ENVIRONMENTAL RELATIONSHIPS 



mum. Here they function with greatest efficiency, reproducing and develop- 

 ing most rapidly, and expending the least energy in counteracting the effects 

 of unfavorable environmental tendencies. In relation to physicochemical 

 factors, the evident harmony between the animal and its environment results 

 from avoidance by the animal of markedly unfavorable conditions, acclimatiza- 

 tion of the individual to prevailing conditions within its limits of tolerance, 

 and gradual adaptation of the species, by selection, through changes in its 

 tolerances. 



The Biotic Environment 



Every environmental situation capable of supporting life, whether it be a 

 sand dune, a fresh-water pond, a marine tide pool, or the intestine of a frog, 

 contains a characteristic population of different kinds of organisms. All these 

 organisms are adapted to the prevailing physical conditions of the habitat, 

 and collectively they form what is termed a community. The fundamental 

 character of the community is determined by the nature of the habitat, that is, 

 by its physical features. Superimposed upon these are the biotic factors, 

 which bind the members of the community together in a complex fabric of 

 action and reaction. In a typical community careful investigation of the 

 animals alone may reveal thousands of individuals, of dozens of species, 

 representing several different phyla. Considering the plant species in addition 

 (and even in animal ecology, the plants may not be ignored), the immense 

 difficulty of establishing clearly the interrelationships within such a com- 

 munity is apparent. The complexity of typical communities, and the num- 

 bers of organisms involved, have indeed been obstacles to rapid progress in 

 this aspect of ecology. Yet from the many excellent studies which have been 

 made there emerge several principles, apparently of universal applicability 

 to problems of the interrelationships between organisms within communities. 

 We shall discuss some of these principles in the paragraphs that follow. 



Food Relations. Only plants of various kinds, and the relatively few 

 species of photosynthetic green protozoans, are capable of utilizing directly 

 the radiant energy of sunlight. All other organisms depend on foods for their 

 energy; that is, on the energy-rich organic compounds contained in the bodies 

 of other organisms. Foods constitute the most important single requirement 

 of animals, and it is not surprising to find that the basic relationship between 

 organisms in a community involves their food requirements and food supplies. 

 As indicated in Figure 19.4, the members of the community are intercon- 

 nected by definite food chains, each animal feeding upon the kind next 

 below it in the chain and itself serving as food for the type next above it. 

 Thus, the fundamental relationships between animals are those of predator 

 and prey. A particular kind of animal may have a place in more than one 

 food chain, and hence the chains intermesh to involve the entire population 

 of a community in what are sometimes termed food webs. 



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