

 ing the decreased fecundity of Sitophilus 

 with increasing density, MacLagan and 

 Dunn's explanation is markedly reminiscent 

 of Pearl's conclusions based on Drosophila. 

 They consider that the "collisions" or 

 increased contacts between weevils in 

 crowded cultures is a fundamental factor 

 and remark that "it operates organically 

 through the reduction of the times avail- 

 able for feeding, ovipositing, and resting; 

 thereby causing adverse effects upon the 

 physiological processes of reproduction" (p. 

 136). 



It should also be mentioned that Crombie 

 (1942), working with the grain beetles 

 Wdzopertha dominica, Orijzaephilus surina- 

 mensis, and Acanthoscleides ohtectus and 

 the grain moth Sitotroga cerealella, reached 

 the general conclusion for all these forms 

 that increased imago density led to a re- 

 duction of fecundity with egg fertihty not 

 affected. When the media were not "con- 

 ditioned" (p. 352) "the reduction of fecun- 

 dity was, it appears, entirely a. result of 

 competition for the oviposition sites usually 

 for two purposes, viz., oviposition and feed- 

 ing. That is to say, at such densities the 

 effect of crowding upon oviposition was of 

 a behaviouristic [coaction] nature" (p. 

 339). 



Another most instructive illustration of 

 population pressure resulting from coactions 

 is afforded by the work of Crombie (1944) 

 on the relation of larval population density 



