ECOSYSTEM RESPONSES TO STRESS 113 



EVALUATIOIM OF AIM ECOSYSTEM'S STRESS RESPONSE 



The examples discussed demonstrate how an empirical approach 

 can be used to evaluate general theories of ecosystem stress response. 

 Microcosms can be used to model ecological processes common to all 

 ecosystems. They provide the advantage of controlling many 

 extraneous variables that complicate attempts at rigorous field 

 experiments. Studies of an ecosystem's response to stress must also 

 be of sufficient duration for dynamics to be observed. This factor, as 

 well as ease of replication, make microcosms a practical and 

 economically feasible approach for studying the theory of stress 

 ecology. A researcher must be aware, however, of the many factors 

 that influence interpretation of a system's response to stress. 

 Foremost is the validity of the microcosm as an accurate analog of 

 the natural systems of primary concern. In addition, such factors as 

 the type of perturbation, the parameters used to characterize the 

 system, and the historical development of the system must be 

 considered in discussing the stress response of any ecosystem. 



Nature of the Stress 



Although theoreticians tend to generalize about ecosystem 

 dynamics, it is not surprising that the specific type of stress to which 

 a system is subjected will greatly influence its response. Figure 2 

 shows the differing responses of replicate microcosms of the 

 21-species system from the diversity experiment after being sub- 

 jected to several types of perturbations. Each trajectory represents 

 two microcosms receiving the same perturbation. The ordinate scale 

 is a subjective evaluation of microcosm health based on a grading 

 system from 1 to 4, with plus (+) or minus (— ) indicating variation 

 within a category; the categories are 1, yellow and brown colors 

 dominant, much debris, metazoans dead; 2, green color dominant 

 but yellow and brown colors common, debris present; 3, normal, no 

 different from controls; and 4, dark green color, suggesting high 

 productivity, structure similar to controls. Stresses on the various 

 microcosms included increasing the pH to 8.8 by adding sodium 

 hydroxide (curve A in the figure); introducing 10 amphipods 

 {Gammarus sp.) into microcosms (curve B); increasing the tempera- 

 ture from 22 to 35°C for 2 weeks and then returning it to normal 

 (curve C); and increasing the temperature to 40°C for 1 week and 

 then returning it to normal (curve D). Although the qualitative 

 nature of the stress is obviously important, curves C and D illustrate 

 the effect of different intensities of the same perturbation. The 



