THE ENVIRONMENT 



299 



an organism to be in a particular area, its critical mini- 

 mum requirements for each factor must be present. 

 This minimum, the threshold, is the lowest limit of a 

 factor that produces or allows a visible effect in an 

 organism, or is the amount of a factor that causes a 

 minimum rate of response. Above the critical mini- 

 mum, rate regularly increases with increase in the 

 factor. However, any factor can become greater than 

 a plant or animal can withstand, or can go beyond the 

 critical maximum. Therefore, any species displays mini- 

 mum and maximum limits oj tolerance^ including a 

 range of conditions that are not limiting, for each 

 environmental feature (Figure 17.1). Anything above 

 or below these limits is lethal. 



-tolerance range- 



I 

 I 

 I 



■stress- 



-optimum 



stress- 



^crit 



; 



limits of tolerance 

 critical minimum critical maximum 



Rgure 17.1 Features in on organism's tolerance to a single ecological 

 factor. 



Certain generalizations can be made about toler- 

 ance. First, it varies among species and within a 

 single species. Within a species tolerance usually 

 changes seasonally, geographically, and individually, 

 and is allied to unique appearing or functioning sea- 

 sonal, geographical, and individual variants within a 

 species. Second, a species may have broad limits of 

 tolerance for one factor and very narrow limits for 

 another. Third, when a species is living near the 

 critical limit for one factor, the limits of tolerance 

 for other factors may be narrowed. An example of 

 this would be a plant requiring more moisture than 

 normal because of excessive heat. 



Fourth, within limits of tolerance there is a narrow 

 optimum ra«^c of conditions for an individual organism. 

 Such optimum conditions may be necessary for spe- 

 cific life processes, for example, reproduction, to take 

 place. Somewhere below the optimum range is the 

 critical minimum, and somewhere above is the criti- 

 cal maximum. Between each critical level and the 

 optimum range is a zone of physiological stress, upper 

 and lower conditions of imperfect functional relation 

 to an ecological factor. 



Fifth, organisms are frequently lound living outside 

 the optimum range (but within their limits of toler- 

 ance) of one or more ecological factors. Sixth, en- 

 vironmental features tend to be most limiting during 

 the period of reproduction, because early stages of the 

 life cycle usually have less tolerance than do adults. 



Finally, individual aspects of an organism's toler- 

 ance to environmental factors are generally heredi- 

 tary. Therefore, tolerance is subject to mutation, 

 natural selection, and evolution. Moreover, organ- 

 isms eventually must mutate to survive ever-present 

 geological and ecological changes. Any mutation, 

 beneficial or detrimental, naturally is acquired by 

 chance, and beneficial mutations become mandatory 

 when environmental changes take organisms beyond 

 the limits of tolerance of any part of their hereditary 

 complex. Life cannot be static; it must evolve or 

 perish. 



ECOLOGICAL AMPLITUDE 



The hereditary complex, or gene pool, that enables 

 the particular reactions of an organism to its environ- 

 ment is expressed as an over-all tolerance range to all 

 factors, called ecological amplitude. However, the gene 

 pool is not uniformly distributed throughout a spe- 

 cies; variation is the rule. Therefore, different gene 

 combinations exist and frequently each leads to a 

 unique ecological amplitude. Some of these indi- 

 vidual gene pools are strongly associated with par- 

 ticular ecological conditions. 



The nature of gene pools is seen in a species having 

 an ecological amplitude that would permit its exist- 

 ence in unoccupied but readily available localities. 

 The organism's ecological amplitude is a set of adap- 

 tations to its environment. These adaptations display 

 unlike potentials for survival and success during the 

 various stages of the life cycle, so adults might prosper 

 in a habitat where other stages might not (Figure 

 17.2). However, the usual reason why a plant or 

 animal does not occur where it could otherwise is 

 competition. Because of organisms already in an area, 

 certain other species in spite of their favorable ecologi- 

 cal amplitudes are unable to occupy the locality. For 

 this reason ecological amplitude does not include the 

 hereditary bases that influence competition. 



The above discussion indicates that ecological 

 amplitude varies throughout the life cycle. In addi- 

 tion, the environment varies. Yet ecological ampli- 

 tude is tolerance to all environmental fluctuations. 



