ECOLOGY AND ZOOGEOGRAPHY 



635 



chain becomes completed when the fish and 

 other organisms die and are destroyed by 

 bacteria, thus making the materials again 

 available to the plants. If a molecule of an 

 element like phosphorus were made radio- 

 active, the path of this tagged element 

 would clearly demonstrate the chainlike 

 nature of the feeding relationships of or- 

 ganisms living in this lake community. The 

 relationship of food chains and the life cycles 

 of parasites would also be demonstrated. 

 Probably no living thing exists which 

 does not serve as a link in some food 

 chain. 



Limiting factors 



The presence and success of an organism 

 or group of organisms depend upon a com- 

 plex of conditions. Any condition, which is 

 less than or exceeds the limits of tolerance, 

 is said to be a limiting factor. By consider- 

 ing all possible limiting factors at once, in- 



stead of focusing upon each one separately, 

 we are able to study their interactions. Thus 

 an animal might be able to tolerate more 

 heat in the presence of water than in its 

 absence. 



Energy transfer 



In any ecosystem energy is continually be- 

 ing transferred from the physical environ- 

 ment to living organisms and from one 

 group of living organisms to another (Fig. 

 445). This is the very basis of life. The 

 account of the web of life illustrates some 

 of the principles involved. It will be recalled 

 that the web of life actually consists of a 

 number of interlinked food chains. There 

 are three types: the predator chain which 

 starts with plants and proceeds from small 

 to larger animals, the parasite chain which 

 goes from large to smaller organisms, and 

 the saprophytic chain which proceeds from 

 dead material to microorganisms. 



PRODUCER 

 ORGANISMS 

 (Large plants) 



ABIOTIC 

 SUBSTANCES 

 (Basic organic 

 and inorganic 

 compounds) 



PRODUCER 

 ORGANISMS- .^ 

 (Phyfoplankton) * 



(Zooplankton) 

 / 



PRIMARY 

 CONSUMERS 

 (Herbivorous 

 animals) 



\ 



\ 



SECONDARY 

 CONSUMERS 

 (Carnivorous 

 animals) 



l._ - 



TERTIARY 



CONSUMERS 



(Carnivorous 



animals) 



REDUCERS 



(Bacteria 



and fungi 



of decay) 



Figure 445. A fresh-water pond ecosystem. An ecosystem is a natural unit that includes living 

 and nonliving parts, interacting to produce a stable system in which the exchange of materials 

 between the living and nonliving follows a circular path. The ecosystem is the largest functional 

 unit in ecology because it includes both the living and nonliving environment. (After Funda- 

 mentals of Ecology by E.P. Odum. Copyright 1953 by W.B. Saunders Company.) 



