valuable clues in tracing the cause-and-effect chain of the impact. 



A drawback of the roadside survey technique used in this study is its 

 strong dependence on observer bias in estimating sample abundances; data 

 gathered by two different observers for the same route at the same time may 

 be considerably different, even if both observers are equally skilled. In 

 order to gain some insight into the importance of observer bias, the per- 

 cent similarity of results from the Circle route between subsequent runs 

 was determined, using the (,oefficient of similarity as defined by Bray and 

 Curtis (1957). Prominence values (Beals 1960) were used in place of sample 

 abundances. 



Between-year similarities for 1968-1979 runs of the Circle Route 

 ranged from 0.32 to 0.89 (figure 26). This wide variation may reflect 

 ecological differences in breeding bird populations from year to year, but 

 is most likely due to changes in observers and observer bias. Figure 17 

 shows an increase in similarity between 1968-1970, between 1972-1975, and 

 between 1977-1979, as the observers became more familiar with the birds 

 and the route. Observers were changed between 1970 and 1972 and again 

 between 1975 and 1977, and these changes are accompanied by a sharp drop 

 in similarity (note: no runs were made in 1971, 1974, or 1976). This 

 demonstrates the extreme importance of maintaining observer continuity 

 for long-term monitoring. 



SMALL MAMMAL COMMUNITY PARAMETERS 



Small mammal capture data for 1979 are summarized in table 17. The 

 two habitats first sampled in 1979, Coulee Trunk and Scoria, were sampled 

 only in October. (Note: the biomass increase data presented in table 18 

 of the first monitoring report are in error; each number should be multi- 

 plied by a factor of two). 



Figure 27 shows year-to-year changes in small mammal biomass for four 

 habitats (biomass estimates obtained for control and experimental sites 

 were averaged for combined spring and fall data). It should be emphasized 

 that 1977 data are not strictly comparable, since a spring-fall trapping 

 regime was not used that year. However, it is clear from this figure that 

 tall coulee shrub habitats yielded the most captures by far during all three 

 years. An overall increase in small mammal captures between 1978 and 1979 

 is apparent. 



Figure 28 shows year-to-year changes in the spring-fall small mammal 

 biomass increase (which is related to production) for the same four habitats. 

 The tall coulee shrub habitats are by far the most productive; the spring- 

 fall biomass increase was remarkably large in 1979. Surprisingly, small 

 mammal biomass in silver sagebrush habitats decreased sharply over the summer 

 of 1979, possibly indicating over-winter use of the dense grass and shrub 

 cover offered by this habitat followed by late spring dispersal or predation. 



65 



