340 



POPULATIONS 



per cent, inclement weather, a density-in- 

 dependent factor, was by far the most im- 

 portant decimating agent, producing about 

 25 per cent of all losses. The factors com- 

 prising the remaining 21 per cent included 

 such items as predation by fox squirrels 

 (4.4 per cent), blue jays (2.4 per cent), 

 and cats (1.9 per cent); sterile and deserted 

 eggs (4.2 per cent); young falling from or 

 dying in the nest (1.6 per cent) .* 



CHEMICAL FACTORS 



A number of factors aflFect populations in 

 a density-independent fashion roughly clas- 

 sifiable as "chemical factors of the environ- 

 ment." These are of particular significance 

 for natural, aquatic groups, but examples 

 also can be found among studies of labora- 

 tory and natural, terrestrial populations. In 

 this section we select a few illustrations 

 sampled rather widely from the literature 

 and dealing with a diverse series of forms. 



Hydrogen Ion Concentration 



There was a time not many years past 

 when ecologists considered pH to be an 

 omnipotent environmental agent. This opin- 

 ion, as mentioned in the Historical section, 

 has been considerably revised, and it has 

 now frequently been found difficult to 

 prove that acidity or alkalinity within 

 ranges commonly experienced in nature 

 have any limiting or stimulating eflFect on 

 populations whatsoever. The underlying 

 chemical basis for pH; the pH values of a 

 number of natural environments; the rela- 

 tion of this factor to the ecological distribu- 

 tion of animals; and certain generalizing 

 statements are to be found in Section II 

 (p. 172). 



Edmondson (1944) studied the distri- 

 bution of sessile rotifers in a number of in- 

 land lakes and attempted to correlate this 

 distribution with chemical factors of the 

 medium and with floral and substrate fac- 

 tors. He found, after appropriate statistical 

 analysis, that many pairs of species are 

 associated together as natural populations 

 more frequently than would be expected on 

 the basis of a hypothesis of independent, 

 random distribution. There were a few 

 cases of apparent mutual exclusion, but 

 these were so rare in comparison with in- 



• Consult also the paper of Baskett ( 1947 ) 

 dealins^ with nesting losses in populations of 

 the ring-necked pheasant (Phasianus colchicus 

 torquatus) . 



stances of associations that the conclusion 

 was reached that "while external factors 

 favorable or unfavorable to species are ol 

 major importance, competition does not ap- 

 preciably affect the composition of the 

 fauna" (p. 64). 



It was shown that many rotifer species 

 are sensitive to the chemical nature of their 

 habitat and frequently occur with higher 

 incidence in those lakes on one side of the 

 median of concentration of specific, dis- 

 solved materials than in lakes on the other 

 side of the median. Both hydrogen ion and 

 bicarbonate concentrations were extensively 

 studied. Edmondson concluded that "some 

 species are very Ukely excluded from lakes 

 by high bicarbonate concentration, but not 

 necessarily high pH" (p. 64). Some rotifers 

 were tolerant as populations of media of 

 considerable alkahnity, but not specifically 

 of higher bicarbonate concentrations. A 

 few forms could not tolerate high concen- 

 trations of either. 



Substratum was an important factor in 

 distribution. Edmondson reports that thirty- 

 two species of rotifers are hmited by "chem- 

 istry" or markedly associated with particu- 

 lar substrates. Of these thirty-two, the dis- 

 tribution of fifteen was correlated with both 

 the chemical and substratal features, fifteen 

 with substrate alone and independent of the 

 chemistry, and two with chemical factors 

 alone. 



For our purposes the general conclusion 

 emerging from this interesting study is that, 

 while a few sessile rotifer populations may 

 be specifically sensitive to hydrogen and/or 

 hydroxyl ions, in most cases in which chem- 

 ical factors are influential at all, it is the 

 bicarbonate concentration that is significant. 

 Thus, although pH may function as an im- 

 portant environmental index, it is not per 

 se a factor of much general import. 



The principal reason for reporting Ed- 

 mondson's study at this point is to strength- 

 en somewhat at the population level our 

 general argument that pH when critically 

 examined may not be so important ecologi- 

 cally as it may seem at first glance. This 

 is not to deny, of course, obvious and 

 proved cases of pH effects, certain of which 

 have been already considered in Section II 

 (p. 172). 



Salinity 



Andrews (1940) discusses the effect of 

 changes in salinity on natural populations 



