114 



LEFFLER 



1 1 2 3456789 10 11 12 



PERTURBATION RECOVERY 

 PERIOD PERIOD 



WEEKS 



Fig. 2 Effects of different perturbations on replicate microcosms as 

 described by a subjective evaluation of microcosm health. The 

 stresses illustrated are A, pH of system increased by addition of 

 sodium hydroxide; B, 10 amphipods introduced to each system; C, 

 temperature increased from 22 to 35 C for 2 weeks; and D, 

 temperature increased from 22 to 40 C for 1 week. (See text for 

 discussion of the evaluation.) 



duration, the frequency of occurrence, and the synergistic effects of 

 different stresses also must be considered. 



Parameters Characterizing the Ecosystem 



Understanding ecosystem behavior depends on the parameters 

 used to characterize the system, but this is seldom pointed out in 

 discussions of ecosystem dynamics. This was obvious in the diversity 

 study illustrated by Table 1, where stability rankings of the four 

 treatments varied greatly depending on which parameter was 

 considered. The relative sensitivity of each parameter is also worth 

 noting, especially if microcosm experiments continue to be used for 

 studies of ecosystem dynamics. To evaluate parameter sensitivity, I 

 examined data from the nutrient— energy subsidy studies. For each 

 parameter and each of the nine treatments, coefficients of variation 

 of the steady-state observations were used to rank the treatments in 

 terms of constancy stability. The median coefficient of variation 

 from the first and second perturbations combined was calculated for 



