POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 355 



fecundity, from being an important cause 

 of population senescence until later. 



It is now fair to conclude that this con- 

 ditioning of the medium, entirely brought 

 about by population activity, contributes 

 significantly to the decline of Tribolium 

 cultures largely through reduction of re- 

 productive rate. It is not yet definitely 

 known just what constitutes conditioning, 

 but the a priori assumption tliat both nutri- 

 tive depletion and an increase in the con- 

 centration of toxic waste products are in- 

 volved seems reasonable. This is a matter 

 for further experimental analysis. There is 

 some collateral evidence supporting tliese 

 two assumptions. First, it has been shown 

 by a number of authors (for summary, see 

 Park and Burrows, 1942) that fecundity 

 performance of Tribolium is highly sensi- 

 tive to quantitative and qualitative changes 

 in nutritive levels of the medium. Second, 

 in addition to the obvious fact that floin: 

 beetles in a dense cultiire release metabolic 

 wastes into the mediinn in proportion to 

 their number and exposure time, it has 

 been shown by Roth and Rowland (1941) 

 and Roth (1943, 1944) that the imagoes, 

 particularly when crowded, liberate a gas 

 from speciaHzed "odoriferous glands" that 

 causes morphological abnormaHties and 

 probably lowers the general well-being of 

 the beetles. This substance has been iden- 

 tified by Alexander and Barton (1943) as 

 "ethylquinone." 



Before a definitive statement about the 

 conditioned flour problem for Tribolium 

 can be made, chemical analyses of the flour 

 must be forthcoming and experiments con- 

 ducted that demonstrate to what extent 

 the observed effects are caused by nutri- 

 tional deficiencies and/or metabolic con- 

 tamination. With these data at hand, an 

 unusually complete analysis of a habitat 

 — > population -^ habitat nexus will exist. 



Some interesting extensions of the con- 

 ditioning problem have been reported by 

 Crombie (1942, 1943). This investigator 

 worked with the granary beetles Tribolium 

 conjusum, Oryzaephilus surinamensis, Rhi- 

 zopertha dominica, and the moth Sitotroga 

 cerealella. 



Crombie's findings are in general agree- 

 ment with the data that fecundity is re- 

 duced by conditioning, but that normal 

 control levels can be restored by reintro- 

 duction of the beetles into fresh medium. 



and that egg fertility is not significantly 

 affected. He concludes further, for Rhizo- 

 pertha at least, that "It is believed that 

 conditioned medium operates upon fecun- 

 dity through 'poisoning,' and that the effect 

 is upon oviposition rather than, as that of 

 starvation, upon egg-production" (1942, p. 

 339). 



Crombie also studied "heterotypic," in 

 contradistinction to "homotypic," condi- 

 tioned medium in relation to the fecundity 

 of these beetles. Homotypic medium (see 

 Alice, 1931) refers to seff-conditioning of 

 the habitat. Heterotypic medium refers to 

 conditioning of the habitat by another, or 

 several other, species. The following hetero- 

 typic situations were explored by Crombie 

 in comparison with homotypic controls: 



Rhizopertha in Ori/zcep/w7«s-conditioned med. 

 Rhizopertha in Siioffogc-conditioned med. 

 Oryzaephilus in rnfooZittm-conditioned med. 

 Oryzaephilus in Rhizopertha-conditioned med. 

 Tribolium. in Orj/zaep/ifZus-conditioned med. 



From these experiments Crombie reached 

 the general conclusion that heterotypic con- 

 ditioning produced no striking change in 

 fecundity over and above (or below) thai 

 displayed by homotypic conditioning when 

 the media were conditioned to the same de- 

 gree. A minor exception to this is noted 

 when the fecundity of Oryzaephilus is as- 

 sayed in Tribolium and Rhizopertha flours. 

 In this case "fecundity in 100 per cent 

 Rhizopertha medium is significantly higher 

 than that in media conditioned to the same 

 degree by Oryzaephilus or Tribolium." 



Conditioning of Water by Goldfishes. An- 

 other facet of the conditioning problem is 

 well illustrated by the investigations of 

 Allee and his collaborators deahng with the 

 conditioning of water by populations ol 

 goldfishes and the demonstration that this 

 is related to growth rate. The roots of this 

 study extend back to the eighteen-fifties 

 when Jabez Hogg (1854) showed that 

 crowding caused snails to be stunted. From 

 then on there has been a steady series of 

 reports showing that growth is reduced by 

 overcrowding. 



On the other hand, a number of reports 

 suggest that increased growth is a result 



