352 POPULATIONS 



by crowding leads to survival. The emi- 

 grating larvae find unexploited niches 

 and develop successfully therein. However, 

 when the total population is "asymptotic" or 

 nearly so, a point is reached "when migra- 

 tion from one grain to another merely leads 

 to death in another place." Parallels to this 

 can be found in natural, as well as other 

 experimental, populations. In fact, we have 

 already discussed one case (that of musk- 

 rats, reported by Errington, page 338) 

 which is similar in some respects. 



Environmental Conditioning 



The discussion thus far has dealt largely 

 with coactions, the focus centering on the 

 aggregate responses between individual or- 

 ganisms of a defined population and the re- 

 lation of these to the population's growth 

 form. We shall now consider "environmen- 

 tal conditioning:" This is defined as a mod- 

 ification of the eflFective environment by 

 population-group activities. Such activities 

 fall into the category of "reactions" as well 

 as "coactions." A situation in which the 

 environment is conditioned by the popula- 

 tion through reactions obtains when numer- 

 ous summed reactions result in the condi- 

 tioning. A situation in which the environ- 

 ment is conditioned by the population 

 through coactions obtains when numerous 

 summed coactions result in the condition- 

 ing. Actually, in most populations both re- 

 actions and coactions play a role in condi- 

 tioning. 



It is helpful to clarify these ideas first by 

 means of a hypothetical, oversimplified ex- 

 ample and then by actual cases. Suppose 

 ten sexually mature green sunfishes of equal 

 size and with similar respiratory rates are 

 living together in an aquarium. Suppose 

 further that each sunfish as an individual 

 member of this population liberates one x 

 unit of carbon dioxide into the water per 

 stated time interval. Such liberation con- 

 stitutes a reaction. The physical habitat is 

 modified by the organism living within it. 

 The total population then liberates lOx 

 units of carbon dioxide over the period. As- 

 sume further, a not unreasonable assump- 

 tion, that the ten fishes so behave towards 

 each other (coactions) that they move 

 about more than they would if isolated, 

 and as a consequence their production of 

 carbon dioxide increases 0.35 units per fish 



per interval.* From this it follows that the 

 total carbon dioxide produced by the popu- 

 lation for the period is 13.5 units, of which 

 10 units result from reactions and 3.5 from 

 coactions. This illustrates, in a somewhat 

 naive fashion perhaps, a population condi- 

 tioning system with both reaction and co 

 action components. 



Should the sunfish population be in- 

 creased from ten to 100 in the same aqua- 

 rium and should the rate of carbon dioxide 

 production per fish remain the same (an un- 

 likely event because of crowding) the con- 

 ditioning resulting from reactions would in- 

 crease from 10 to 100 units. The amount 

 resulting from coactions would also in- 

 crease, either at the same rate (i.e., 0.35 X 

 100 = 35 units) or, more hkely, at a higher 

 rate since the coactions would compound 

 with increase in densitv. The total carbon 

 dioxide conditioning thus has density-de- 

 pendent aspects in terms of both reaction 

 and coaction. It should be repeated that 

 this model is oversimphfied for purposes of 

 illustration. 



On the basis of research carried out on 

 various natural and experimental popula- 

 tions and on various processes, conditioning 

 can be considered by actual cases under 

 the following practical categories: (1) re- 

 duction of the available food supply; (2) 

 partial distribution of available food; (3) 

 addition of contaminants to the environ- 

 ment; (4) liberation of a "growth-promot- 

 ing," or some other needed, substance, to 

 the environment; (5) fixation by the popu- 

 lation of toxic substances ("detoxification") : 

 (6) osmotic regulation of the aquatic envi- 

 ronment; (7) physical conditioning of the 

 substratum; (8) compound conditioning: 

 combinations of certain of the above as, for 

 example, categories one, two, and three. 



Selected examples will now be presented, 

 and it will be indicated how they illustrate 

 the above points. 



Flour Conditioning by Tribolium Popu- 

 lations. Because conditioning of flour by 

 populations of the beetle Tribolium confu- 

 sum has been extensively studied and be- 

 cause it affords an illustration of condition- 

 ing probably resulting both from reduction 



* It would be as reasonable to suppose that 

 the coactions would quiet rather than agitate 

 the fishes (as in a winter aggregation). In this 

 event the group rate of carbon dioxide produc- 

 tion would be reduced. See Shlaifer's ( 1938, 

 1939) reports on goldfishes. 



