THERMAL ECOLOGY AND STRESS 335 



are, we believe, analogous. Such an assertion is more meaningful if 

 ecosystem stability is viewed as the capacity to maintain equilibrium 

 or to return to equilibrium after the system has been perturbed. 

 Certainly a perturbed system can be considered in terms of stress if 

 ecosystem stability is perceived in this way. 



Brett (1958) proposed a more acceptable definition of stress: "a 

 state produced by any environmental or other factor which extends 

 the adaptive response of an animal beyond normal range, or which 

 disturbs the normal functioning process to such an extent that, in 

 either case, the chances for survival are significantly reduced." The 

 obvious utility of this definition rests with the implication that stress 

 can be viewed in terms of chance or probability, which indicates an 

 implicit assumption that stress can be quantified. Although this is 

 clearly a positive aspect of Brett's definition, there are several 

 objections similar to those raised for Selye's definition. Brett does 

 not consider plants nor ecosystems, and, in addition, he implies that 

 the outcome of stress must be viewed in negative terms. This is not 

 always the case; although it is clearly negative for an individual 

 organism if it dies as a result of exhaustion caused by stress, it is not 

 necessarily negative for the success or survival of a population. For 

 example, mortality of some individuals in a crowded population may 

 actually serve to ensure species survival if space and/or nutrient 

 resources are limiting. It is interesting to note that a similar 

 mechanism may operate among some host— parasite systems, enhanc- 

 ing survival potential of both host and parasite species in an 

 evolutionary sense. Pimentel (1961) and Pimentel and Bellotti 

 (1976) noted that "co-evolution in a host and/or parasite toward a 

 balanced supply— demand economy and regulation of parasite num- 

 bers is possible" through operation of a genetic feedback mechanism. 

 Thus, if virulent parasites are eliminated by parasite-induced host 

 mortality, the probability of propagating less-virulent parasites is 

 enhanced, and survival and reproduction of more-resistant hosts may 

 also be concomitantly increased. Another positive aspect of stress 

 was suggested by Gibbons (1976) when he indicated that a process 

 which may ultimately be negative at the population level can actually 

 be preceded by an enhancement of the same process. Reviewing 

 available literature, he noted, for example, that increases in tempera- 

 ture caused by release of thermal effluent in aquatic systems were 

 shown initially to increase both primary and secondary productivity 

 (Gibbons, 1970) and to alter a variety of species interactions (Saks 

 etal., 1974). 



All these objections to Selye's and Brett's definitions of stress 

 were also discussed by Esch, Gibbons, and Bourque (1975), who, in 



