ECOSYSTEM RESPONSES TO STRESS 

 IN AQUATIC MICROCOSMS 



JOHN W. LEFFLER* 



Institute of Ecology, University of Georgia, Athens, Georgia 



ABSTRACT 



Ecosystem stress is defined as any change in exogenous inputs which produces 

 an output significantly different from the system's normal response. Any 

 discussion of stress requires a description of the state space in which the 

 ecosystem normally functions. This normal operating range can be either static 

 or dynamic, depending on the time scale of interest. Ecosystem stress responses 

 can be discussed in terms of the system's relative stability. Hypotheses regarding 

 such responses can be empirically evaluated by use of microcosm models. 

 Experiments using aquatic microcosms to evaluate the stabilizing effects of 

 species diversity and nutrient availability are described as examples of this 

 technique. Contrary to previously proposed hypotheses, no relationships 

 between diversity, nutrient availability, or system mass and ecosystem stability 

 or between ecosystem resistance and resilience stabilities were demonstrated. 

 Data from microcosm studies illustrate the importance of specifying the nature 

 of the perturbation, the parameters used to characterize the system, and the 

 prior history of the system in discussions of an ecosystem's response to stress. 



Although stress is a term wdth an obvious meaning, it is difficult to 

 define precisely. Dictionaries provide definitions such as "the action 

 on a body of any system of balanced forces whereby strain or 

 deformation results" and "the internal resistance or reaction of an 

 elastic body to the extenial forces applied to it" (Barnhart and Stein, 

 1964). Medical textbooks consider stress to be any force that elicits 

 increased levels of epinephrine, glucocorticoids, blood glucose, blood 

 coagulation, arterial pressure, and cellular metabolism (Guyton, 

 1971; Smith, Jones, and Hunt, 1972). In most definitions of stress, it 



♦Present address: Department of Biology, Ferrum College, Ferrum, Virginia. 



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