116 



LEFFLER 



System History 



A third consideration in the assessment of an ecosystem's stress 

 response is the effect of history. We might expect that prior exposure 

 to stress would alter a system's future response to similar distur- 

 bances. Data from the nutrient— energy subsidy experiment were 

 examined for such evidence. Coefficients of variation of the 

 steady-state observations were compared by treatment for each of 

 the two perturbations to evaluate the increase or decrease in 

 steady -state variability before and after the first perturbation of each 

 of the nine treatments. The binomial test was used to detect whether 

 a significant (P < 0.100) change in constancy stability had occurred. 

 As shown in Table 5, only the P/R ratio was changed significantly; 

 the perturbation caused a decrease in steady -state constancy. Similar 

 procedures allowed comparisons of resistance, resilience, and total 

 relative stability after the first and second stress periods. The 

 occurrence of a prior disturbance caused night respiration to be less 

 resistant to a second perturbation, but the P/R ratio became more 

 resistant. Resilience and total relative stability were also affected for 



TABLE 5 



EFFECTS OF PRIOR PERTURBATION ON AN 



ECOSYSTEM'S RESPONSE TO STRESS IN THE 



MICROCOSM NUTRIENT-ENERGY 



SUBSIDY EXPERIMENT* 



*The coefficient of variation of steady-state observations, the 

 relative resistance, the relative resilience, and the total relative stability 

 of each of the nine treatments were compared for the first and second 

 perturbations. Significant trends for all treatments were detected with 

 the binomial test at the P=^ 0.100 level. Plus ( + ) indicates that the 

 stability of the microcosms was greater for the second stress than for 

 the first; minus ( — ) indicates that the stability was less for the second 

 stress than for the first; and NS is no significant change. 



