POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



345 



well be a combination of both. It seems 

 that the answer as to wliich of these possi- 

 biUties is actually true would be forthcom- 

 ing only after investigation of the particu- 

 lar situation. 



It is also obvious that the chain of events 

 in nature may be even more complicated. 

 A cUmatic condition may operate so as to 

 affect the food supply; for example, favor- 

 able rainfall could bring about lush vege- 

 tation. This in turn could be exploited by 

 members of a herbivorous species, even 

 though there was much more food avail- 

 able than could be utilized. 



This discussion leads us to the point 

 that food furnishes such an inextricable 

 meeting ground in ecology between the 

 physical and biotic environments that it is 

 difficult, and possibly not even worthwhile, 

 to attempt a further breakdown. This is a 

 well-recognized issue. Chapman (1931) 

 speaks to the point as follows: 



"It cannot be said that nutrition is a purely 

 physical factor of ecology for the reason that 

 no animal [except probably the phytoflagel- 

 lates], so far as we are able to discern, is able 

 to live upon a diet which does not contain 

 some compounds which have been synthesized 



< 



CD 

 LlJ 



LxJ 



275- 



250 - 



— 225 



X 



"Z. 



3 200 



o 



CO '^5 



(T 



=) 

 O 

 X 150 



-400 



cn 



UJ 

 CD 



-300 3 



-200 



1902 1903 1904 1905 1906 1907 1908 



YEARS 



Fig. 127. Catch of mackerel in May plotted against sunshine in February in British fishing 



grounds. (After Allen.) 



This matter requires brief elaboration 

 here primarily to set the stage for the more 

 detailed treatment of Food Chains that fol- 

 lows in the section devoted to Communities. 



There is some reason for thinking of food 

 in terms of quantity, or amount required by 

 a population, and quahty, or the kinds of 

 food substances needed by a population. 

 Both quantity and quality may be directly 

 affected by soil and chmate in the case of 

 terrestrial species and by climate, substra- 

 tum, and physicochemical properties of the 

 water in the case of aquatic species. In addi- 

 tion, population size and/or density along 

 with their consequent competitive pres- 

 sures can influence, and be influenced by, 

 both quantity and quality of food. 



by another organism. We have therefore ar- 

 rived at the point where physical autecology 

 and biotic autecology are merged, and it is an 

 arbitrary matter as to whether this chapter is 

 to be included under physical autecology or 

 biotic autecology. All animals are dependent 

 upon at least one other organism for the 

 preparation of their food . . . There is prob- 

 ably no better example in all nature of the 

 interdependence of organisms than that of 

 nutrition itself" (p. 155)." 



" Actually this question is probably not so 

 closed a case as Chapman's quotation implies, 

 particularly so far as the protista are con- 

 cerned. Mast and Pace ( 1933 ) present sug- 

 gestive evidence for "chemoautotrophic nutri- 

 tion" in Chilomonas paramecium (p. 358). 

 Such forms apparently grow in a medium 

 without photosynthesis. 



